Page 13 - LN- EXCRETION

Regulation of Kidney Functions
To maintain homeostasis, the regulation of water and solute contents of the body fluids is
performed by the kidneys. The vertebrate kidney is very flexible in its functioning. It
excretes larger quantities of dilute urine when water is abundant in the body tissues and
small amounts of concentrated urine when there is shortage of water.
Hormones acts as an important signalling molecules in controlling the regulatory processes
in the kidneys. The functioning of the kidneys is efficiency monitored and regulated by
hormonal feedback mechanisms involving hypothalamus, JGA (Juxtaglomerular Apparatus)
and to a certain extent, the heart.
Regulation of the functioning of kidneys can be discussed under the following headings
Regulation by the Hypothalamus
Excessive loss of fluid from the body activates osmoreceptors, which stimulate the
hypothalamus to release ADH or vasopressin form the neurohypophysis. ADH facilitates
water reabsorption from posterior parts of tubule. An increase in body fluid volume can
switch off the osmoreceptors and suppresses the ADH release to complete the feed back.
ADH also causes constrictory effects on blood vessels.
This causes an increase in blood pressure, which in turn increase the glomerular blood flow
and thereby the GFR (Glomerular Filtration Rate).
Regulation by the Juxtaglomerular Apparatus (JGA)
As blood pressure/glomerular blood flow /GFR decreases, the cells of the JGA release the
enzyme renin.
Renin converts angiotensinogen in blood to Angiotensin I and Angiotensin II (active form).
This mechanism is generally known as the Renin-angiotensin mechanism.
Angiotensin has following effects
(a) Raises blood pressure by constricting blood vessels (being a powerful vasaconstrictor)
and thereby, GFR.
(b) Activates the adrenal cortex to release aldosterone.
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(c) 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.
Regulation by the Heart
Atrial Natriuretic Factor (ANF) produced by the atria of heart can cause vasodilation (dilation
of blood vessels) and thereby, decrease the blood pressure.
ANF inhibits NaCl reabsorption and concentration of urine.
Micturition
Urine is produced and drained continuously by the nephron into the renal pelvis from here,
it is carried down to the ureters and then into the urinary bladder.
The bladder serves to store the urine temporarily till a voluntary signal is given by the
Central Nervous System (CNS). As urine collects, the muscular walls of the bladder distend
to accommodate it. .
The stretch receptors on the walls of the bladder set up reflexes (send signals to the (CNS)
by stimulating the sensory nerve ending in the bladder). It causes an urge to pass out urine.
The act of expulsion of urine involves the coordinated contraction (as CNS passes on motor
messages) of the smooth muscle of the bladder wall and simultaneous relaxation of the
internal and external urethral sphincters. The process of release of urine is called micturition
and the neural mechanism causing it is called the micturition reflex.
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