19 EXCRETORY PRODUCTS AND THEIR ELIMINATION
EXCRETORY PRODUCTS AND THEIR ELIMINATION
EXCRETORY PRODUCTS AND THEIR ELIMINATION
Human Excretory System
Anatomically, the human excretory system consists of a pair of kidneys, a pair of ureters, urinary bladder and the urethra. The kidneys contain tiny, numerous structures called nephrons.
These are termed as the functional unit of the kidneys and are responsible for the separation of water, filter toxins and replenish necessary elements back into the bloodstream.
Ammonotelism:
The animals which excrete ammonia are called ammonotelic and excretion of ammonia is known as ammonotelism eg. Amoeba, sycon, hydra, liver fluke, tapeworm, Leech, Prawn, bony fishes etc.
Ureotelism:
Excretion of urea is known as ureotelism and the animals which excrete urea are ureotelic animals eg. mammals, many terrertrial amphibians and marine fishes and sting rays etc.
Uricotelism:
Excretion of uric-acid is known as uricotelism and the animals are called uricotelic eg. most insects, land snails, lizards, snakes and birds.
Kidneys
Kidneys are reddish brown bean shaped structure situated between last thoracic and lumber vertebra. Each kidney has a notch on its inner side called hilum through which ureter, blood vessels and nerves enter.
- Inside the hilum has broad funnel shaped space called renal pelvis with projection called calyces.
- Inside the kidney are two zone- outer cortex and inner medulla. Medulla is divided into medullary pyramids projecting into calyx.
- Cortex extends between medullary pyramids as renal column called Columns of Bertini.
- The functional unit of kidney is nephron. Each kidney contains about one million nephrons.
- Each nephron has two parts- the glomerulus and renal tubules. Glomerulus is the tuft of capillaries formed by afferent arteriole. Blood from glomerulus is carried away by efferent arteriole.
- Renal tubules starts with Bowman’s capsule continue with tubular parts divided into Proximal Convoluted tubules, Henle’s loop and Distal Convoluted tubule.
- The malpighian tubules, PCT and DCT of nephron are situated in cortical region where as loops of Henle’s into medulla.
Types of Nephrons
Juxtamedullary Nephron:
About 15% of total nephrons, Glomeruli are found in inner region of cortex, large in size, long loop of Henle and found deep in medulla, associated with vasa recta control plasma volume when water supply is short.
Cortical Nephron:
About 85% of total nephron mainly lie in renal cortex, glomeruli found in outer cortex, short loop of Henle, extends very little in medulla. They do not have vasa recta or vasa recta is highly reduced.
Urine formation
- Glomerular Filtration (Filtration of blood by glomerulus).
- Reabsorption (Reabsorption by renal tubules).
- Secretion (Tubular cells secretes H+, K+ ammonia into filtrate).
Glomerular capillaries:
Glomerular capillaries blood pressure cause filtration of blood through 3 layers (endothelium of glomerular blood vessels, epithelium of Bowman’s capsule and basement layer between two membranes as ultra-filtration.
glomerular filtration rate (GFR):
The amount of filtrate formed by kidneys per minute is called glomerular filtration rate (GFR) which is 125 ml/minute.
Glomerular Filtration rate:
Glomerular Filtration rate is controlled by Juxta glomerular apparatus (JGA).
Reabsorption:
99% of filtrate has to be reabsorbed by renal tubules called reabsorption.
Function of Tubules
-
Proximal Convoluted Tubules (PCT):
- All the important nutrients, 70-80% electrolytes and water are reabsorbed.
-
Henle’s Loop:
- Maintains high osmolarity of medullary interstitial fluid.
-
Distal Convoluted Tubules (DCT):
- Conditional reabsorption of Na+ and water. Maintains pH and sodium- potassium balance.
-
Collecting Duct:
- Large amount of water is reabsorbed to produce concentrated urine.
Mechanism of concentration of urine:
The flow of filtrate in two limbs of Henle’s loop is in opposite direction to form counter current. The flow of blood in two limbs of vasa recta increase the osmolarity towards the inner medullary interstitium in the inner medulla.
The transport of substance facilitated by special arrangement of Henle’s loop and vasa recta is called counter current mechanism.
Regulation of kidney function
- Functioning of kidney is monitored by hormonal feedback mechanism of hypothalamus and JGA. Change in blood volume, body fluid and ion concentration activates the osmoreceptors in the body that stimulate the hypothalamus to release ADH or vasopressin hormones. The ADH facilitates water absorption in tubules.
- Decrease in glomerular blood pressure activate JG cells to release renin which converts angiotensinogen to angiotensin I and II that increase the glomerular blood pressure and release of aldosterone that increase absorption of Na+ ions and water.
Micturition
- The process of expulsion of urine from the urinary bladder is called micturition. The neural mechanism that causes it is called micturition reflex. Urine formed in nephron is stored in urinary bladder till a voluntary signal is given by CNS. This initiates the contraction of smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter causing the release of urine.
- Lungs, liver and skin also play important role in process of excretion. Lungs remove CO2 and water, liver eliminates bile containing substances like bilirubin, biliverdin. Sweat glands remove NaCl, small amount of urea and lactic acid. Sebaceous glands excrete sterol, hydrocarbons and waxes.
Disorders of Excretory System
Uremia:
There is high concentration of non-protein nitrogen (urea, uric acid, creatinine). Urea can be removed by hemodialysis.
Renal failure:
Also known as kidney failure where glomerular filtration is ceased and both kidney stops working. Kidney transplant is the ultimate method in correction of acute kidney failure.
Renal Calculi:
Formation of stone or insoluble mass of crystallized salts formed within the kidney.
Glomerulonephritis (Bright’s Disease):
Inflammation of glomeruli of kidney due to entry of protein or red blood corpuscles in to filtrate due to injury.