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Excretion &
internal environment of the body. In vertebrates the two functions of excretion and osmoregulation are • Excretion is the elimination to waste products from
performed by kidneys and their associated structures in urinary system.
• Waste products are unwanted and toxic by-products
• The organs which form, store and void the urine which are removed to maintain homeostasis and constitute urinary system.
protect the body from their toxicity, e.g., jaundice in case of bile pigments, uremia in case of urea.
• Perspiration is another excretory process which removes salts and water although the primary purpose • Waste products (which do not accumulate in the body) are nitrogenous materials, CO , pigments, excess of water, inorganic salts, vitamins and hormones. Carbon dioxide and some water are excreted by lungs. Other ExcrEtory products waste materials are removed in the urine.
• The organs and tissues participating in the excretion • Excretory substances are produced during metabolism of waste products constitute the excretory system. of nitrogeneous substances like amino acids and A major function of the excretory system is the
nucleic acids.
excretion of nitrogenous waste. These include ammonia, urea and uric acid.
• Metabolism of carbohydrates and fats produces CO and H O which are easy to remove. Their excretion • Excretion also helps to maintain a constant body is effected through lungs (expired air), skin (sweat) or temperature by removing excess heat and maintain kidneys (urine).
a constant internal environment in association with the other system of the body.
• Other excretory products are pigments, mostly formed by the breakdown of haemoglobin; drugs etc.
• Defaecation is elimination of undigested food residue
from alimentary canal while secretion is the discharge
• Protein metabolism produces nitrogenous waste of specially synthesised product, e.g., hormone by material such as ammonia, which is the basic endocrine gland, saliva from salivary gland.
nitrogenous catabolite of protein, formed by breakdown of amino acids. Removal of the amino • Osmoregulation (term coined by Haber) is the
group (NH ) is known as deamination and it converts
regulation of osmotic concentration, that is, water the amino acid into a keto acid. In vertebrates, contect and salt and cells.
deamination takes place in the liver. Ammonia thus
• Both excretion and osmoregulation are helpful in produced is highly toxic and cannot be stored within maintaining homeostasis or constant favourable the body. It needs to be eliminated immediately.
Objective biology Table: Excretory organs of different animals.
General body surface General body surface In the above three, contractile vacuole is also there which is not really an excretory organ.
It is specially for water balance & helps to get rid of extra water that diffuse into the cell.
Flame cells (Solenocytes) Larva of platyhelminthmiracidium, redia larva, Renette cell (excretory cell) Chloragogen cells Earthworm megascolex Malpighian tubules, uricose gland, Spider, Scorpion (arachnida) Prawn (crustacea) Special glands called rectal glands reabsorb water and ions and urine which are mixed with faeces. This is an adaptation of dry habitat.
Tubefeet (podia) & dermal branchea Starfish(thin walls of gills) • Depending upon the form in which the ammonia acid (formed during fat metabolism) in food with the or nitrogenous waste is excreted from the body, the amino acid ornithine.
organisms are grouped as under into three categories:
- Hippuric acid: It is formed when benzoic acid is
ammonotelic, uricotelic and ureotelic.
combined with glycine. It is less toxic. • Besides these (ammonia, urea and uric acid) there - Creatinine: Mammals contain a small quantity of
are a few other types of minor nitrogenous excretory creatinine in their blood (1 mg/100 ml) which is a products, like— derivative of creatine. The excess is eliminated along - Trimethylamine oxide (TMAO): Marine teleost
fishes excrete a large proportion of their nitrogen - Creatine: Mammals also excrete creatine which is
as trimethylamine oxide (TMAO). Large amounts synthesized in the liver from three amino acids— of this compound is also stored in their body for arginine, glycine and methionine.
osmoregulation i.e. to minimize loss of water and entry of salts.
- Guanine: Spiders excrete almost exclusively a
• Animals excreting their nitrogenous waste in the chemical called guanine. It is even less soluble as form of ammonia are known as ammonotelic. This compared to uric acid and hence requires no water phenomenon is known as ammonotelism.
for elimination. It is a metabolic waste of nucleotide • Ammonia is highly soluble in water with which metabolism. It is also found in penguins.
it forms ammonium hydroxide (NH OH) which - Ornithuric acid: It is excreted in small amount by
injures cells directly by alkaline caustic action. Hence birds and is formed by a combination of benzoic excretion of ammonia requires large amounts of Objective biology

water to be lost from the body. That is why such a uric acid. Part of uric acid is oxidised further to form
mode is suitable for aquatic organisms which have a allantoin and allantoic acid. Teleost fish excrete
constant access to water. allantoate or hydration product of allantoin. In most • No energy is required to produce ammonia.
fishes and amphibians, allantoate is hydrolysed to urea and glyoxylate. Some marine invertebrates have • E.g. all aquatic invertebrates, bony fishes and aquatic gone a step further by hydrolysing urea to ammonia and carbon dioxide. Creatine is formed in liver from • Ammonia is the first metabolic waste product of
amino acids. Creatinine is produced from creatine.
protein metabolism.
• Conversion of ammonia to uric acid and its subsequent elimination requires lesser amount of Aminotelism is the excretion of amino acids water as it is comparatively less soluble in water which cannot be metabolised due to their being and less toxic as compared to ammonia. Hence, it in excess. The animals performing aminotelism is observed in terrestrial animals that do not have are called aminotelic, e.g. some molluscs (Pila, Unio, Limnaea) and some echinoderms (starfish, constant access to water or rather have limited access to water.
• In anurans (amphibians) the larval tadpoles excrete ammonia, while the adults produce urea.
• Animals that excrete their nitrogenous waste mainly in the form of urea are known as ureotelic and the phenomenon is known as ureotelism.
• Animals which excrete their nitrogenous waste • Urea can be stored in body for considerable periods mainly in the form of uric acid and urates are of time, and is least toxic. It is eliminated in the form known as uricotelic. The phenomenon is known as
• Ureotelism is exhibited by semi-terrestrial • All terrestrial animals like insects, reptiles, and birds animals, e.g. some earthworms, adult amphibians, excrete uric acid.
elasmobranch (cartilagineous fishes) and mammals.
• Uric acid (C H N O ) (which require more energy) • Frog like other amphibians is ammonotelic in tadpole is produced by degradation of purines (e.g. guanine) state and ureotelic in mature state. Earthworm in liver and kidneys to some extent. In uricotelic is similarly ammonotelic when sufficient water is animals, excess nitrogen is first used in synthesis available and ureotelic when water availability is of purines. A purine is changed to xanthine (from
hypoxanthine or guanine) which is then oxidised to
Check your GRASP
1. A person has taken very high protein rich diet; his urine
c. excretion through alimentary canal will eliminate more of
d. reutilizing water of urine 4. Most terrestrial insects get rid of bulk of their nitrogenous
wastes as
2. Vertebrates kidney and contractile vacuole of protozoans
resemble as both excrete
5. The flame cell system of helminthes work
d. nitrogenous wastes a. to remove ammonium ions and uric acids 3. The passing out of almost solid excretory wastes in reptiles
b. to remove uric acid is an indication of
c. to regulate pH b. excretion through skin d. for osmoregulation Objective biology 6. Renal gland is the excretory organ of
18. Excess bile pigments in urine indicate
b. diabetes insipidus d. al of these.
7. Deamination is first step in urea formation which means
19. Ammonia is the end product of
a. reduction of ammonia a. glucose breakdown b. oxidation of ammonia b. fatty acid metabolism c. addition of amino group to organic molecule c. protein metabolism d. removal of amino group from amino acid d. breakdown of biogenic amines 8. Antennary glands are excretory organs of
20. Pterydines are the excretory products of some
9. Protonephridia are present in platyhelminthes and
21. The liver of which one of the following is richest source
of arginase enzyme ?
b. cartilaginous fish d. platyhelminthes only 10. Excretory organs of protochordate Amphioxus are
22. Excretion is required for maintaining homeostasis of body
a. malpighian tubules b. protonephridia fluids through regulation of their
d. none of these.
a. volume, composition, pH and osmotic potential 11. Major source of ammonia produced by kidney comes
c. composition and pH d. osmotic potential 23. Uric acid is excreted in
12. Uric acid is the end product of metabolism of
b. glomerular acids 24. In human beings, gout is caused by
a. deficiency of iodine b. excessive secretion of thyroid 13. Trimethylamine is excreted by
c. excessive liberation of uric acid a. marine teleosts d. deposition of uric acid 25. Deamination is the process in which
c. fresh water fish a. poisonous urea is removed from the blood and it occurs in kidneys 14. The kidneys resembles contractile vacuoles of protozoans in
b. amino acids are absorbed from digested food, and a. expel ing of excess water it occurs in intestinal vil i b. expel ing out salts c. amino acids are broken down to produce urea and c. expel ing out excess glucose it occurs in liver d. expel ing out urea and uric acid d. amino acids are synthesised and it occurs in 15. In one of the following groups the toxic substance benzoic
acid after combining with amino acids glycine forms hippuric
26. In aquatic organisms the waste end product of nitrogen
acid as material in
16. Which is most toxic excretory product and require 600 ml
27. Excretory products of mammalian embryo are eliminated by
of water for throwing one gm nitrogen ?
b. amniotic fluid 17. Which one of these animal excretes amino acid without
28. The snakes living in deserts are mainly
Objective biology 29. The excretory product of excess metabolism of creatine
38. Excretory product of birds and reptiles is
and guanine is
30. In Rabbit and other terrestial mammals, the main nitrogenous
waste material is
39. Sea Gulls excrete salts through
d. urea and uric acid 31. Nitrogenous waste products are eliminated mainly as
40. In Amoeba, NH is excreted through
a. urea in tadpole and ammonia in adult frog b. ammonia in tadpole and urea in adult frog b. plasma membrane c. urea in both tadpole and adult frog c. contractile vacuole d. urea in tadpole and uric acid in adult frog d. al of these.
32. In Prawn, excretion is carried out by
41. Malphigian tubule are the excretory organs in
b. malpighian tubules b. platyhelminthes 33. Excretion is
a. removal of substances not required by body b. removal of useless substances and substances present 42. Ammonia is changed to uric acid in the liver of
a. ammonotelic animals c. formation of substances having some role in body b. uricotelic animals d. al of the above c. ureotelic animals 34. The animal which retain urea for hypertonicity is
d. ornithotelic animals 43. Which one is false ?
a. nephrons perform excretion through filtration, 35. Aquatic animals are mostly ammonotelic because
reabsorption and secretion a. ammonia helps in checking inflow of water into body b. nephridia are accessory excretory organs in Prawn b. excretion of ammonia requires large amount of water c. tapeworm have excretory flame cel s which is available to these animals d. nephrons begin with Bowman's capsule having c. water contains less nitrogen d. these get less light 44. Which of the following is likely to accumulate in dangerous
36. Excretion of nitrogenous waste product mainly as uric acid
proportion in the blood of a person whose kidney is not
by birds is helpful in
working properly ?
a. conserving body heat b. conserving water d. sodium chloride c. eliminating excess water 45. Bony fishes are
d. eliminating excess body heat 37. Which one is the most soluble in water ?
Objective biology

• Human beings are ureotelic i.e. excreting their • When acid loss exceeds acid gain, alkalosis occurs.
nitrogenous waste as urea.
When gain exceeds loss acidosis occurs. There are
various renal responses to acidosis and alkalosis.
• Responses to acidosis:
- Bicarbonate is added to the blood plasma by tubular
 Tubular cells reabsorb more bicarbonate from the tubular fluid.  Collecting duct cells secrete more hydrogen and generate more bicarbonate. - Ammoniagenesis leads to increased buffer formation
(in the form of NH ) • Responses to alkalosis:
- Excretion of bicarbonate in urine.
Fig. Excretory system.
 This is caused by lowered rate of hydrogen ion secretion from the tubular epithelial cells.  This is also caused by lowered rates of glutamine • Kidney are mesodermal in origin and developed
metabolism and ammonia excretion.
from nephrostomes of early embryo.
• Kidney also secretes hormones like – erythropoietin • Kidney are excretory and homeostatic organ.
(which regulates red blood cell production in the bone marrow); renin (which is a key part of the renin- • Man possesses a pair of dark red, bean shaped, angiotensin-aldosterone system) and active form of metanephric kidneys located in the peritoneal cavity
vitamin D, calcitriol, and prostaglandins.
outside the coelom (retroperitoneal).
• The kidneys are situated below the diaphragm on the • Kidney is covered by peritoneum on the ventral
left and right sides, and are partially protected by the eleventh and twelfth pairs of ribs.
• As the primary organs of excretion, the kidneys
• The right kidney is lower than the left because the
perform two major functions. They filter the blood,
liver takes up much space on that side.
removing water, ions, and nitrogenous wastes and forming urine and in turn regulate the volume, osmotic • Each kidney is surrounded by layers of fibrous pressure, and pH of the blood and body fluids.
connective tissue and a mass of adipose (fat) tissue which cushions it and holds it in place.
• The body is very sensitive to its pH level. Outside the range of pH that is compatible with life, proteins are • The kidney consists of an outer cortex and an inner
denatured and digested, enzymes lose their ability to medulla. They both contains the uriniferous tubules
function, and the body is unable to sustain itself.
or nephrons (the structural and functional unit of
• The kidneys maintain acid-base homeostasis by regulating the pH of the blood plasma. Gains and • The renal cortex is granular in appearance because
losses of acid and base must be balanced.
the tubules here are much convoluted (proximal and distal convoluted tubules) and contain malphigian - Sources of acid gain: carbon dioxide (since CO and
H O form H CO , carbonic acid, in the presence of carbonic anhydrase); production of nonvolatile • The cortex overlies the bases of medullary pyramids acids from the metabolism of proteins and other and dips down between them. These displaced organic molecules; and loss of bicarbonate in faeces portions of cortex which invade the medullary territory are called the renal columns of Bertini.
- Sources of acid loss: use of hydrogen ions in the
• Cortex is sub-divided into alternating radial tracts metabolism of various organic anions; and loss of known as rays and labyrinths. The rays are radially acid in the vomitus or urine straight since they contain the straight parts of Objective biology The kidney cells that make EPO are specialized and are The measurement of EPO in the blood can indicate bone sensitive to low oxygen levels in the blood coming into the marrow disorders or kidney disease. Normal levels of EPO kidney. These cells release erythropoietin when the oxy- are 0 to 19 mU/ml (milliunits per millilitre). Elevated levels gen level is low in the kidney. Erythropoietin stimulates can be seen in polycythemia rubra vera, a disorder charac- the bone marrow to produce more red blood cells which in terized by an excess of red blood cells. Lower than normal turn increases the oxygen-carrying capacity of the blood. values of EPO are seen in chronic renal failure. Erythropoietin or EPO is a glycoprotein hormone that is a Using recombinant DNA technology, EPO has been syn- cytokine for erythrocyte (red blood cell) precursors in the thetically produced for use in persons with certain types bone marrow. Erythropoietin is available as a therapeutic of anaemia: anemia due to kidney failure, anaemia sec- agent produced by recombinant DNA technology in mam- ondary to AZT treatment of AIDS, and anaemia associated malian cell culture. It is used in treating anemia resulting with cancer.
from chronic renal failure or from cancer chemotherapy. EPO has been much misused as a performance-enhanc- Its use is also believed to be common as a blood doping ing drug in endurance athletes such as reportedly cyclists, agent in endurance sports such as bicycle racing, triathlons long-distance runners, speed skaters, and Nordic (cross- and marathon running.
country) skiers. When misused in such situations, EPO is EPO is produced not only in the kidney but also, to a less- thought to be especially dangerous (perhaps because de- er extent, in the liver. The EPO gene has been found on hydration can further increase the viscosity of the blood, human chromosome 7 (in band 7q21). Different DNA se- increasing the risk for heart attacks and strokes. EPO has quences flanking the EPO gene act to control liver versus been banned by the Tour, the Olympics, and other sports kidney production of EPO. proximal tubules and the collecting ducts. They • Nephrons are connected to collecting ducts which are continuous with striated medulla, hence called pass through the renal pyramids and open into the medullary rays. The labyrinths have a granular renal pelvis.
appearance because the convoluted tubules, which • Urine formed in the nephrons and collecting ducts compose them, are cut irregularly.
empties into the renal pelvis and is carried away from • Renal medulla mainly contains loop of Henle,
the kidney by the ureter.
collecting tubules and ducts of Bellini.
• The medulla appears striated since the tubules run through it in a straight course radiating towards the • The medulla consists of 10 to 15 multilobular conical • It is the basic and ancestral form. Such kidney is found masses, the renal pyramids, whose bases are adjacent
today in larvae of certain cyclostomes (Myxine), but to the cortex and whose apices form the papillae. do not occur in any adult vertebrate.
These papillae project into cup shaped channels • Glomeruili are only present in some of the posterior • The spaces between calyces are part of the renal sinus and are filled with adipose tissue, vessels and nerves.
• The calyces join to form the pelvis which is the
• Pronephros the most primitive excretory organs
expanded beginning of the ureter.
that develop in vertebrate, corresponding to the first stage of kidney development.
• The medial concave border of a kidney contains a notch known as hilus through which the renal artery
• The pronephros develops in the anterior nephrotomes enters and the renal vein and ureter leave the kidney.
of all vertebrates. • The structural and functional unit of a kidney is
• It is a paired organ, consisting of a series of nephrons the nephron or the uriniferous tubule.
filtering urine from both the pericardium fluids via Objective biology openings called nephrostomes and blood from the • Urinary bladder is sac like structure which stores
urine temporarily. • The organ is active in adult forms of some primitive • Bladder has three parts– apex, fundus (or body)
fish, like lampreys or hagfish. It is present at the embryo of more advanced fish and at the larval stage • Body has triangular area called trigone. Neck regions
of amphibians.
possesses two sphincters– involuntary internal • In human beings, it is rudimentary and replaced by sphincter and voluntary external sphincter.
mesonephros after 3.5 weeks.
• Ureters and urinary bladder are lined by flexible
• Urethra is a membranous tube, which conduct urine
• The mesonephros (Latin for "middle kidney") serves
to the exterior.
as the main excretory organ of aquatic vertebrates
and as a temporary kidney in higher vertebrates. • The urethral sphincters keep the urethra closed except during voiding of urine.
• The mesonephros is also called the Wolffian body after Caspar Friedrich Wolff who described it in 1759.
• Urethra is concerned with the release of urine as well as semen (sperms + glandular secretion) through an • The mesonephros is composed of the mesonephric aperture called urinogenital aperture at the tip of the duct (also called the Wolffian duct), mesonephric tubules, and associated capillary tufts. A single tubule and its associated capillary tuft is called a mesonephric • In the male, urethra has 4 parts– excretory unit; these units are similar in structure and - Urinary urethra (preprostatic urethra). It is 1.0–
function to nephrons of the adult kidney. 1.5 cm long part which lies between urinary bladder • The mesonephros is derived from intermediate and point of union with ejaculatory ducts.
- Prostatic urethra. It is about 2.5 cm long and is
• Mesoderm in the vertebrate embryo.
surrounded by prostate gland.
• In human males, the mesonephros gives rise to the - Membranous urethra. It is about 2.5 cm long,
efferent ductules of the testis, the epididymis, vas without any covering.
deferens, seminal vesicle, and vestigial structures such as the appendix testis, appendix epididymis, and - Penile urethra. It is approximately 15cm long
present inside the copulatory organ penis.
• The mesonephros largely regresses in human females, though vestigial structures such as Gartner's cysts, the structurE of nEphron or epoophoron, and paroophoron are common.
urInIfErous tubulE • E.g. Fish and frog but in reptiles, birds and mammals • Nephrons or uriniferous tubule are morphological
it is functional in embryo.
and physiological unit of kidneys.
- Nephrons eliminate wastes from the body, regulate
blood volume and pressure, control levels of • Also called posterior kidney.
electrolytes and metabolites, and regulate blood pH. • Most advanced kidney in which loop of henle is Its functions are vital to life and are regulated
by the endocrine system by hormones such as
• E.g. Reptiles, birds & mammals.
antidiuretic hormone, aldosterone, and parathyroid ureters, urinary bladder and urethra • Two types of nephrons present in kidney are:
cortical and juxtamedullary nephrons.
• Ureters are thin muscular tubes emerge out from
the hilum of kidneys. Urine enters the ureter from • Cortical nephrons (70–80%) close to kidney
the renal pelvis and is conducted along the ureter by surface, have a shorter loop of Henle and peritubular peristaltic waves on its walls.
capillary network.
• Ureter develops in the region of calyces.
• Juxtamedullary nephrons (20–30%) at the junction
Objective biology of renal cortex and medulla, have a longer loop of • In addition, foot processes have a negatively-
Henle and vasa recta.
charged coat (glycocalyx) that limits the filtration of
• A nephron consists of two parts– an initial filtering
negatively-charged molecules, such as albumin.
component (the renal corpuscle) and a long tubule • The parietal layer of Bowman's capsule is lined by a (renal tubule) – both made of simple cuboidal
single layer of squamous epithelium.
• Between the visceral and parietal layers is Bowman's space, into which the filtrate enters after passing through the podocytes' filtration slits. • The renal corpuscle filters out large solutes from
• Unlike the visceral layer, the parietal layer does not the blood, delivering water and small solutes to
function in filtration. Rather, the filtration barrier the renal tubule for modification.
is formed by three components: the diaphragms of • The renal corpuscle (or Malphigian corpuscle) is the filtration slits, the thick glomerular basement composed of a glomerulus and Bowman's capsule.
membrane, and the glycocalyx secreted by Where blood is filtered to begin the process of urine • Podocytes are special, less flattened cells which line • The malpighian corpuscle named after Marcello
the concavity of Bowman's capsule.
Malpighi (1628 - 1694), an Italian physician and
• Podocytes prevents filteration of large macromolecules that might pass through basement membrane and • The nephron begins as a double-walled blind cup called Bowman's capsule (lined by squamous
epithelium) which surrounds a network of capillaries known as glomerulus.
• Attached to each Bowman's capsule is a long, thin • Glomerulus is a capillary (fenestrated) tuft that
tubule (which functions as dialysis unit) with three
receives its blood supply from an afferent arteriole
distinct regions– proximal convoluted tubule,
of the renal circulation.
loop of Henle and distal convoluted tubule.
• Blood enters glomerular capillaries through afferent
• The first region is called the proximal convoluted
arteriole and leaves through efferent arteriole.
tubule. ‘Proximal' means that it is near Bowman's
• The diameter of afferent arteriole is much more
capsule, and ‘convoluted' describes its coiled and than that of efferent arteriole.
looped shape.
• Bowman's capsule is composed of visceral (simple
• Proximal convoluted tubules (PCT) or pars
squamous epithelial cells) (inner) and parietal
convoluta is about 14 mm long and lined by a single
(simple squamous epithelial cells) (outer) layers.
layer of cubical cells.
• Bowman's capsule is named after Sir William Bowman
• Cells of the proximal convoluted tubule have (1816–1892), a British surgeon and anatomest.
numerous microvilli and mitochondria which provide surface area and energy and closeness of • The visceral layer lies just beneath the thickened
blood capillaries.
glomerular basement membrane and is made of
podocytes which send foot processes over the length
• The proximal convoluted tubule connects to the of the glomerulus. second region, the loop of Henle.
• Foot processes interdigitate with one another
• The loop of Henle is a U-shaped or hair pin tube that forming filtration slits that, in contrast to those dips deeply into the medulla within a renal pyramid in the glomeruluar endothelium, are spanned by and then loops back towards the cortex.
diaphragms. The size of the filtration slits restricts • Its primary role is to concentrate the salt in the
the passage of large molecules (eg, albumin) and cells interstitium; the tissue surrounding the loop.
(e.g., red blood cells and platelets). As a result, the • The loop of Henle is described as having a descending
filtrate leaving the Bowman's capsule is very similar
limb and an ascending limb. These limbs have
to blood plasma in composition as it passes into the
different properties and play different roles in urine proximal convoluted tubule.
Objective biology • Descending limb is permeable to water but
the macula densa. The juxtaglomerular cells, the
completely impermeable to salt, and thus only macula densa and the lacis cells (specialised glandular indirectly contributes to the concentration of the cells present at the vascular angle formed by the interstitium. As the filtrate descends deeper into the afferent and efferent arterioles whose significance hypertonic interstitium of the renal medulla, water at this location is unknown) are collectively known flows freely out of the descending limb by osmosis as the juxtaglomerular apparatus. Juxtaglomerular
until the tonicity of the filtrate and interstitium apparatus is one component of tubuloglomerular
feedback mechanism that regulates renal blood flow • Longer descending limbs allow more time for water and glomerular filteration rate.
to flow out of the filtrate, so longer limbs make the • Juxtaglomerular cells are the site of renin synthesis
filtrate more hypertonic than shorter limbs.
and secretion and thus plays a critical role in renin
• Unlike the descending limb, the ascending limb
of Henle's loop is impermeable to water, a critical
• The third region of the nephron tubule is called feature of the counter-current exchange mechanism the distal convoluted tubule. ‘Distal' means that employed by the loop. The ascending limb actively
it is farther from Bowman's capsule than the other pumps sodium out of the filtrate, generating
the hypertonic interstitium that drives counter-
• The distal convoluted tubule is similar to the proximal current exchange. In passing through the ascending
convoluted tubule in structure and function. Cells limb, the filtrate grows hypotonic since it has lost lining the tubule have numerous mitochondria, much of its sodium content. This hypotonic filtrate enabling active transport to take place by the energy is passed to the distal convoluted tubule in the renal supplied by ATP. • Distal convoluted tubules from many nephrons all • The thick ascending limb of the loop of Henle reaches connect to a common tube, the collecting duct
the glomerulus of the nephron from which the tubule which empties into the renal pelvis.
arises and passes close to its afferent arteriole and efferent arteriole. • The collecting duct has important functions in
regulating the composition of urine, as water, ions, • These limbs operates counter current mechanisms to and nutrients are reabsorbed from the filtrate in the made urine hypertonic.
nephron tubules and collecting ducts.
• Counter-current multiplier system is an active
• This reabsorption prevents the loss of useful nutrients, process which is responsible for the production of ions, and water, and provides an opportunity for concentrated urine in the collecting ducts of the tubule cells to regulate the composition of blood and nephrons. Sodium and chloride ions are actively the body fluids.
pumped from the ascending limb of the loop but water is retained, since the ascending limb is impermeable • The collecting duct system begins in the renal cortex to water. This creates a concentration gradient in the and extends deep into the medulla. As the urine medulla in which the concentration of sodium and travels down the collecting duct system, it passes by chloride is greatest in the region of the bend of the the medullary interstitium which has a high sodium loop. Fluid passing from the loop of Henle to the concentration as a result of the loop of Henle's distal tubule is less concentrated than that entering counter-current multiplier system. the loop, but because of the high osmotic pressure in • Though the collecting duct is normally impermeable the medulla water diffuses out of the collecting ducts, to water, it becomes permeable in the presence of producing a concentrated urine.
antidiuretic hormone (ADH).
• Peritubular capillaries called vasa rectae are present • The collecting ducts unite with each other in the around loop of Henle. They help to retain reabsorbed medulla to form still larger ducts of Bellini. The ions and urea in the interstitial fluid, and help in ducts of Bellini open into renal pelvis.
maintaining high osmotic pressure in the medulla.
• The walls of the afferent arterioles contain the renin-
Nephron's Blood Supply
secreting juxtaglomerular cells. At this point, the
• There is an intimate association between the tubular epithelium is modified histologically to form blood vessels and the nephrons of the kidney. This Objective biology association permits both extensive filtration from • Both kidneys produce glomerular filtrate at the rate the blood and selective reabsorption back into the of about 125 ml/min or 180 lt/day, equivalent to ten times the blood volume daily.
• After entering each kidney, the renal artery branches
• Measuring the GFR (glomerular filtration rate) is a repeatedly, forming smaller and smaller arteries, until diagnostic test of kidney function. A decrease GFR tiny arterioles reach each of the 1 million nephrons.
may be a sign of renal failure.
• An afferent arteriole delivers blood to the glomerulus
• Water and dissolved substances are present in the capillaries for filtration, an efferent arterioles drains
filtrate at about the same concentrations as they are filtered blood away from the same glomerulus.
in the blood.
• The efferent arteriole connects to a second network • If the glomerular filtrate were excreted from the body of capillaries, the peritubular capillaries, which
unchanged, persons would be in constant danger are closely associated with the nephron tubule. It of both dehydration and starvation. Persons would is into these peritubular capillaries that water, ions need to spend most of their life drinking and eating and nutrients are reabsorbed from the filtrate in to compensate for water and nutrient losses.
the nephron tubule. After leaving the vicinity of the • Fortunately, humans do not excrete the glomerular nephron, blood flows through progressively larger filtrate. Water and other useful materials are veins until reaches the renal vein, which leaves the
reabsorbed from the filtrate, and only a small volume kidney and returns blood to the inferior vena cava.
of concentrated urine is actually formed.
process of urine formation • Glomerular filtration occurs because the pressure of the blood flowing in the glomerular capillaries is • The formation of urine is the result of the following higher than the pressure of the filtrate in Bowman's capsule. In other words, blood pressure drives - ultra filtration or glomerular filteration of
glomerular filtration, and because the process takes the blood plasma by the glomeruli; advantage of a pressure gradient, glomerular filtration does not require the expenditure of energy by kidney - selective reabsorption by the tubules (useful
substances such as sugar, salts, water are selectively reabsorbed from the glomerular filtrate to maintain • To prevent the rate of glomerular filtration from the internal environment); and changing when blood pressure is altered as a result - secretion by the tubules (the tubules secrete
of exercise or other conditions in the body, a certain certain substances like urea, uric acid, anions etc. degree of self-regulation over filtration occurs in the from the blood into the tubular lumen for excretion into the urine).
• Specialized cells in the nephron wall sense changes in blood pressure and rectrete chemicals that change Glomerular Filtration of Blood
the diameter of the arterioles connected to the • Glomerular filtration is the first of the three processes glomerular capillaries. Changing the size of these that form urine.
vessels alters the amount of blood flowing through the glomerulus, maintaining a relatively stable rate of • Urine formation begins with filtration of blood glomerular filtration and urine formation.
through the epithelial walls of the glomerulus and Bowman's capsule.
• GFR is auto-regulated by myogenic mechanism (increase in blood pressure tends to stretch the • The fluid portion of the blood, which consists of afferent arteriole which responds by contraction; water, urea, ions, nutrients and small proteins, is reducing the diameter & hence blood flow) and able to move across the capillary wall. Blood cells juxtaglomerular apparatus (which responds to low and larger proteins, however, cannot cross and are blood pressure by secreting renin).
retained in the blood.
• Effective Filteration Pressure (EFP) is determined • The molecules that leave the blood and enter the glomerular capsule are called the glomerular filtrate.
It is also termed as primary urine.
- GHP (Glomerular Hydrostatic Pressure) - It is
blood pressure in a glomerular capillaries due Objective biology to narrower efferent arteriole and is the chief concentration (water is reabsorbed by osmosis and determinant of EFR. Its value is 75 mmHg.
urea by simple diffusion).
- BCOP (Blood Colloid Osmotic Pressure) - It is
• Water is reabsorbed in all parts of the tubules except osmotic pressure created in the blood of glomerular the ascending loop of Henle.
capillaries due to plasma proteins. Its value is 30 • Glucose and amino acids are reabsorbed by active - CHP (Capsular Hydrostatic Pressure) - It is caused
by fluid occupying the Bowman's capsule and resists • The reabsorption of Na+ occurs by both passive and filteration. Its value is 20 mmHg.
active transport. Na+ moves passively by diffusion from the filtrate into tubule cells but is actively • Hence, EFP = GHP - (BCOP + CHP) transported out of the tubule cells on its way to the • = 75 - (30 + 20) = 25 mm Hg.
• About 1250 ml of blood circulates per minute through • Renal threshold of a substance is its highest the two kidneys, and out of it 650-700 ml is plasma. concentration in the blood upto which it is totally This is called Renal Plasma Flow (RPF).
reabsorbed from the glomerular filtrate.
• Filtration Fraction (FF) is the fraction of the plasma. • High threshold substances are almost completely • passing through kidneys which is filtered at the absorbed from nephric filtrate, e.g., glucose, amino glomerulus, i.e., the ratio of GFR to renal plasma flow acids, vitamin C, Na+, water. Glucose has a threshold value of 180 mg/100 ml.
• Therefore FF = = = 0.17 • Low threshold substances (in which only very small reabsorption occurs) are urea, uric acid, xanthin, Selective Tubular Reabsorption
phosphate and non-threshold substances are not al all reabsorbed, e.g., creatinine, hippuric acid • Tubular reabsorption is the second process in the formation of urine from filtrate. As a result of tubular reabsorption, much of the filtrate passes out of the nephron tubule and returns to the blood through the • Certain chemicals in the blood that are not removed by filtration from the glomerular capillaries are removed by a third process of urine formation called • As much as 99 per cent of the material in the filtrate tubular secretion.
is reabsorbed, preventing the loss of water, nutrients, and ions from the body. As a consequence of tubular • These chemicals are removed from the blood in the reabsorption urine contains mostly waste materials • peritubular capillaries to the nephron tubule by both and excess water.
passive and active transport.
• Reabsorption occurs within cell, three regions of the • After entering the proximal or distal convoluted nephron and in the collecting duct, but most of it tubules, the chemicals are mixed with the glomerular takes place within the proximal convoluted tubule.
filtrate and eliminated from the body with the urine.
• The epithelial cells of the proximal convoluted tubule • The chemicals moved by tubular secretion include have numerous microvilli which increase the surface those foreign body and the ions and molecules that area available for reabsorption.
are toxic at elevated levels.
• During reabsorption, molecules move out of the • Ions removed from the blood by tubular secretion lumen of the tubule and enter the tubule's epithelial include potassium (K+), hydrogen (H+) and cells. They then pass out of the epithelial cells, cross ammonium (NH4+).
into the peritubular capillaries, and enter the blood.
• The secretion of H+ is an important way in which • Depending on the type of molecule being reabsorbed, kidneys help control blood pH.
movement into and out of epithelial cells occurs by • In addition to ions, other molecules, such as food passive transport or active transport.
preservatives, medicines such as penicillin and • Water and urea, for example, are reabsorbed metabolic by– products such as creatine, are removed by passive transport, by which they move from from the blood by tubular secretion.
regions of higher concentration to regions of lower Objective biology • The kidneys also rid the body of harmful drugs • When water content in the body is low, the posterior such as marijuana, cocaine and heroin by tubular pituitary secretes ADH, which makes the walls of DCT, collecting tubules and collecting ducts permeable to water. Water is reabsorbed in the Route of urine flow
surrounding tissue having hypertonic fluid and hypertonic urine is eliminated.
role of hormones in urine formation • Under the deficiency of ADH, a disease called diabetes • Certain hormones and hormone like substances are insipidus is caused in which the output of urine may intimately related to renal function.
reach 3-–40 litre/day in place of normal 1.2–1.8 litre/day. Frequent urination (due to inadequately • Hormones are important signalling molecules reabsorbed water from the collecting duct) and thrust controlling the kidneys in the regulatory processes.
is the symptoms of the disease.
• Four major hormones help to maintain homeostasis by regulating the concentration and amount of urine excreted. They are: antidiuretichormone (ADH), • Aldosterone is a hormone secreted by the outer layer aldosterone, angiotensin II, atrial natriuretic peptide of the adrenal gland (cortex part), a gland which sits like a cap above the kidney.
• Water reabsorption is controlled by antidiuretic • Decreased blood volume and interstitial fluid level, hormone (ADH) in negative feedback and aldosterone resulting in decreased blood pressure, trigger regulates the transfer of sodium from the nephron to • When aldosterone is present in the blood, all the Na+ in the filtrate is reabsorbed by the epithelial cells of the collecting duct.
• ADH increases the reabsorption of water by the distal tubule and collecting duct.
• When aldosterone is absent, some Na+ remains in the filtrate and is excreted with the urine.
• ADH or vasopressin (synthesized in hypothalamus and secreted by posterior pituitary gland) are • The release of aldosterone is controlled by negative produced outside the kidney and travel to the kidney via the blood as chemical messengers.
• Retaining Na+, raises the osmotic pressure of the blood and reduces water loss from the body.
• ADH regulates water excretion by increasing the permeability of the collecting ducts to water and salt and by accelerating water and ion transfer in a direction determined by the osmotic gradient.
• As blood pressure decreases, the cells of the juxtaglomerular apparatus release the enzyme renin • The receptors at the base of the brain form part of the feedback mechanism that (1) stimulates ADH and activate the renin-angiotensin-aldosterone output if the osmotic concentration of extracellular pathway (RAAS). The juxtaglomerular apparatus is fluid (ECF) is high, so as to concentrate the urine, a small group of cells situated in the area where the and (2) reduces ADH output and so dilutes the urine renal tubule links up with the afferent and efferent if osmotic concentration of ECF (extra cellular fluid) arterioles. Renin converts angiotensinogen into and of plasma falls.
angiotensin I. Angiotensin converting enzyme (ACE) then converts angiotensin I into angiotensin II, a • Alcohol inhibits the release of ADH and caffeine peptide hormone that is the active form. Angiotensin interferes with ADH action and sodium reabsorption II has the following effects: thus both artificially dilute urine to be produced.
- Increases the synthesis and release of aldosterone
• Drugs (called diuretics) increase the production of - Raises blood pressure directly by constricting blood
dilute urine and prevent the excessive water retention and tissue swelling (edema) that may accompany congestive heart failure, high blood pressure and other - Stimulates sodium reabsorption by the proximal
convoluted tubules Objective biology 1. Which of the following statements about metanephric
c. raises the concentration of Na– in the blood leaving kidneys is true ?
a. they become functional at the end of the eighth d. al of the above 9. Refer the figure and identify the marked alphabet which
b. they are active throughout fetal development represents the sites at which tubular fluid osmolality exceeds
c. they are the third pair of kidneys to develop that of plasma by greatest amount
d. al of the above are true 2. Which of the following statements about the renal pyramids
is false ?
a. they are located in the renal medul a
b. they contain glomeruli
c. they contain col ecting ducts
d. they empty urine into the calyces
3. Sodium reabsorption from the distal tubule will be increased
if there is an increase in
a. plasma potassium
c. urine flow rate d. plasma osmolality 4. Antidiuretic hormone promotes the retention of water by
stimulating
a. the active transport of water
b. the active transport of chloride c. the active transport of sodium 10. The kidneys help maintain acid-base balance by
d. the permeability of the col ecting duct a. the secretion of H– in the distal regions of the nephron b. the action of carbonic anhydrase within the apical cel 5. Aldosterone stimulates sodium reabsorption and potassium
membranes and the cytoplasm of the tubule cel s secretion in
c. the buffering action of phosphates and ammonia in a. the proximal convoluted tubule b. the descending limb of the nephron loop d. al of the above.
c. the ascending limb of the nephron loop 11. The detrusor muscle is located in
d. the cortical col ecting duct 6. Diuretic drugs that act in the nephron loop
c. the urinary bladder a. inhibit active sodium transport and promote the 12. Glucose reabsorption occurs in the
excretion of salt and water a. proximal tubule b. cause an increased flow of filtrate to the distal c. distal tubule d. medul ary col ecting duct convoluted tubule 13. Ammonia is an effective and important urinary buffer for
c. cause an increased secretion of potassium into the which of the following reasons ?
a. its production in the kidney decreases during chronic d. al of the above 7. Reabsorption of water through the tubules occurs by
b. only the afferent arteriole is constricted b. active transport c. only the efferent arteriole is constricted c. facilitated diffusion d. the efferent and afferent arterioles are both 8. The countercurrent exchange in the vasa recta
a. removes Na+ from the extracel ular fluid 14. Sodium reabsorption from the distal tubule will be increased
b. maintains high concentrations of NaCl in the if there is an increase in
extracel ular fluid a. plasma potassium c. urine flow rate d. plasma osmolality Objective biology

MCQ's From Competitive Exams 18. Assertion: The final reabsorption of water from urine into
1. If excess water passes out from the tissue without being
blood occurs through the collecting duct of a mammalian
restored by the kidneys, the cells would
nephron, resulting in the production of hyperosmotic
urine.
a. burst open and dieb. take water from the plasma Reason: The loop of Henle is responsible for the formation c. not be affected at al of a sodium gradient across the depth of the cortical d. shriver and die intersitium of a mammalian kidney.
(Year 1994) a. If both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.
2. Which one of the four parts mentioned below does not
b. If both Assertion and Reason are true but Reason is constitute a part of single uriniferous tubule?
not a correct explanation of the Assertion.
a. distal convoluted tubule b. col ecting duct c. If Assertion is true but the Reason is false.
c. Bowman's capsule d. If both Assertion and Reason are false.
(Year 1996) (Year 1994) 19. Waste product of adenine and guanine metabolism are
3. Two examples in which the nitrogenous waste are excreted
excreted by man as
from body in the form of uric acid are
a. birds and lizards b. frogs and bony fishes c. insects and bony fishes d. mammals and mol uscs (Year 1997) (Year 1994, 2004) 20. Assertion: In the descending limb of loops of Henle, the
4. The ornithine cycle removes two waste products from the
urine is hypertonic, while in ascending limb of loops of
blood in liver. These products are
Henle, the urine is hypotonic.
a. CO and ammonia b. ammonia and uric acid Reason: Descending limb is impermeable to Na+, while d. ammonia and urea.
ascending limb is impermeable to H O.
(Year 1996, 2005) a. If both Assertion and Reason are true and the Reason 5. In ureotelic animals, urea is formed by
is a correct explanation of the Assertion.
b. If both Assertion and Reason are true but Reason is c. ornithine cycle d. Cori cycle.
not a correct explanation of the Assertion.
(Year 1997) c. If Assertion is true but the Reason is false.
6. The basic functional unit of human kidney is
d. If both Assertion and Reason are false.
(Year 1997) 21. The cells named podocytes occur in
(Year 1997) a. glomerulus of kidney b. wal of capil aries 7. A condition of failure of kidney to form urine is called
c. neck region of nephrons d. large intestine.
d. none of these.
(Year 1998) (Year 1998) 22. The function of renin is
8. Solenocytes are the main excretory structure in
a. stimulation of corpus luteum a. echenodermalia b. platyhelminthes c. to reduce blood pressured. degradation of angiotensinogen.
(Year 1999) 17. Where the conversion of harmful prussic acid into potassium
23. The end-product of ornithine cycle is
sulphocyanide takes place ?
a. carbon dioxide (Year 1999) (Year 1996) Objective biology

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