Kidneys
Functions
Excretion: nitrogenous wastes (urea, uric acid), excess salts,
excess water.
Osmoregulation: maintaining the blood at a suitable constant
concentration.
Homeostasis: maintaining a suitable constant internal
environment to sustain efficient metabolism.
Urinary System
Textbook Diagram: urinary system.
The kidneys are a pair of fist-sized red-brown bean-shaped
structures.
The kidneys are attached to the back wall of the abdominal
cavity.
They lie on either side of the backbone just above the pelvis.
Each kidney receives a good supply of oxygenated blood from the
renal artery, a branch of the dorsal aorta.
The renal vein takes the deoxygenated blood from the kidneys to
the inferior vena cava.
The blood in the renal vein has less oxygen, salt, urea and
uric acid than the renal artery.
Urine is carried to the bladder along the ureter by peristalsis
for temporary in the bladder.
A sphincter muscle at the junction of the bladder and urethra
regulates the retention and release of urine.
Urine is channelled to the exterior along the urethra.
Kidney Structure
Textbook Diagram: longitudinal section of a kidney showing its
internal structure.
A smooth thin protective cover called the capsule surrounds
each kidney.
Below the capsule is a thick reddish granular layer, the
cortex.
The central part of the series of triangular structures is
reddish-brown, the renal pyramids, the tips of which project into the upper
expanded end of the ureter known as the pelvis.
Nephron
Textbook diagram: structure and blood supply of the nephron.
The nephron or renal tubule is the functional unit of the
kidney.
The nephron has a number of functionally distinct parts.
Each human kidney has about one million nephrons.
Urine is manufactured by the nephrons.
Production of Urine: Filtration and Selective Reabsorption
Details of Urine Formation
Filtration
The glomerulus functions as a filter.
The glomerular capillary walls are porous.
The red blood cells, white cells, platelets and plasma proteins
are too big to pass through the pores.
Therefore the glomerulus filters the blood.
The filtrate passing into Bowman’s Capsule.
Glomerular filtrate composition is water, glucose, amino acids,
vitamins, salts, urea, and uric acid.
About 20% of the plasma volume passes out of the glomerulus.
The filtration is much higher than expected.
The blood pressure is unusually high in the glomerulus.
The blood pressure is generated by the pumping action of the
heart.
The high blood pressure in the glomerulus is due to:
The arrangement of blood vessels: arteriole —> capillaries
—> arteriole
This arrangement is unusual - normally low-pressure venules
follow capillaries.
The efferent arteriole is narrower than the afferent arteriole.
The higher than normal filtration at the glomerular capillaries
is known as ultrafiltration.
Selective Reabsorption
Much useful material was lost from the blood into Bowman’s
Capsule.
The ‘useful’ materials are taken back into the blood from the
nephron.
By de-selection, urea and uric acid remain in the nephron and
are excreted in the urine.
Urine is the unabsorbed glomerular filtrate.
Proximal Convoluted Tubule (PCT)
Total reabsorption of glucose and amino acids.
Four fifths of the salts and water are reabsorbed.
Glucose, amino acids and salts are reabsorbed by active
transport.
Water is reabsorbed by osmosis.
The cells lining the PCT are rich in mitochondria, which supply
the ATP for active transport.
The Loop of Henle
This structure allows the kidney to reabsorb extra water in
times of water stress. As a result it is possible for the kidney to produce
hypertonic urine, i.e., more concentrated than blood plasma. A Loop of Henle is
only present in mammals and birds — the only animals able to produce hypertonic
urine.
About 5% of the water from the glomerular filtrate is
reabsorbed from the Loop of Henle by osmosis.
The main function of the Loop is to develop an increasingly
concentrated medulla. It accomplishes this by acting as a ‘hairpin counter
current multiplier’. This allows extra water, if needed, to be absorbed from
the collecting duct under the influence of ADH hormone.
Distal Convoluted Tubule (DCT)
Reabsorption of water is by osmosis.
The amount varies depending on the need of the body.
Water reabsorption by the DCT is under the influence of ADH
(antidiuretic hormone).
Reabsorption of salt is by active transport.
The amount of salt reabsorbed depends on the needs of the body.
The role of the DCT is crucial in osmoregulation.
Osmoregulation is a major process in homeostasis.
Osmoregulation
Blood concentration is kept in check by varying the amount of
water and salt reabsorbed by the kidneys nephrons.
Blood Concentration Rising
Cause: salty food, water loss due to sweating, inadequate water
intake.
Response: increases water reabsorption, decreases salt
reabsorption.
Blood Concentration Falling
Cause: excessive water intake, cold weather (sweating less than
usual), diet very low on salt.
Response: decreases water reabsorption, increases salt
reabsorption.
Note: the greater the excess protein in the diet the greater is
the urea content of the urine.
Regulation of Body Fluids by the Kidney
The kidney maintains the blood at the correct composition and
concentration by excretion and osmoregulation.
As a result all the other body fluids are kept at optimum
condition i.e. tissue fluid and cell cytoplasm.
Role of ADH (antidiuretic hormone)
ADH is secreted to increase water reabsorption by the DCT and
collecting duct when blood concentration rises.
Osmoreceptors in the brain’s hypothalamus detect the increase
in osmotic pressure of the blood.
This stimulates the pituitary to increase the secretion of ADH
into the blood.
ADH is transported everywhere throughout the body in the blood.
The DCT and collecting duct are the target tissues of this
hormone.
ADH causes these parts of the nephron to become more permeable
to water.
Extra water is now reabsorbed into the blood reducing its
concentration back to normal.
The loss of extra water from the filtrate reduces the volume
but increases the concentration of the urine.
Major Homeostatic Organs: kidneys, liver, lungs, skin and
brain.
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