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Animals accumulate ammonia, urea, uric acid, carbon dioxide, water

and ions like Na

, K

, Cl

–

, phosphate, sulphate, etc., either by metabolic

activities or by other means like excess ingestion. These substances have

to be removed totally or partially. In this chapter, you will learn the

mechanisms of elimination of these substances with special emphasis on

common nitrogenous wastes. Ammonia, urea and uric acid are the major

forms of nitrogenous wastes excreted by the animals. Ammonia is the

most toxic form and requires large amount of water for its elimination,

whereas uric acid, being the least toxic, can be removed with a minimum

loss of water.

The process of excreting ammonia is Ammonotelism. Many bony fishes,

aquatic amphibians and aquatic insects are ammonotelic in nature.

Ammonia, as it is readily soluble, is generally excreted by diffusion across

body surfaces or through gill surfaces (in fish) as ammonium ions. Kidneys

do not play any significant role in its removal. Terrestrial adaptation

necessitated the production of lesser toxic nitrogenous wastes like urea

and uric acid for conservation of water. Mammals, many terrestrial

amphibians and marine fishes mainly excrete urea and are called ureotelic

animals. Ammonia produced by metabolism is converted into urea in the

liver of these animals and released into the blood which is filtered and

excreted out by the kidneys. Some amount of urea may be retained in the

kidney matrix of some of these animals to maintain a desired osmolarity.

Reptiles, birds, land snails and insects excrete nitrogenous wastes as uric

acid in the form of pellet or paste with a minimum loss of water and are

called uricotelic animals.

XCRETORY

RODUCTS AND

THEIR

LIMINATION

HAPTER

19.1 Human

Excretory

System

19.2 Urine Formation

19.3 Function of the

Tubules

19.4 Mechanism of

Concentration of

the Filtrate

19.5 Regulation of

Kidney Function

19.6 Micturition

19.7 Role of other

Organs in

Excretion

19.8 Disorders of the

Excretory

System

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A survey of animal kingdom presents a variety of excretory structures.

In most of the invertebrates, these structures are simple tubular forms

whereas vertebrates have complex tubular organs called kidneys. Some

of these structures are mentioned here. Protonephridia or flame cells are

the excretory structures in Platyhelminthes (Flatworms, e.g., Planaria),

rotifers, some annelids and the cephalochordate – Amphioxus.

Protonephridia are primarily concerned with ionic and fluid volume

regulation, i.e., osmoregulation. Nephridia are the tubular excretory

structures of earthworms and other annelids. Nephridia help to remove

nitrogenous wastes and maintain a fluid and ionic balance. Malpighian

tubules are the excretory structures of most of the insects including

cockroaches. Malpighian tubules help in the removal of nitrogenous

wastes and osmoregulation. Antennal glands or green glands perform

the excretory function in crustaceans like prawns.

19.1 HUMAN EXCRETORY SYSTEM

In humans, the excretory system consists

of a pair of kidneys, one pair of ureters, a

urinary bladder and a urethra (Figure

19.1). Kidneys are reddish brown, bean

shaped structures situated between the

levels of last thoracic and third lumbar

vertebra close to the dorsal inner wall of

the abdominal cavity. Each kidney of an

adult human measures 10-12 cm in

length, 5-7 cm in width, 2-3 cm in

thickness with an average weight of 120-

170 g. Towards the centre of the inner

concave surface of the kidney is a notch

called hilum through which ureter, blood

vessels and nerves enter. Inner to the hilum

is a broad funnel shaped space called the

renal pelvis with projections called calyces.

The outer layer of kidney is a tough

capsule. Inside the kidney, there are two

zones, an outer cortex and an inner

medulla. The medulla is divided into a few

conical masses (medullary pyramids)

projecting into the calyces (sing.: calyx).

The cortex extends in between the

Figure 19.1 Human Urinary system

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Figure 19.3 A diagrammatic representation of a nephron showing blood vessels,

duct and tubule

medullary pyramids as renal columns called

Columns of Bertini (Figure 19.2).

Each kidney has nearly one million

complex tubular structures called nephrons

(Figure 19.3), which are the functional units.

Each nephron has two parts – the

glomerulus and the renal tubule.

Glomerulus is a tuft of capillaries formed by

the afferent arteriole – a fine branch of renal

artery. Blood from the glomerulus is carried

away by an efferent arteriole.

The renal tubule begins with a double

walled cup-like structure called Bowman’s

capsule, which encloses the glomerulus.

Glomerulus alongwith Bowman’s capsule, is

called the malpighian body or renal

corpuscle (Figure 19.4). The tubule

continues further to form a highly coiled

network – proximal convoluted tubule

Figure 19.2 Longitudinal section (Diagrammatic)

of Kidney

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(PCT). A hairpin shaped Henle’s loop is the

next part of the tubule which has a

descending and an ascending limb. The

ascending limb continues as another highly

coiled tubular region called distal

convoluted tubule (DCT). The DCTs of many

nephrons open into a straight tube called

collecting duct

, many of which converge and

open into the renal pelvis through medullary

pyramids in the calyces.

The Malpighian corpuscle, PCT and DCT

of the nephron are situated in the cortical

region of the kidney whereas the loop of Henle

dips into the medulla. In majority of

nephrons, the loop of Henle is too short and

extends only very little into the medulla. Such

nephrons are called cortical nephrons. In

some of the nephrons, the loop of Henle is

very long and runs deep into the medulla.

These nephrons are called juxta medullary

nephrons.

The efferent arteriole emerging from the glomerulus forms a fine

capillary network around the renal tubule called the peritubular

capillaries. A minute vessel of this network runs parallel to the Henle’s

loop forming a ‘U’ shaped vasa recta. Vasa recta is absent or highly

reduced in cortical nephrons.

19.2 URINE FORMATION

Urine formation involves three main processes namely, glomerular

filtration, reabsorption and secretion, that takes place in different parts of

the nephron.

The first step in urine formation is the filtration of blood, which is carried

out by the glomerulus and is called glomerular filtration. On an average,

1100-1200 ml of blood is filtered by the kidneys per minute which constitute

roughly 1/5

of the blood pumped out by each ventricle of the heart in a

minute. The glomerular capillary blood pressure causes filtration of blood

through 3 layers, i.e., the endothelium of glomerular blood vessels, the

epithelium of Bowman’s capsule and a basement membrane between these

two layers. The epithelial cells of Bowman’s capsule called podocytes are

arranged in an intricate manner so as to leave some minute spaces called

filtration slits or slit pores. Blood is filtered so finely through these

membranes, that almost all the constituents of the plasma except the

proteins pass onto the lumen of the Bowman’s capsule. Therefore, it is

considered as a process of ultra filtration.

Afferent arteriole

Efferent

arteriole

Bowman’s

capsule

Proximal

convolutedtubule

Figure 19.4 Malpighian body (renal corpuscle)

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The amount of the filtrate formed by the kidneys per minute is called

glomerular filtration rate (GFR). GFR in a healthy individual is

approximately 125 ml/minute, i.e., 180 litres per day !

The kidneys have built-in mechanisms for the regulation of glomerular

filtration rate. One such efficient mechanism is carried out by juxta

glomerular apparatus (JGA). JGA is a special sensitive region formed by

cellular modifications in the distal convoluted tubule and the afferent

arteriole at the location of their contact. A fall in GFR can activate the JG

cells to release renin which can stimulate the glomerular blood flow and

thereby the GFR back to normal.

A comparison of the volume of the filtrate formed per day (180 litres

per day) with that of the urine released (1.5 litres), suggest that nearly 99

per cent of the filtrate has to be reabsorbed by the renal tubules. This

process is called reabsorption. The tubular epithelial cells in different

segments of nephron perform this either by active or passive mechanisms.

For example, substances like glucose, amino acids, Na

, etc., in the filtrate

are reabsorbed actively whereas the nitrogenous wastes are absorbed by

passive transport. Reabsorption of water also occurs passively in the initial

segments of the nephron (Figure 19.5).

During urine formation, the tubular cells secrete substances like H

and ammonia into the filtrate. Tubular secretion is also an important

step in urine formation as it helps in the maintenance of ionic and acid

base balance of body fluids.

19.3 FUNCTION OF THE TUBULES

Proximal Convoluted Tubule (PCT): PCT is lined by simple cuboidal

brush border epithelium which increases the surface area for reabsorption.

Nearly all of the essential nutrients, and 70-80 per cent of electrolytes

and water are reabsorbed by this segment. PCT also helps to maintain

the pH and ionic balance of the body fluids by selective secretion of

hydrogen ions, ammonia and potassium ions into the filtrate and by

absorption of HCO

–

from it.

Henle’s Loop: Reabsorption is minimum in its ascending limb.

However, this region plays a significant role in the maintenance of high

osmolarity of medullary interstitial fluid. The descending limb of loop of

Henle is permeable to water but almost impermeable to electrolytes. This

concentrates the filtrate as it moves down. The ascending limb is

impermeable to water but allows transport of electrolytes actively or

passively. Therefore, as the concentrated filtrate pass upward, it gets

diluted due to the passage of electrolytes to the medullary fluid.

Distal Convoluted Tubule (DCT): Conditional reabsorption of Na

and water takes place in this segment. DCT is also capable of reabsorption

of HCO

–

and selective secretion of hydrogen and potassium ions and

to maintain the pH and sodium-potassium balance in blood.

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Collecting Duct: This long duct extends from the cortex of the kidney

to the inner parts of the medulla. Large amounts of water could be

reabsorbed from this region to produce a concentrated urine. This segment

allows passage of small amounts of urea into the medullary interstitium

to keep up the osmolarity. It also plays a role in the maintenance of pH

and ionic balance of blood by the selective secretion of H

and K

ions

(Figure 19.5).

19.4 MECHANISM OF CONCENTRATION OF THE FILTRATE

Mammals have the ability to produce a concentrated urine. The Henle’s

loop and vasa recta play a significant role in this. The flow of filtrate in

the two limbs of Henle’s loop is in opposite directions and thus forms a

counter current. The flow of blood through the two limbs of vasa recta is

Figure 19.5 Reabsorption and secretion of major substances at different parts of

the nephron (Arrows indicate direction of movement of materials.)

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also in a counter current pattern. The proximity between the Henle’s loop

and vasa recta, as well as the counter current in them help in maintaining

an increasing osmolarity towards the inner medullary interstitium, i.e.,

from 300 mOsmolL

–1

in the cortex to about 1200 mOsmolL

–1

in the inner

medulla. This gradient is mainly caused by NaCl and urea. NaCl is

transported by the ascending limb of Henle’s loop which is exchanged

with the descending limb of vasa recta. NaCl is returned to the interstitium

by the ascending portion of vasa recta. Similarly, small amounts of urea

enter the thin segment of the ascending limb of Henle’s loop which is

transported back to the interstitium by the collecting tubule. The above

described transport of substances facilitated by the special arrangement

of Henle’s loop and vasa recta is called the counter current mechanism

(Figure. 19.6). This mechanism helps to maintain a concentration gradient

Figure 19.6 Diagrammatic representation of a nephron and vasa recta showing

counter current mechanisms

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in the medullary interstitium. Presence of such interstitial gradient helps

in an easy passage of water from the collecting tubule thereby

concentrating the filtrate (urine). Human kidneys can produce urine nearly

four times concentrated than the initial filtrate formed.

19.5 REGULATION OF KIDNEY FUNCTION

The functioning of the kidneys is efficiently monitored and regulated by

hormonal feedback mechanisms involving the hypothalamus, JGA and

to a certain extent, the heart.

Osmoreceptors in the body are activated by changes in blood volume,

body fluid volume and ionic concentration. An excessive loss of fluid from

the body can activate these receptors which stimulate the hypothalamus

to release antidiuretic hormone (ADH) or vasopressin from the

neurohypophysis. ADH facilitates water reabsorption from latter parts of

the tubule, thereby preventing diuresis. An increase in body fluid volume

can switch off the osmoreceptors and suppress the ADH release to complete

the feedback. ADH can also affect the kidney function by its constrictory

effects on blood vessels. This causes an increase in blood pressure. An

increase in blood pressure can increase the glomerular blood flow and

thereby the GFR.

The JGA plays a complex regulatory role. A fall in glomerular blood

flow/glomerular blood pressure/GFR can activate the JG cells to release

renin which converts angiotensinogen in blood to angiotensin I and

further to angiotensin II. Angiotensin II, being a powerful

vasoconstrictor, increases the glomerular blood pressure and thereby

GFR. Angiotensin II also activates the adrenal cortex to release

Aldosterone. Aldosterone causes reabsorption of Na

and water from

the distal parts of the tubule. This also leads to an increase in blood

pressure and GFR. This complex mechanism is generally known as

the Renin-Angiotensin mechanism.

An increase in blood flow to the atria of the heart can cause the release

of Atrial Natriuretic Factor (ANF). ANF can cause vasodilation (dilation of

blood vessels) and thereby decrease the blood pressure. ANF mechanism,

therefore, acts as a check on the renin-angiotensin mechanism.

19.6 MICTURITION

Urine formed by the nephrons is ultimately carried to the urinary bladder

where it is stored till a voluntary signal is given by the central nervous

system (CNS). This signal is initiated by the stretching of the urinary bladder

as it gets filled with urine. In response, the stretch receptors on the walls

of the bladder send signals to the CNS. The CNS passes on motor messages

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to initiate the contraction of smooth muscles of the bladder and

simultaneous relaxation of the urethral sphincter causing the release of

urine. The process of release of urine is called micturition and the neural

mechanisms causing it is called the micturition reflex. An adult human

excretes, on an average, 1 to 1.5 litres of urine per day. The urine formed

is a light yellow coloured watery fluid which is slightly acidic (pH-6.0)

and has a characterestic odour. On an average, 25-30 gm of urea is

excreted out per day. Various conditions can affect the characteristics of

urine. Analysis of urine helps in clinical diagnosis of many metabolic

discorders as well as malfunctioning of the kidney. For example, presence

of glucose (Glycosuria) and ketone bodies (Ketonuria) in urine are

indicative of diabetes mellitus.

19.7 ROLE OF OTHER ORGANS IN EXCRETION

Other than the kidneys, lungs, liver and skin also help in the elimination

of excretory wastes.

Our lungs remove large amounts of CO

(approximately 200mL/

minute) and also significant quantities of water every day. Liver, the largest

gland in our body, secretes bile-containing substances like bilirubin,

biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs.

Most of these substances ultimately pass out alongwith digestive wastes.

The sweat and sebaceous glands in the skin can eliminate certain

substances through their secretions. Sweat produced by the sweat

glands is a watery fluid containing NaCl, small amounts of urea, lactic

acid, etc. Though the primary function of sweat is to facilitate a cooling

effect on the body surface, it also helps in the removal of some of the

wastes mentioned above. Sebaceous glands eliminate certain

substances like sterols, hydrocarbons and waxes through sebum. This

secretion provides a protective oily covering for the skin. Do you know

that small amounts of nitrogenous wastes could be eliminated through

saliva too?

19.8 DISORDERS OF THE EXCRETORY

SYSTEM

Malfunctioning of kidneys can lead to accumulation of urea in blood, a

condition called uremia, which is highly harmful and may lead to kidney

failure. In such patients, urea can be removed by a process called

hemodialysis. During the process of haemodialysis, the blood drained

from a convenient artery is pumped into a dialysing unit called artificial

kidney. Blood drained from a convenient artery is pumped into a dialysing

unit after adding an anticoagulant like heparin. The unit contains a coiled

cellophane tube surrounded by a fluid (dialysing fluid) having the same

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SUMMARY

Many nitrogen containing substances, ions, CO

, water, etc., that accumulate in

the body have to be eliminated. Nature of nitrogenous wastes formed and their

excretion vary among animals, mainly depending on the habitat (availability of

water). Ammonia, urea and uric acid are the major nitrogenous wastes excreted.

Protonephridia, nephridia, malpighian tubules, green glands and the kidneys are

the common excretory organs in animals. They not only eliminate nitrogenous wastes

but also help in the maintenance of ionic and acid-base balance of body fluids.

In humans, the excretory system consists of one pair of kidneys, a pair of ureters,

a urinary bladder and a urethra. Each kidney has over a million tubular structures

called nephrons. Nephron is the functional unit of kidney and has two portions –

glomerulus and renal tubule. Glomerulus is a tuft of capillaries formed from afferent

arterioles, fine branches of renal artery. The renal tubule starts with a double walled

Bowman’s capsule and is further differentiated into a proximal convoluted tubule

(PCT), Henle’s loop (HL) and distal convoluted tubule (DCT). The DCTs of many

nephrons join to a common collecting duct many of which ultimately open into the

renal pelvis through the medullary pyramids. The Bowman’s capsule encloses the

glomerulus to form Malpighian or renal corpuscle.

Urine formation involves three main processes, i.e., filtration, reabsorption and

secretion. Filtration is a non-selective process performed by the glomerulus using

the glomerular capillary blood pressure. About 1200 ml of blood is filtered by the

glomerulus per minute to form 125 ml of filtrate in the Bowman’s capsule per

composition as that of plasma except the nitrogenous wastes. The porous

cellophane membrance of the tube allows the passage of molecules based

on concentration gradient. As nitrogenous wastes are absent in the

dialysing fluid, these substances freely move out, thereby clearing the

blood. The cleared blood is pumped back to the body through a vein

after adding anti-heparin to it. This method is a boon for thousands of

uremic patients all over the world.

Kidney transplantation is the ultimate method in the correction of

acute renal failures (kidney failure). A functioning kidney is used in

transplantation from a donor, preferably a close relative, to minimise its

chances of rejection by the immune system of the host. Modern clinical

procedures have increased the success rate of such a complicated

technique.

Renal calculi: Stone or insoluble mass of crystallised salts (oxalates,

etc.) formed within the kidney.

Glomerulonephritis: Inflammation of glomeruli of kidney.

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minute (GFR). JGA, a specialised portion of the nephrons, plays a significant role

in the regulation of GFR. Nearly 99 per cent reabsorption of the filtrate takes place

through different parts of the nephrons. PCT is the major site of reabsorption and

selective secretion. HL primarily helps to maintain osmolar gradient

(300 mOsmolL

–1

-1200 mOsmolL

–1

) within the kidney interstitium. DCT and

collecting duct allow extensive reabsorption of water and certain electrolytes, which

help in osmoregulation: H

, K

and NH

could be secreted into the filtrate by the

tubules to maintain the ionic balance and pH of body fluids.

A counter current mechanism operates between the two limbs of the loop of

Henle and those of vasa recta (capillary parallel to Henle’s loop). The filtrate gets

concentrated as it moves down the descending limb but is diluted by the ascending

limb. Electrolytes and urea are retained in the interstitium by this arrangement.

DCT and collecting duct concentrate the filtrate about four times, i.e., from 300

mOsmolL

–1

to 1200 mOsmolL

–1

, an excellent mechanism of conservation of water.

Urine is stored in the urinary bladder till a voluntary signal from CNS carries out

its release through urethra, i.e., micturition. Skin, lungs and liver also assist in

excretion.

EXERCISES

1. Define Glomerular Filtration Rate (GFR)

2. Explain the autoregulatory mechanism of GFR.

3. Indicate whether the following statements are true or false :

(a) Micturition is carried out by a reflex.

(b) ADH helps in water elimination, making the urine hypotonic.

(d) Henle’s loop plays an important role in concentrating the urine.

(e) Glucose is actively reabsorbed in the proximal convoluted tubule.

4. Give a brief account of the counter current mechanism.

5. Describe the role of liver, lungs and skin in excretion.

6. Explain micturition.

7. Match the items of column I with those of column II :

Column I Column II

(a) Ammonotelism (i) Birds

(b) Bowman’s capsule (ii) Water reabsorption

(d) Uricotelism (iv) Urinary bladder

(d) ADH (v) Renal tubule

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8. What is meant by the term osmoregulation?

9. Terrestrial animals are generally either ureotelic or uricotelic, not ammonotelic,

why ?

10. What is the significance of juxta glomerular apparatus (JGA) in kidney function?

11. Name the following:

(a) A chordate animal having flame cells as excretory structures

(b) Cortical portions projecting between the medullary pyramids in the human

kidney

12. Fill in the gaps :

(a) Ascending limb of Henle’s loop is _______ to water whereas the descending

limb is _______ to it.

(b) Reabsorption of water from distal parts of the tubules is facilitated by hormone

_______.

(d) A healthy adult human excretes (on an average) _______ gm of urea/day.

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