Download Excretion Excretion: is the process by which the products of

Excretion
Excretion:
is the process by which the products of metabolism are removed from the
body.
The excretory system of man consists of two kidneys, two ureters,
urinary bladder, urethra, and excretory opening.
Structure of the Kidney
• It is located in the anterior part of the abdominal cavity on each
side of the vertebral column.
• Its outer surface is convex and the inner surface, the hilum, is
concave, in which pass the renal artery, vein, nerves and
lymphatics alongside the pelvis (funnel-shaped upper end of the
ureter).
• The pelvis is formed by the joining together of 4-6 major calyces,
each of which is in turn formed by several short branches of minor
calyces, which project together forming the pyramids.
• Each kidney consists of about one million of nephrons.
• Each nephron consists of a Malpighian body and a uriniferous
tubule.
• Malpighian body is made up of a glomerulus and a Bowman’s
capsule.
• The glomerulus is a mass of convoluted blood capillaries (afferent
and efferent arterioles).
• Bowman’s capsule opens into a narrow neck region of the tubule.
The uriniferous tubule is divided into three parts; 1) proximal
convoluted tubule, 2) narrow U-shaped tube having descending and
ascending limbs which are known as Henel’s loop, and 3) distal
convoluted tubule which opens into a collecting tubule.
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• The collecting tubules of all the excretory units in each kidney are
joined together forming a duct of Bellini, which runs up to the
pelvis.
Physiology of Excretion
Physiology of excretion by the kidneys involves the following steps:
1. Glomerular filtration:
- The efferent arterioles are narrower than the afferent ones hence
blood enters into glomerulus under higher pressure.
- Blood is filtered through the Bowman’s capsule into the
proximal convoluted tubule.
- The filtrate is known as the primary urine and it contains
glucose, urea, amino acids, sodium and potassium salts, and a
large quantity of water.
- So, the glomerulus functions as a filter and it discriminates
between different molecules in plasma on the basis of size. In
some diseases the permeability of the glomerulus increases and
plasma albumen appears in urine.
2. Selective reabsorptio:
- Useful substances are reabsorbed selectively along the tubules
of the nephrons. The reabsorbed substances are glucose, water,
mineral salts, and amino acids.
- The concentrated fluid enters the collecting tubules is known as
the secondary urine.
- Secondary urine is collected into the urinary bladder through the
ureters, and passes outside the body from time to time through
the urethra, mechanically by micturition.
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3. Tubular secretion:
- Tubules can collect specific waste materials from blood (e.g.
creatinin and drugs) and secrete them in the lumen to be added
to the filtrate.
- The urine of normal man contains inorganic salts and organic
compounds, such as urea, uric acid, creatinin and small amounts
of oxalic acid and amino acids.
- Normal urine is acidic. It becomes more acidic in exercise,
fasting, and diseases.
- It is about 1.5 liters per day.
Regulation of Urine Formation
I.
Nervous control.
Nerve impulses transmitted along the nerve fibers from the central
nervous system to the kidneys, cause constriction or dilation of renal
blood vessels. So, the permeability of the glomerular walls and absorptive
capacity of the epithelia of the uriniferous tubules are altered.
II.
Endocrine (chemical) control.
1. Antidiuretic hormone (ADH) secreted by the pituitary gland
increases the permeability of collecting tubules to water.
2. Aldosterone hormone secreted by the adrenal glands regulates Na+
and K+ excretion and retention, so regulates the amount of
excreted water. Thyroxine reduces water reabsorption.
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