PinkMonkey Online Study Guide-Biology
The primary urine (glomerular filtrate) enters the proximal
convoluted tubule where its volume is reduced by about 80%. This is accomplished
by active reabsorption of many useful substances like sodium chloride,
glucose, amino acids, etc. from the tubular fluid. The water diffuses
out of tubular fluid into surrounding tissue (cortical interstitium).
This is called obligatory reabsorption of water. Thus, in proximal
tubule there is a drastic reduction in fluid-volume without any change
in its osmolarity (maintains osmotic balance).
As the tubular fluid passes down the descending limb
of Henle’s loop, water leaves and sodium enters the tubular fluid, the
fluid becomes progressively more concentrated and hyperosmotic
to blood. The tubular fluid passes through the hair-pin bend and enters
the ascending limb of Henle’s loop. The walls of the ascending limb do
not permit diffusion of water and salts. In the ascending limb the chloride
ions are actively transported out of the nephron and into the surrounding
tissue and sodium ions follow passively.
During ultrafiltration, certain harmful substances like
ammonia, urea, potassium, hydrogen ions etc. may escape filtration and
pass in the surrounding capillary network. Such substances are secreted
back into the distal convoluted tubule and then into the common collecting
duct. This results in the formation of secondary urine, which is
then excreted. Once urine is in the collecting duct, no more absorbtion
of water or salts takes place.
About 1600 liters of blood passes through the human
kidney daily. The amount of plasma in the human body is only about
three liters. Thus every drop of plasma in the blood passes through
the kidneys, where its contents are monitored, checked and altered,
about 560 times in a day. About 180 liters of filtrate passes through
the nephron in a day, at the rate of about 125 ml per minute. Most
of it is reabsorbed, so that only about one liter of urine is produced
and excreted from the body daily.
iii) Regulation of kidney function (Hormonal control)
The amount of salts, water and production of urine depends
upon kidney functions like filtration, reabsorption, and secretion. These
are regulated by certain hormones from the pituitary and adrenal gland.
a) Antidiuretic Hormone (ADH): Under conditions
of water diuresis (i.e., when there is no need for water conservation),
the hyposomotic tubular fluid (urine) passes into the collecting tubule
unchanged in its osmolarity. When water needs to be conserved by the body,
ADH (also called vasopressin) is secreted from the brain, and ,
the distal convoluted tubule and collecting tubules reabsorb more water,
so that urine becomes more concentrated. The water level in blood is restored
in this manner. In the absence of ADH, reabsorption of water is reduced,
leading to excretion of increased amounts of more dilute urine.
b) Aldosterone: Reabsorption of sodium ions is
controlled by the hormone aldosterone, produced in the cortex
of the adrenal gland. Increased aldosterone production results in increased
reabsorption of sodium ions, whereas lowered levels of aldosterone causes
increased excretion of sodium.
Table of Contents
Ammonotelism, Ureotelism and Uricotelism
Excretory System of Man
Skin and Lungs as Accessory Excretory Organs