1.Define Glomerular
Filtration Rate (GFR).
Solution. The amount of
filtrate formed by the kidneys per minute is called glomerular filtration rate
(GFR). It is approximately 125 mL/min. in a healthy person.
2.Explain the autoregulatory
mechanism of GFR.
Solution. The kidneys
have built-in mechanisms for the regulation of glomerular filtration rate. One
such efficient mechanism is carried out by juxta glomerular apparatus (JGA). JGA
is a special sensitive region formed by cellular modifications in the distal
convoluted tubule and the afferent arteriole at the location of their contact. A
fall in GFR can activate the JG cells to release renin which can stimulate the
glomerular blood flow and thereby the GFR back to normal.
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3.Indicate whether the
following statements are true or false.
(a)
Micturition is carried out by a reflex.
(b)
ADH helps in water elimination, making the urine
hypotonic.
(c) Protein-free fluid is filtered
from blood plasma into the Bowman’s capsule.
(d)
Henle’s loop plays an important role in concentrating the
urine.
(e)
Glucose is actively reabsorbed in the proximal convoluted
tubule.
Solution.(a) True (b)
False (c) True (d) True (e) True
4.Give a brief account of the
counter current mechanism.
Solution.
The kidneys have a special mechanism for concentrating the urine, it is
called counter current mechanism. The mechanism is said to be a counter current
mechanism because the out flow (in the ascending limb) of Henle’s loop runs
parallel to and in the opposite direction of the inflow (in the descending limb)
and vasa recta. As the mechanism begins to function, the ascending limb of loop
of Henle actively transports chloride and sodium ions out into the vasa recta
from where it is secreted into the interstitial fluid. As a result the
interstitial fluid around the loop of Henle contains large quantities of NaCl.
The filtrate passes from the ascending limb of loop of Henle and enters a
collecting duct. The collecting duct passes adjacent to the loop of Henle where
the interstitial fluid contains large amounts of NaCl. The high osmotic pressure
created by NaCl causes water to diffuse out of the collecting duct in the
interstitial fluid and eventually to the blood of vasa recta. The filtrate
becomes greatly concentrated and is now called urine. A similar counter current
mechanism, operates between the interstitial fluid and blood passing through the
vasa recta. As the blood capillary runs along the ascending limb of loop of
Henle, NaCl diffuses out of the blood. The direction is reversed as the blood
capillary passes along the descending limb of Henle. The blood flows in the vasa
recta around the loop of Henle from ascending to the descending side while the
fluid passing through the loop of Henle goes in the opposite direction. The
arrangement helps to maintain the concentration gradient of NaCl.
The
‘overall function of counter current mechanism is to concentrate sodium chloride
in the interstitial fluid and thereby cause water to diffuse out of the
collecting ducts and concentrate the urine.
5.Describe the role of liver,
lungs and skin in excretion.
Solution.
Other than the kidneys, lungs, liver and skin also help in the elimination of
excretory wastes. Lungs remove large amounts of C02 (18 litres/day) and also
significant quantities of water every day. Liver secretes bile which contains
substances like bilirubin, biliverdin, cholesterol, degraded steroid hormones,
vitamins and drugs. Most of these substances ultimately pass out along with
digestive wastes. The sweat and sebaceous glands in the skin can eliminate
certain substances through their secretions. Sweat produced by the sweat glands
is a watery fluid containing NaCl, small amounts of urea, lactic acid etc.
Sebaceous glands eliminate certain substances like sterols, hydrocarbons and
waxes through sebum.
6.Explain
micturition.
Solution. The process of
passing out urine from the urinary bladder is called micturition. Urine formed
by the nephrons is ultimately carried to the urinary bladder where it is stored.
This causes stretching of the wall of bladder that leads to the stimulation of
stretch receptors on the walls of the bladder. This sends signal to the CNS. The
CNS passes on motor messages to initiate the contraction of smooth muscles of
the bladder and simultaneous relaxation of the urethral sphincter causing the
release of urine.
7.Match the items of column I
with those of column II.
Column
I
Column
II
(a)
Ammonotelism
(i)Birds
(b)
Bowman’s capsule (ii)Water
reabsorption
(c) Micturition
(iii)Bony
fish
(d)
Uricotelism
(iv)Urinary bladder
(e)
ADH
(v)Renal tubule
Solution.
(a) – (iii), (b) – (v), (c) – (iv), (d) – (i), (e) – (ii)
8.What is meant by the term
osmoregulation?
Solution. The regulation
of water and solute contents of the body fluids by the kidney is called
osmoregualtion.
9.Terrestrialanimalsaregenerallyeitherureotelic
or uricotelic, not ammonotelic, why?
Solution.
Ammonotelic animals are aquatic animals that excrete ammonia which is highly
soluble in water, thus large amount of water is also excreted. Terrestrial
animals cannot afford to lose such large quantities of water from their bodies
as they live in environment having water scarcity. They, therefore, excrete
either urea (ureotelic) or uric acid (uricotelic) as these are less soluble in
water.
10. What is the significance
of juxta glomerular apparatus (JGA) in kidney function?
Solution. Juxta
glomerular apparatus (JGA) is a special sensitive region formed by cellular
modifications in the distal convoluted tubule and the afferent arteriole at the
location of their contact. The JGA plays a complex regulatory role. A fall in
glomerular blood flow/ glomerular blood pressure/GFR can activate the JG cells
to release renin which converts angiotensinogen in blood to angiotensin I and
further to angiotensin II. Angiotensin II, being a powerful vasoconstrictor,
increases the glomerular blood pressure and thereby GFR. Angiotensin II also
activates the adrenal cortex to release aldosterone. Aldosterone causes
reabsorption of Na+ and water from the distal parts of the tubule. This also
leads to an increase in blood pressure and GFR.
11 .Name the
following.
(a) A chordate animal having flame
cells as excretory structures.
(b)
Cortical portions projecting between the medullary pyramids in the human
kidney.
(c) A
loop of capillary running parallel to the Henle’s loop.
Solution. (a)
Cephalochordate – Amphioxus
(b) Columns of Bertini
(c) Vasa recta
12.Fill in the
gaps.
(a)
Ascending limb of Henle’s loop is________to water whereas the descending limb
is________to it.
(b) Reabsorption of water from distal
parts of the tubules is facilitated by hormone________
(c) Dialysis fluid contains all the
constituents as in plasma except________
(d)
A healthy adult human excretes (on an average)________gm of
urea/day.
Solution.
(a)
Ascending limb of Henle’s loop is impermeable to water whereas the
descending limb is permeable to
it.
(b) Reabsorption of water from distal parts of the tubules is facilitated
by hormone ADH.
(c) Dialysis
fluid contains all the constituents as in plasma except nitrogenous
wastes.
(d) A healthy adult human excretes (on an average) 25 – 30 gm of urea/day.