Friday, November 27, 2020

Excretory Products and their Elimination | Plus 1 | Exam Capsule Notes

EXCRETORY PRODUCTS: CHAPTER AT A GLANCE
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.



Kidney: Covered by renal capsule. Blood vessels, nerves, ureter etc. enter the kidney through hilum. Hilum leads to renal pelvis with renal calyces. A kidney has outer cortex & inner medulla. Medulla consists of medullary pyramids.

Nephron: Structural & functional units of kidney. 

A nephron has 2 parts:
  • 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.
2. Reabsorption: 
  • 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.
3. Tubular Secretion:
  • 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.
2. Regulation by JGA (Renin-Angiotensin mechanism): 
  • 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. 
3. Regulation by ANF: 
  • 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.
Hemodialysis:
  • 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.

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