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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. manmals, 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 and snakes and birds.
Nephrons : The structural and functional unit of kidneys. Each kidney contains about one
million of nephrons.
Structure of Nephron : A nephron consists of Glomerulus, Bowman’s capsule, PCT (Proximal convo-
luted tubule). JG A (Juxaglomerular Appartus) and the collecting duct. (Refer fig.,
19.3, page 292
Structure of Kidney : Size 10-12 cm in length, 5-7 cm in width, 2-3 cm thick, average weight about
120-170 g
• The blood vessels, ureter and nerves enter in the kidney through hilum
(a notch).
The outer layer is a tough capsule.
• The outer zone of Kidney is cortex and the inner is medulla.
• The medulla is divided into few conical masses (medulllary pyramids)
projecting into calyces.
•
The cortex extends between medullary pyramids called columns of Bertini.
Refer figure 19.2, page 292
Glomerular Filtration : The filtration of blood in glomerulus, about 1100-1200 ml of blood is
filtered by the kidney per minute.
Glomerular Filtration Rate (GFR) : The amount of filtrate formed by the kidney perminute. In a healthy individual it
is about 125 ml/minute, ie 180 litres per day.
(i) 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 recta, control plasma volume when water supply is short.
(ii) Cortical Nephron :- About 25% 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.
(i) PCT :- absorption of all essential nutrients and 70-80% of electrolytes and
water, helps to maintain the pH and ionic balance of body fluids by selective secretion
of H
+
, ammoni and K
into filtrate.
(ii) Henle’s Loop :- reabsorption in this segment is minimum, it plays a sig-
+
nificant role in maintenance of high as molarity of medullary interstitial fluid.
(iii) DCT :- conditional reabsorption of Na
absorption of HCO
3
–
and selective secretion of H
+
+
and water takes place here, re-
and K
and ammonia to maintain
the pH and sodium-potassium balance is blood.
(iv) Collecting duct :- Large amount of water is absorbed from this region to
produce concentrated urine, it plays a role in maintenance of pH and ionic balance of
blood by selective secretion of H
+
and K
ions.
Mechanism of concentration of the Filtrate (Countercurrent Mechanism) : +
Refer fig 19.6 page 296 (NCERT - Class XI Biology)
• This mechanism is said to be countercurrent mechanism because the out flow
(in the ascending limb) runs parallel to and in the opposite direction of the
inflow (in the descending limb).
•
NaCl is transported by the ascending limb of Henle’s loop which is
exchanged with the discending limb of vasa recta.
•
NaCl is returned to the interstitium by the ascending portion of vasa recta.
•
Henle’s loop and vasa recta as well as the counter current in them help to
maintain an increasing osmolarity towards the inner medullary interstitium ie
from 300 mOsmol/L in cortex to about 1200 mOsmol/L in inner medulla.
•
Small amount of urea enter the thin segment of ascending limb of Henle’s loop
which is transported back to the interstitium by the collecting tubule.
•
This mechanism helps to maintain a concentration gradient in the medullary
tubule interstitium.
• It helps in an easy passage of water from the collecting tubule to concentrate
the filtrate ie urine.
Micturition : The expulsion of urine from the urinary bladder. It is a reflex process but can be
controlled voluntarily to some extent in grown up children and adults.
•
The CNS (Central Nervous System) sends the signal which cause the stretch-
ing of the urinary bladder when it gets filled with urine.
•
Inresponse, the stretch receptors on the walls of the bladder sends signals to
the CNS.
•
The CNS passes on motor massage to initiate the contraction of smooth
muscles of the bladder and simultaneous relexation of the urethral spnicter
causing the release of urine.
•
An adult human excretes on an average 1 to 1.5 litres of urine per day.
•
On an average 25-30 gram of urea is excreted out per day.
Role of other organs in excretion :
•
Lungs :- removes CO
(18L/day) and water.
•
Liver :- secretes bilirubin, biliverdin etc. helps to eliminate these substances
2
alongwith cholesterol,vitamins, drugs and degraded steroid hormones through
digestive wastes.
•
Sweat and sebaceous glands :- These glands of skin help to eliminate small
amount of urea, NaCl and lactic acid etc. through sweat while subaceous
glands help to eliminate some substances like steroids, hydrocarbons and waxes
through sebum.
•
Saliva :- It can help to eliminate small amount of nitrogenous wastes.
Disorders of Excertory system :
•
Uremia :- The accumulation of urea in blood due to malfunctioning of kidney.
•
Hemodialysis :- The process of removal of urea from the blood artificially. In
this process the blood from an artery is passed into dialysing unit after addding
an anticoagulant like heparin. The blood passes through coiled cellophane
tube surrounding by dialysing fluid. The nitrogenous wastes from the concentration
gradient and the blood becomes clear.This
blood is pumped back to
the
body through vein after adding anti-heparin to it.
•
Renal calculi :- The formation of insoluble mass of crystallised salts (oxalates
or phosphates of calcium.
•
Glomerulonephritis :- Inflammation of glomeruli of kindney.
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