Revision: Class 11 >> Excretory Products and Their Elimination NEET (UG) Excretory Products and Their Elimination

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

• 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.

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.

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. 

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.

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.

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

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 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.

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 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.

• PCT (Proximal Convoluted Tubule) reabsorbs all nutrients and 70–80% of water and electrolytes, and helps maintain pH balance by secretion of H⁺, NH₃, and K⁺.
• Loop of Henle maintains medullary osmolarity.
  • Descending limb: permeable to water only
  • Ascending limb: permeable to electrolytes, not water
• DCT (Distal Convoluted Tubule) shows selective reabsorption of Na⁺, water, and HCO₃⁻ and secretion of H⁺, K⁺, NH₃ to maintain ionic balance.
• Collecting duct reabsorbs large amounts of water and some urea, helping in urine concentration and osmolarity maintenance.
• Counter-current mechanism (Henle’s loop + vasa recta) creates an osmotic gradient (300 → 1200 mOsm/L) in the medulla.
• This gradient allows maximum water reabsorption from the collecting duct, resulting in concentrated urine.

• 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.

• Mammals produce concentrated urine with the help of Henle's loop and vasa recta through a counter current mechanism.
• Filtrate in Henle's loop and blood in the vasa recta both flow in opposite directions (counter-current).
• Counter current maintains osmolarity from 300 mOsmol/L (cortex) to 1200 mOsmol/L (inner medulla).
• The gradient is mainly created by NaCl (transported by the ascending limb) and urea (transported by the collecting tubule).
• Interstitial gradient allows water to move out of the collecting tubule, concentrating urine up to 4 times more than the initial filtrate.

• 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.

• Micturition is the process of urine release, controlled by the micturition reflex and the central nervous system (CNS).
• When the bladder is full, the CNS signals the bladder muscles to contract and the urethral sphincter to relax, allowing urine to pass out.
• An adult excretes about 1–1.5 L of urine per day.
• Urine is light yellow, slightly acidic (pH ~6), and contains about 25–30 g of urea daily.
• The presence of glucose or ketone bodies in urine indicates disorders like diabetes mellitus.

• 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.