Revision: Human Physiology >> Excretory Products and Their Elimination Biology (Theory) ISC (Science) ISC Class 11 CISCE

Kidneys are the primary excretory organs, eliminating nitrogenous wastes (chiefly urea) from the blood and throwing it out in the form of urine.

During different metabolic activities taking place in our body, the body produces many substances of which some are useful and some are useless.
If retained in the body the unwanted substances may become poisonous and cause much harm and in severe cases, even death. The organs which remove these unwanted and toxic substances from the body are called excretory organs.

During different metabolic activities taking place in our body, the body produces many substances, of which some are useful and some are useless.
The process of removing useless and harmful metabolic waste substances is called excretion.

The process of removal of chemical wastes (Mainly Nitrogenous) from the body is known as ''excretion''. It plays an important role in maintaining the homeostatic (steady-state) condition of the body.

The artificial process which cleans and filters the blood in a person where one or both the kidney may stop working properly is called dialysis.

Inside the kidney, there are millions of microscopic tubes called renal tubules or nephrons. It is the structural and functional unit of the kidney.

Tubular secretion is the active transfer of certain substances, such as ions and drugs, from the blood into the renal tubule during urine formation.

Ultrafiltration is the process in which blood is filtered under high pressure in the glomerulus, allowing water and small solutes to pass into the Bowman’s capsule.

Selective absorption is the process by which only useful substances like glucose, some salts, and water are absorbed from the renal tubule back into the blood, without disturbing its normal concentration.

Glomerular filtrate is the fluid formed after ultrafiltration that enters the renal tubule and contains water, urea, salts, glucose, and other small molecules.

The blood flows through the glomerulus under great pressure which causes the liquid part of the blood to filter out from the glomerulus into the renal tubule. This filtration under high force is called Ultrafiltration.

Glomerular filtration rate is the amount of glomerular filtrate formed in all the nephrons of both kidneys per minute. In a healthy individual, it is about 125 ml/minute.

The function of the kidney that involves regulating the composition of blood, including the percentage of water and salts, while removing wastes like urea, is called osmoregulation.

Dialysis is a medical process in which nitrogenous wastes and toxic substances are removed from the blood using an artificial machine when the kidneys fail to function properly.

An artificial kidney is a dialysis machine that removes urea and excess salts from the blood when both kidneys fail, and returns the purified blood back into the body.

• The human excretory system consists of a pair of kidneys, two ureters, a urinary bladder and a urethra.
• Kidneys are dark red, bean-shaped, retroperitoneal structures located from the 12th thoracic to the 3rd lumbar vertebra. Size: 10-12 cm × 5-7 cm × 2-3 cm; weight: 150 g (males), 135 g (females).
• Ureters are narrow tubular structures made of transitional epithelium that carry urine from the kidneys to the urinary bladder.
• The urinary bladder is a pear-shaped, hollow, muscular organ in the pelvic cavity, lined by transitional epithelium, and acts as a reservoir of urine.
• Two sphincters exist between the bladder and the urethra: the internal sphincter (involuntary, detrusor muscles) and the external sphincter (voluntary, striated muscles).
• The urethra is a canal-like structure that opens to the exterior via the urethral orifice, much longer in males than in females.
• The aorta supplies oxygenated blood to the kidneys; the inferior vena cava carries deoxygenated blood away from the kidneys.

1. Kidneys help maintain the balance of water and salts in the blood, a process known as osmoregulation.
2. In summer, more water is lost through sweat, so urine becomes thicker and less frequent due to increased water reabsorption.
3. In winter, water loss through sweat is less, so more urine is passed, and it is more diluted.
4. In cholera, severe water loss through vomiting and diarrhoea reduces water absorption into the blood.
5. Kidneys reabsorb almost all available water, but urea may also accumulate, leading to uremia (urea poisoning).
6. Immediate treatment includes oral rehydration solution (ORS) or glucose-saline drips to restore fluid balance.

• Kidney function is regulated by hormonal feedback involving the hypothalamus, JGA and the heart. Osmoreceptors detect changes in blood volume and ionic concentration.
• Low body fluid → osmoreceptors activated → hypothalamus → neurohypophysis releases ADH → DCT and collecting duct reabsorb more water → prevents diuresis.
• High body fluid → osmoreceptors are suppressed → ADH decreases → more water is excreted in urine.
• Low GFR → JGA releases renin → angiotensinogen → angiotensin I → angiotensin II (vasoconstrictor) → increases BP and GFR.
• Angiotensin II → adrenal cortex releases aldosterone → increases Na⁺ and water reabsorption → raises blood volume and BP.
• Increased blood flow to the heart atria → releases ANF → vasodilation → decreases BP.
• ANF acts as a check on the renin-angiotensin mechanism - both work in opposite directions to maintain BP balance.

1. Kidneys are the main excretory organs that remove urea and other nitrogenous wastes from the blood in the form of urine.
2. Skin eliminates small amounts of urea, salts, and water through sweat, mainly for body cooling.
3. Lungs excrete carbon dioxide through exhaled air, playing a key role in gaseous waste removal.
4. Liver helps in detoxification by converting ammonia to urea and breaking down toxins like alcohol and drugs.