NEPHRON
The functional unit of the kidney,comprising bowman’s
capsule , the proximal and distal tubules, the loop of Henle and the collecting
duct which conveys urine to the renal pelvis.
The nephron consists of a tubule closed at one end,
the other end opening into a collecting tubule.
The closed or blind end is indented to form the
cup-shaped glomerular capsule(Bowman’s capsule) which almost completely
encloses a network of arterial capillaries, the glomerulus.
Continuing from the glomerular capsule the remainder
of the nephron is about 3cm long and is described in three parts.
1. The
proximal convoluted tubule
2. The
medullary loop (loop of henle)
3. The
distal convoluted tubule, leading into a collecting duct.
The collecting ducts unite, forming larger ducts
that empty into the minor calyces.
After entering the kidney at the hilum the renal
artery divides into smaller arteries and arterioles.
In the cortex an arteriole, the afferent arteriole,
enters each glomerular capsule then subdivides into a cluster of capillaries
forming the glomerulus.
The blood vessel leading away from the glomerulus is
the efeerent arteriole; it breaks up into a second capillary network to supply
oxygen and nutrients to the remainder of the nephron.
Venous blood drained from this capillary bed
eventually leaves the kidney in the renal vein which empties the into the
inferior venecava.
The blood pressure in the glomerulus is higher than
in other capillaries because the diameter of the afferent arteriole is greater
than that of the efferent arteriole.
The walls of the glomerulus and the glomerular
capsule consist of a single layer of flattened epithelial cells. The glomerular
walls are more permeable than those of other capillaries.
The remainder of the nephron and the collecting
tubule are formedby a single layer of highly specialized cells.
The nerve supply to the blood vessels of the kidney
consists of sympathetic and parasympathetic nerves.
The presence of both branches of the autonomic
nervous system permits control of renal blood vessel diameter and renal blood
flow independently of autoregulation.
Formation of urine .
The kidneys form urine which passes through the
ureters to the bladder for storage prior to excretion.
The composition of urine reflects the activities of
the nephrons in the maintenance of homeostasis.
Waste products of protein metabolism are excreted ,
electrolyte balance is maintained and the pH (acid-base balance) is maintained
by the excretion of hydrogen ions.
There are three processes involved in the formation
of urine.
1. Simple
filtration
2. Selective
reabsorption :
3. Secretion
1. Simple
filtration :
Filtration takes place through the semipermeable
walls of the glomerulus and glomerular capsule. The filtrate in the glomerulus
is very similar in composition to plasma with the important exception of plasma
proteins.
Filtration is assisted by the difference between the
blood pressure in the glomerulus and the pressure of the filtrate in the
glomerular capsule .
The diameter of the efferent arteriole is less than
that of the afferent arteriole, a capillary hydrostatic pressure of about
7.3kPa builds up in the glomerulus. This pressure is opposed by the osmotic
pressure of the blood, about 4 kPa (30mmHg), and by filtrate hydrostatic pressure
of about 2kpa (15mmHg) in the glomerular capsule.
2. Selective
reabsorption :
Selective reabsorption is the
process by which the composition and volume of the glomerular filtrate are
altered during its passage through the convoluted tubules, the medullary loop
and the collecting tubule.
3. Secretion
:
Filtration occurs as the blood
flows through the glomerulus; Substances not required and foreign materials,
e.g drugs including penicillin and aspirin, may not be cleared from the blood
by filtration because of the short time it remains in the glomerulus.
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