removal of metabolic waste products from the body of the animals is
is the process by which the osmotic pressure is regulated or kept
constant in the blood and the tissue fluids.
and excretion together help the body to maintain its physiological
equilibrium also called as homeostasis.
process of breaking down of amino acids into forms in which they can
be excreted is called deamination. In vertebrates the process of
deamination takes place in the liver.
elimination of nitrogenous wastes in the form of ammonia is called
as ammonotelism. Animals eliminating nitrogenous wastes in the form
of ammonia are called ammonotelic. Animals like aquatic
invertebrates, bony fishes and aquatic amphibians like salamanders
and tadpoles are ammonotelic.
elimination of nitrogenous wastes in the form of urea is called
ureotelism. Animals excreting nitrogenous wastes in the form of urea
are called ureotelic. Mammals such as man, whale, seal, camel,
kangaroo, rat, etc; amphibians like toad and frog, fishes like shark
and sting-ray and reptiles like terrapin and turtle are ureotelic.
elimination of nitrogenous wastes in the form of uric acid is called
uricotelism. Animals that excrete uric acid are called uricotelic.
Birds, land reptiles, insects, land snails, etc., are called
excretory organs (excretory system) of man consists of a pair of
kidneys, a pair of ureters, a urinary bladder and urethra.
- Kidneys are a pair of purplish brown, bean-shaped organs
lying on the dorsal side of the abdominal cavity outside the
peritoneum, one on either side of the vertebral column.
- Between the right and left kidneys run two main blood
vessels namely dorsal aorta and inferior vena cava.
- Each kidney bears a longitudinal opening on its concave
side called hilium.
- Renal arteries, renal veins and nerves enter or leave the
kidney at the hilius.
- The ureter also originates from the hilius.
- The human kidney is enclosed in a capsule of fibrous
connective tissue, called renal capsule.
- The kidney tissue can be divided into two regions; cortex - the outer region and
medulla - the inner region.
- The hilium leads to a funnel-shaped space, the renal
pelvis. The ureter is connected to the pelvis.
- The urine in the natural state is filled in the hollow
regions of the kidney.
- Nephrons are the structural and functional units of
kidney. Each kidney has around million nephrons.
- A nephron along with the connecting tubule is called
- Ureters are a pair of narrow, muscular tubes, which
originate from the renal pelvis and open into the urinary bladder.
- Urinary bladder is median and unpaired.
- It is a muscular sack with smooth muscles. It lies behind
the pubis bone of the pelvic girdle.
- Ureters from both the kidneys open into the urinary
- The urinary bladder stores the urine till it is expelled
through the urethra.
The Structure of Uriniferous Tubule Nephron:
- The urethra is a median tube arising from the lower end of
- The opening of the bladder is guarded by a sphincter
muscle called urethral sphincter.
- The opening of this sphincter, which is under voluntary
control, results in micturition(urination) and in turn emptying of
- In females, the urethra is relatively shorter than in
Circulation of Blood Within the Kidney
- The nephron is the structural and functional unit of
- The nephron is a thin, coiled duct, lined by a single
layer of epithelial cells.
- The proximal end of nephron is blind (ie., it has no
opening anywhere). The distal end of the nephron opens into a
- The proximal and distal ends of nephron both lie in the
cortex of the kidney whereas the middle region lies in the medulla.
- The nephron consists of two parts, viz. Malpighian
corpuscle and Renal tubule.
- Malpighian Corpuscle
- The malpighian corpuscle is the anterior part of the
nephron. It consists of double walled cup-shaped structure called
Bowman's capsule. The Bowman's capsule is the widest part of the nephron.
It is made up of two layers, the outer parietal and the inner
visceral. Both the layers are continuous along the rim of the cup.
The Bowman's capsule encloses a knot of blood capillaries called
glomerulus. The glomerulus and the Bowman's capsule are intimately
connected with each other both structurally and functionally.
Together, they are called renal corpuscle or malpighian corpuscle.
- Renal Tubule:
The renal tubule arises from the base of the Bowman's capsule and
has an outer diameter of about 60 micrometers. The connection
between the renal tubule and the Bowman's capsule is called the
neck. The renal tubule is differentiated into four regions, viz.
the proximal convoluted tubule, Henle's loop, the distal convoluted
tubule and the collecting tubule.
The circulation of blood within the kidney can be represented thus,
Renal artery Arterial
capillary network Venule
separation of urine from the blood occurs in the following three
This is the first stage of urine formation and it takes place in
the renal corpuscle. The main structures involved in the filtration
are the glomerulus and the visceral layer of the Bowman's capsule.
There are present pores of about 0.1 micrometre in diameter on the
endothelium of glomerular capillaries. These pores are large enough
to let through all constituents of blood plasma except large
proteins. The blood cells are bigger in size and hence do not pass
through these openings. Endothelial cells rest on the thin basement
membrane. On the other side of this thin basement membrane are
situated the epithelial cells of the visceral layer of Bowman's
capsule. The membrane is made up of a layer of fine filaments
called glomerular slit diaphragm. The substances pass through the
endothelium and the slit diaphragm of the basement membrane. The
filtrate can thus reach the urinary space of the Bowman's capsule
through the gaps that are present in the visceral epithelial layer.
Along with it the substances pass from the blood into the lumen of
nephron. This process is called ultrafiltration.
Two processes are involved in the reabsorption process, namely,
osmosis and active transport.
From the capillaries, the cells of the tubules absorb substances
like potassium ions, ammonia ions, hydrogen ions, creatinine, etc
and secrete them into the lumen of the tubule. This process is
called tubular secretion. The resulting fluid in the tubule forms
Kidney and Osmoregulation
- The amount of urine produced varies depending on the time
of the day (lesser during night) and the food and fluids consumed.
- Normal urine is slightly acidic with a pH range of 5 to 7.
The Skin as the Excretory Organ
- The main function of reabsorption is to maintain the water
balance of the blood and other body fluids. According to the needs
of the body the urine produced may be hypotonic or hypertonic in
comparison with the body fluids. This is mainly due to the hormonal
control of reabsorption of water in the renal tubules.
- Antidiuretic hormone (ADH), a hormone secreted by the
pituitary gland is produced when the water content of the body
fluids is less than normal. The effect of ADH is mainly on the
permeability of the distal tubule and the collecting duct.
The two glands present in the skin which help the skin to perform as
an excretory organ are:
Lung as an Excretory Organ
- sweat glands
- sebaceous glands
It throws carbon dioxide out of the body. Water is lost from the
lung surfaces by evaporation, thus helping in osmoregulation.