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INTERMEDIATE BIOLOGY

EXCRETORY SYSTEM

DEFINITION
Excretion is the elimination of metabolic wastes like ammonia, urea, uric acid etc
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TYPES OF EXCRETION
1)AMMONOTELISM
-Process of excretion of NH3
-Ammonotelic animals:
#Aquatic invertebrates
#aquatic insects
#bony fishes
#aquatic amphibians e
-NH3 is highly toxic
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-NH3 is readily soluble in water and is excreted by diffusion through body surface or gill surfaces
(in fishes)
as ammonium ions
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2
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-Ureotelic animals
#Cartilaginous fishes
#terrestrial & semi-aquatic amphibians (frogs, toads etc
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-Some amount of urea may be retained in the kidney matrix of some animals to maintain a
desired osmolarity
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-Uricotelic animals
#Insects
#some land crustaceans
#land snails
#terrestrial reptiles
#birds
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EXCETORY ORGANS IN VARIOUS ORGANISMS
1)PROTONEPHRIDIA(FLAME CELLS)
-In Flatworms, rotifers,some annelids & cephalochordate (Amphioxus)
-Protonephridia are primarily for osmoregulation
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3)MALPHIGHIAN TUBULES
-In Insects
-Help in the removal of nitrogenous wastes and osmoregulation
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)
5)KIDNEYS
-In higher animals
-It includes kidneys, ureters, urinary bladder & urethra
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-Length: 10-12 cm
-width: 5-7 cm
-thickness: 2-3 cm
-Average weight:120-170 gm
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- On the concave side of kidney, there is an opening (hilum or hilus) through which blood
vessels, nerves, lymphatic ducts and ureter enter the kidney
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- A kidney has outer cortex & inner medulla
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- Cortex extendsin between the medullary pyramids asrenal columns (Columns of Bertini)
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#NEPHRON
- Nephrons are the structural & functional units of kidney
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o GLOMERULUS

-A tuft of capillaries formed by afferent arteriole (a fine branch of renal artery)
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o RENAL TUBULES
-It begins with a double walled cup-like
Bowman’s capsule,which encloses the glomerulus
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- Henle’s loop is hairpin-shaped
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- The DCTs of many nephrons open into a collecting duct
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-They converge and open into the renal pelvis through medullary pyramids in the calyces
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-Loop of Henle dips into medulla
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-A minute vessel of this network runs parallel to Henle’s loop forming a ‘U’ shaped vasa recta
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CORTICAL NEPHRONS (85%)
-In this, the Henle’s loop is short and extends only very little into the medulla
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2
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-Vasa recta present
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GLOMERULAR FILTRATION (ULTRAFILTRATION)
- The glomerular capillary blood pressure causes filtration of blood through 3 layers, i
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endothelium of glomerular blood vessels, epithelium of Bowman’s capsule & a basement
membrane between these 2 layers
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- Almost all constituents of the blood plasma except the proteins pass onto the lumen of the
Bowman’s capsule
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-It constitutes 1/5th of the blood pumped out by each ventricle of the heart in a minute
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- Normal GFR = 125 ml/minute, i
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, 180 litres/day
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REABSORPTION
- 180 litres of glomerular filtrate is produced daily
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-So normal volume of urine released is 1
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- From the filtrate, glucose, amino acids, Na+, etc
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-Simple cuboidal brush border epithelium of PCT increases surface area for absorption
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-Descending limb is permeable to water but almost impermeable to electrolytes
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-In ascending limb, minimum reabsorption occurs
- It is impermeable to water but allows transport of electrolytes
-So, filtrate gets diluted
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- Collecting duct extends from cortex to inner parts of medulla
-It reabsorbs large amount of water to concentrate urine
- It also allows passage of small amounts of urea into medullary interstitium to keep up the
osmolarity
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TUBULAR SECRETION
- Cells of PCT & DCT maintain ionic (Na-K balance) and acid-base balance (pH) of body fluids
by selective secretion of H+, K+ & NH3 into the filtrate and absorption of HCO -from it
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#MECHANISM OF CONCENTRATION OF FILTRATE
- Henle’s loop & vasa recta help to concentrate the urine
- The 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 (counter current pattern)
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- Due to the counter current and proximity between Henle’s loop & vasa recta, osmolarity
increases from cortex (300 mOsmolL-1) to the inner medullary interstitium (1200 mOsmolL-1)
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- NaCl is transported by ascending limb of Henle’s loop that is exchanged with descending limb
of vasa recta
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-Similarly, small amount of urea enters the thin segment of the ascending limb of Henle’s loop
which is transported
back to the interstitium by the collecting tubule
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- Thus DCT & collecting duct produce urine four times concentrated than the initial filtrate
formed (i
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300Gradual filling of urinary bladder causes stretching
- As a result, stretch receptors on its wall send impulses to CNS
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- It causes the contraction of smooth muscles of the bladder and simultaneous relaxation of the
urethral sphincter
- It results in micturition (release of urine)
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- An adult human excretes 1 to 1
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- Urine is a light yellow coloured watery fluid, slightly acidic (pH-6
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- Various conditions affect the

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