Revision: 11th Std >> Excretion and Osmoregulation MAH-MHT CET (PCM/PCB) Excretion and Osmoregulation

Organs which are concerned with the formation, storage and elimination of urine constitute the 'excretory system'.

The kidney while removing wastes like urea from the blood also regulates its composition, i.e., the percentage of water and salts. This function is called osmoregulation.

• The process of removal of chemical wastes (mainly nitrogenous wastes) from the body is known as 'excretion' (ex: out, crete: flow).
• Excretion is the process of removal of harmful and unwanted nitrogenous waste products from the body.
• Excretion is defined as the process by which organisms expel metabolic waste products and other toxic substances from the body.

Micturition is the process of expelling urine out of the body through the urethra by opening the sphincter muscles and passing urine, involving the relaxation of the sphincter muscles between the urinary bladder and urethra. 

Excretion is the process that biological organisms use to expel or eliminate the waste products produced by their metabolism.

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.

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

The kidney is composed of an enormous number of minute tubules called uriniferous tubules or nephrons or renal tubules or just kidney tubules. These are the structural and functional units of the kidney. 

The apex of each pyramid in the medulla of the kidney that projects into the pelvis is called the papilla.

Bowman's capsule is a thin-walled, cup-shaped structure in the nephron that surrounds the glomerulus and collects the filtrate from the blood.

The glomerulus is a knot-like network of blood capillaries located inside the Bowman's capsule, where blood filtration occurs.

The Malpighian capsule is the combined structure of the Bowman's capsule and glomerulus, forming the filtration unit of the nephron.

The separation of small molecules from large molecules in a solution by interposing a semipermeable membrane between the solution and water using artificial machine known as haemodialyser.

Kidney transplantation is the surgical transfer of a healthy kidney into a patient with end-stage renal disease.

• 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.

• Humans have two bean-shaped kidneys located on either side of the vertebral column (from the 12th thoracic to the 3rd lumbar vertebra).
• Kidneys help in maintaining homeostasis by regulating water balance (osmoregulation) and pH of body fluids.
• They also secrete erythropoietin, a hormone important for red blood cell production.
• Each kidney has an outer cortex and an inner medulla; the medulla contains conical structures called renal pyramids.
• Cortex extends into medulla, forming renal columns (columns of Bertini), and pyramids open into minor calyces through renal papilla.
• Kidneys are protected by renal capsule, adipose capsule (fat layer), and renal fascia, and contain nephrons, which are functional units for urine formation.

• Nephrons are the structural and functional units of the kidney, each consisting of a glomerulus and a renal tubule, and measuring about 4–6 cm in length.
• The Malpighian corpuscle (glomerulus + Bowman’s capsule) is the site of ultrafiltration, where blood enters through the afferent arteriole and leaves via the efferent arteriole.
• The proximal convoluted tubule (PCT), lined with cuboidal cells and microvilli, is the main site of selective reabsorption of water, ions, and nutrients.
• The loop of Henle extends into the medulla and helps in concentration of urine; its descending limb is permeable to water, while the ascending limb is impermeable to water but allows movement of electrolytes.
• The distal convoluted tubule (DCT) carries out tubular secretion and regulation of ions, and opens into the collecting duct, which reabsorbs water and transports urine to the renal pelvis.
• Nephrons are of two types: cortical nephrons with short loops of Henle and juxtamedullary nephrons with long loops that play a key role in urine concentration.
• The kidneys receive rich blood supply and filter large volumes of blood daily, with most filtrate reabsorbed and about 1–1.5 litres of urine excreted per day.

• Urine formation occurs in three main steps: ultrafiltration, selective reabsorption, and tubular secretion.
• Ultrafiltration takes place in the glomerulus due to high pressure, filtering blood plasma without proteins and cells to form glomerular filtrate.
• The filtrate contains water, glucose, amino acids, urea, and salts, and passes into the proximal convoluted tubule (PCT).
• Selective reabsorption occurs mainly in PCT, where useful substances like glucose, amino acids, ions, and most water (about 99%) are reabsorbed.
• Tubular secretion occurs in the distal convoluted tubule (DCT) and the collecting duct, where wastes like H⁺, K⁺, and creatinine are added to the filtrate.
• These processes help in maintaining water balance, electrolyte balance, and pH of the body while forming urine.

• Urine is a pale yellow, slightly acidic liquid due to the pigment urochrome, and its composition depends on diet and fluid intake.
• ADH (antidiuretic hormone) regulates water balance by increasing water reabsorption in the DCT and the collecting duct, reducing urine volume.
• Osmoreceptors in the hypothalamus detect changes in blood osmolarity and control ADH secretion by negative feedback.
• RAAS (Renin-Angiotensin-Aldosterone System) regulates blood pressure and volume by increasing reabsorption of Na⁺ and water.
• ADH and RAAS work together to maintain fluid balance, osmolarity, and blood pressure in the body.
• ANP (Atrial Natriuretic Peptide) opposes RAAS by increasing excretion of Na⁺ and water, thus lowering blood volume and pressure.

• Besides kidneys, organs like skin and lungs also help in the excretion of waste products.
• Sweat glands in the skin excrete water, salts (NaCl), urea, and lactic acid, and also help in thermoregulation.
• Sebaceous glands secrete sebum, which lubricates and protects the skin from infection and damage.
• Lungs excrete carbon dioxide (CO₂) and water vapour produced during cellular respiration.
• Thus, skin and lungs assist in the removal of wastes and help maintain the internal balance of the body.

• Haemodialysis is an artificial method of filtering blood when kidney function falls below 5–7%.
• In this process, blood is taken out of the body and passed through a semipermeable membrane (cellophane tube) in a dialysis machine.
• The tube is placed in dialysate fluid, which is similar to normal blood plasma, allowing only excess salts and wastes to diffuse out.
• Waste substances move from blood into dialysate, and purified blood is returned to the body.
• Anticoagulant (heparin) is added to prevent clotting, and the process is slow as blood flows gradually through the tube.

• Peritoneal dialysis uses the peritoneum (lining of the abdominal cavity) as a natural semipermeable membrane for filtration.
• Dialysate fluid is introduced into the abdominal cavity through a catheter.
• Waste substances and excess solutes diffuse from blood into the dialysate fluid.
• The used fluid is drained out after some time, and the process can be repeated as needed.
• It can be done at home and is convenient, but it is less efficient than haemodialysis.

• Kidney transplantation is the replacement of a non-functional kidney with a healthy kidney in end-stage renal disease.
• The donor kidney can be obtained from a deceased (cadaveric) donor or a living donor.
• Living donors can be genetically related or non-related to the patient.
• Immunosuppressant drugs are given after a transplant to prevent rejection of the new kidney.