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.
19.1 Human
19.2 Urine Formation
19.3 Function of the
19.4 Mechanism of
Concentration of
the Filtrate
19.5 Regulation of
Kidney Function
19.6 Micturition
19.7 Role of other
Organs in
19.8 Disorders of the
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.
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
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
(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
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.
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
Figure 19.4 Malpighian body (renal corpuscle)
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.
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.