Types of Excretion
- Ammonotelism: Excretion of NH3. E.g. Aquatic invertebrates, bony fishes, aquatic amphibians.
- Ureotelism: Excretion of urea. E.g. Cartilaginous fishes, amphibians, mammals.
- Uricotelism: Excretion of uric acid. E.g. Insects, terrestrial reptiles & birds.
HUMAN EXCRETORY SYSTEM
Includes kidneys, ureters, urinary bladder & urethra.
Nephron: Structural & functional units of kidney.
A nephron has 2 parts:
Types of nephrons: Cortical (85%) & Juxtamedullary (15%).
1. Glomerular filtration (ultrafiltration):
1. Regulation by ADH (vasopressin):
- Glomerulus: Capillary network.
- Renal tubule: Bowman’s capsule + Proximal convoluted tubule (PCT) + Henle’s loop + Distal convoluted tubule (DCT).
Glomerulus + Bowman’s capsule = Malpighian body.
Types of nephrons: Cortical (85%) & Juxtamedullary (15%).
URINE FORMATION
1. Glomerular filtration (ultrafiltration):
- In glomerulus, blood is filtered through 3 layers- endothelium of glomerulus, basement membrane & epithelium of Bowman’s capsule.
- Epithelial cells (podocytes) of Bowman’s capsule form filtration slits.
- Glomerular filtration rate (GFR): Amount of glomerular filtrate formed per minute.
- Normal GFR = 125 ml/minute, i.e., 180 litres/day.
- 99% of filtrate is reabsorbed by the renal tubules. So normal volume of urine released is 1.5 litre.
- PCT reabsorbs most of the nutrients and 70-80% electrolytes & water.
- In DCT: Conditional reabsorption of Na+ & water.
- Collecting duct reabsorbs water to concentrate urine.
- PCT, DCT & Collecting duct maintain ionic and acid-base balance (pH) of body fluids by secretion of H+, K+ & NH3 into filtrate and absorption of HCO3- from it.
Mechanism of concentration of the filtrate
- Henle’s loop & vasa recta help to concentrate the urine.
- Flow of filtrate in the 2 limbs of Henle’s loop and the flow of blood through the 2 limbs of vasa recta are in opposite directions. This is called Counter current mechanism.
- Due to the counter current, osmolarity increases from cortex (300 mOsmolL-1) to the inner medullary interstitium (1200 mOsmolL-1). This gradient is caused by NaCl & urea.
- DCT & collecting duct produce urine four times concentrated than the initial filtrate formed.
MICTURITION
- It is the release of urine.
- Filled urinary bladder → stretch receptors send impulses to CNS → motor messages → urinary bladder contracts → micturition (1 - 1.5 litre urine (25-30 gm urea) per day).
- Micturition reflex: Neural mechanism of micturition.
- Urine analysis helps in clinical diagnosis of metabolic disorders and malfunctioning of the kidney.
REGULATION OF THE KIDNEY FUNCTION
1. Regulation by ADH (vasopressin):
- Hypothalamus to release ADH. It stimulates water reabsorption from DCT & collecting duct.
- Thus, ADH prevents diuresis and increases body fluid volume.
- ADH constricts blood vessels to increase BP. This increases the glomerular blood flow and GFR.
- JGA (Juxta glomerular apparatus) is a region in nephron.
- Fall in glomerular blood flow/glomerular BP/GFR → activates JG cells → renin.
- Renin converts angiotensinogen → angiotensin I → angiotensin II (vasoconstrictor) → increases glomerular BP & GFR.
- Angiotensin II → adrenal cortex → Aldosterone → reabsorption of Na+ & water from distal parts of tubule.
- When blood flow increases, the atria of heart releases Atrial Natriuretic Factor (ANF).
- It causes vasodilation → BP decreases.
DISORDERS OF EXCRETORY SYSTEM
- Uremia: Accumulation of urea in blood due to kidney failure.
- Renal calculi: Stone of crystallized salts (oxalates, etc.) formed within the kidney.
- Glomerulonephritis: Inflammation of glomeruli.
- Process of removal of urea in patients with uremia.
- Blood from artery (+ anticoagulant like heparin) → dialyzing unit → cellophane tube → passage of molecules → Purified blood (+ anti-heparin) → pumped back through a vein.
Kidney transplantation:
- For acute renal failures.
- Receiving kidney from a close relative minimizes chances of rejection by immune system of host.