EXCRETION

DEFINITION: Excretion is the biological process by which an organism gets rid of toxic waste products of
metabolism.

EXCRETORY OR URINARY SYSTEM IN HUMANS: It consists of organs that take part in separation, conduction,
temporary storage and elimination of nitrogenous wastes as urine.

1. KIDNEYS: It consists of 2 kidney which are like rajma bean. They are located towards the back on each
side of backbone in a lower part of abdominal cavity. Left kidney is slightly higher than the right kidney. Each
kidney is convex on outer side and concaved on the inner side. Concave side has a entry of renal artery, exit of
renal vein and ureter. Renal artery brings oxygenated blood along with waste products. Renal vein carries
deoxygenated blood.
2. URETER: Urine formed in kidney passes into two tubes known as ureter.
3. URINARY BLADDER: It is the storage organ for collection of urine.
4. URETHRA: The urine comes out through urethra.

STRUCTURE OF NEPHRON: Nephron is the structural and functional unit of kidney. Single kidney contains
about one million of nephrons and 3 cm each in length.
1. BOWMAN’S CAPSULE: Each nephron has a cup shaped upper end called Bowman’s capsule. It is
named after Sir William Bowman, a British surgeon.
2. GLOMERULUS (PLURAL – GLOMERULI): Bowman’s capsule contains a bundle of blood capillaries which
is called glomerulus. One end of glomerulus is attached to renal artery and other end of glomerulus
comes out of Bowman’s capsule as blood capillary and surrounds tubule of nephron. Bowman’s
capsule and glomerulus are collectively known as Malphigian capsule.
3. TUBULE: The lower end of Bowman’s capsule is tube shaped and it is called tubule. One end of tubule
is connected to Bowman’s capsule and other end is connected to collecting duct of kidney.

FORMATION OF URINE:

1. ULTRAFILTRATION OR GLOMERULAR FILTRATION: The blood entering into the glomerulus carries
waste materials which are filtered through glomerulus. Filtration is possible because the wall of
glomerulus and Bowman’s capsule are very thin and selectively permeable. Only small molecules like
glucose, amino acids, salts, urea and water, etc. pass through the glomerulus. Large molecules like
protein and blood cells cannot pass through the glomerulus. Now the liquid entering into the tubule is
called glomerular filtrate., which contains Na+, K+, Cl- ions, glucose, amino acid, urea, uric acid and large
amount of water.
2. REABSORPTION: When the glomerular filtrate passes through the tubule then the useful substances
such as amino acids, glucose , most salts and water are reabsorbed into the blood through blood
capillaries surrounding the tubule. Urea, uric acid and excess water remain behind in the tubule. Now
the liquid left in the tubule is known as urine which is carried to the collecting duct..
3. TUBULAR SECRETION: It is active secretion of waste products by the blood capillaries into the urinary
tubule. It ensures removal of all the waste products from blood like urea, uric acid, extra salts into
tubule to maintain a proper concentration and pH of the urine.

FUNCTION OF KIDNEY

1. WAST PRODUCTS: Excretion of nitrogenous and other waste products.

2. TOXIC CHEMICALS: The system takes part in expelling toxic chemicals that enter our body.
3. WATER BALANCE: Maintenance of water balance in the body by producing dilute urine in excess when
intake is high & concentrated. Urine in case lesser than water intake.
4. EXCESS MATERIALS: Elimination of excess water soluble vitamins, drugs and other substances.
5. REGULATION OF SALT CONTENT: Regulation of salt content in the body by excreting salts when in
excess and retaining them when deficient.
6. MAINTENANCE OF pH.

EXCRETION IN PLANTS:
They have adopted varied strategies to protect their living cells from waste products.

1. The plants get rid of gaseous waste products through stomata in leaves and lenticels in stems..
2. The plants get rid of stored solid and liquid wastes by shedding of leaves, peeling of barks and felling of
fruits..
3. WATER: They get rid of excess of water by transpiration.
4. The plants get rid of wastes by secreting them in the form of gums and resins.
5. BARK: Bark consists of dead cells which is peeled off periodically. Tannin and other waste are deposited
in the bark. Tannins are raw materials for dyes and inks.

DIALYSIS:

If the kidneys of a person has stopped working due to an injury, infection, very high blood pressure, very high
blood sugar then the waste from the person’s blood can be removed periodically on a kidney machine
(by a procedure called dialysis).
The procedure used for cleaning the blood of a person by separating the waste substance urea from it is called
dialysis.
The blood from an artery in the patient’s arm is made to flow into the dialyser of dialysis machine made of
long tubes of selectively permeable membrane (like cellulose) which are coiled in a tank containing
dialyzing solution. The dialyzing solution contains water, glucose and salts in similar concentrations to
those in normal blood. As the patient’s blood passes through the dialyzing solution, most of the wastes
like urea in it is pass through the selectively permeable cellulose tubes into the dialyzing solution. The
clean blood is pumped back into a vein of the patient’s arm.