· Excretory and Reproductive, together in vertebrates as the urinogenital system, as they are logically interrelated to some extent because excretory ducts (urinogenital ducts in the male) also used for the discharge of gametes
· In rabbits, the excretory organs include a pair of kidneys, a pair of ureters and a urinary bladder. They are similar to both the sexes.

1. Kidneys
· The chief organs of excretion are the two metanephric kidneys.

Morphology of Kidney:
· The two kidneys are small, reddish-brown, bean-shaped organs attached to the dorsal abdominal wall behind the pericardium on either side of the vertebral column.
· In mammals, the kidneys occupy much anterior position as against the metanephric kidneys of reptiles and birds which remain in the posterior part of the abdominal cavity.
· The right kidney is attached more anterior than the left. This asymmetry is just the reverse of that found in man.
· Each kidney is covered by peritoneum on its ventral surface only and is embedded in a mass of fat.
· The outer side of the kidney is convex and the inner side is deeply concave or notched bearing a depression or pit called hilum or hilus.
· The renal artery enters while the renal vein and ureter leave each kidney through its hilus.

The internal structure of the Kidney:
· The L.S. of kidney shows the following parts:
· Capsule- An outer thin envelope of tough fibrous connective tissue.
· Cortex- This is the outer region dotted or homogeneous looking and lighter in colour.
· Medulla- This is the inner region, striated and darker in colour.
· Pelvis- This is a large funnel-shaped space towards the concavity or hilus of the kidney. It collects urine which is drained off by the ureter.
· Pyramid- In the kidney of a rat or sheep, the medulla projects into the pelvis as a single conical process, the pyramid. But in the kidney of a rabbit or man, the medulla is lobulated forming a number of pyramids projecting towards the hilus. The cortex is continued inside between pyramids forming renal columns of Bertini.

· Histology:
· Histologically a kidney is made of innumerable, fine, long, much convoluted tubular units, called uriniferous tubules or nephrons.
· These are embedded in connective tissue along with blood vessels, lymph vessels, nerves and muscle fibres.
· One human kidney may contain about one million nephrons, each approximately 35 mm in length.
L.S. of human kidney

Structure of a Nephron
· A nephron or uriniferous tubule is made of two parts: 
(a) a Malpighian capsule and
(b) a Renal tubule

A. Malpighian capsule:
· The proximal end of each nephron forms a blind or closed, enlarged and double-walled cup, the Bowman's capsule, in the cortex.
· The hollow of the cup contains a tuft or knot of blood capillaries, called a glomerulus.
· This composite structure including the Borman’s capsule surrounding the glomerulus is known as a renal corpuscle or Malpighian capsule.

B. Renal Tubule: 
· The tubule leading from Bowman's capsule is differentiated in mammals into three regions.

Structure of nephron

1. The part leaving the cup makes a few coils in the cortex and is known as the proximal convoluted tubule.
2. It straightens out and descends down into the medulla forming the loop of Henle. This is the thinnest part of the tubule.
3. The ascending limb of the loop re-enters the cortex to form more coils and this is called the distal convoluted tubule. It joins one of the larger collecting ducts collectively forming the pyramid and finally opening into the pelvis.
· The wall of the Bowman's capsule is made of a single layer of squamous epithelial cells, and that of the tubule of cuboidal epithelial cells which become ciliated at intervals.

Blood supply of kidney
· The renal artery after entering the kidney through hilus branches into smaller vessels called arterioles.
· An afferent arteriole entering a glomerulus is about twice bigger in diameter than the efferent arteriole which leaves the glomerulus.
· The efferent arteriole then breaks up into a capillary network closely investing the uriniferous tubule. These capillaries reunite forming a venule.
· All the venules ultimately join to form the renal vein which comes out of the kidney through the hilus.
· Inside the kidney, the blood vessels run the junction of cortex and medulla, the sending branches into the glomeruli and receiving branches from the uriniferous tubule.
The pelvis of each kidney is directly continued into a thick-walled, narrow, cylindrical tube, ureter. It runs from the hilus of the kidney along the dorsal abdominal wall. Both the ureters open posteriorly into the neck of the urinary bladder on its dorsolateral surface.

Urinary Bladder
· It is a median, oval or pear-shaped, muscular sac situated below the rectum connected to the abdominal wall by a suspensory ligament.
· The posterior narrower part of the bladder, or neck has a circular sphincter muscle.
· The neck of the bladder opens into a thick-walled muscular duct, the urethra.
· In males, it is much longer and called the urinogenital canal which traverses and opens at the tip penis.
· In females, the urethra is small and combines with a vagina to form the vestibule which opens exterior by a slit-like vulva.

The Physiology of Excretion
· Kidneys performs several functions in the body, such as:
(1) Urine formation and
(2) Homeostasis.

· The chief nitrogenous product in mammals is urea, formed in the liver, it reaches es kidneys through blood circulation and in the form of a watery solution, called Urine.

· Urine formation involves the following three steps:
a. Glomerular filtration:
· The diameter of the afferent arteriole entering the glomerulus is greater than that of the efferent arteriole leaving it.
· This means more blood enter than can leave so that the blood in the glomerulus is relatively under great hydrostatic pressure.
· Since the walls of the glomerulus and Bowman’s capsule are extremely thin, permeable and in contact, most of the constituents of blood, the proteins, fats and cells, are filtered into the cavity of the capsule.
· The mechanism is known as pressure filtration or ultra-filtration, which is purely a physical process.
· The filtered out fluid is known as glomerular or capsular filtrate. It contains mostly urea dissolved in the water besides glucose, amino acids, sodium and potassium ions, uric acid, and so forth.
· The remaining constituents of blood, that is, cells, and fats, pass on through the efferent arteriole without getting filtered.

b. Tubular reabsorption:

· The ultrafiltrate contains many substances useful to the body.
· As the filtrate passes down the extremely long uriniferous due to ciliary action, many selected substances are taken back into the blood of the capillaries network which surrounds the tubule in close contact.
· This is called tubular reabsorption or selective absorption.
· In mammals, all the glucose, amino and some urea are reabsorbed in the proximal convoluted tubule.
· Sodium chloride and bicarbonate are reabsorbed in both the proximal and distal convoluted tubules.
· About 99% of water in the filtrate is reabsorbed at many levels including the loop of Henle.
· Tubular reabsorption occurs in two ways- Urea and water are reabsorbed by passive diffusion.
· Other substances (amino acids, sugars, salts, etc.) are actively taken up by the tubular cells and secreted into the capillary blood against a concentration (diffusion) gradient. This process is called active transport and requires the expenditure of energy by the tubular cells.
· If the kidney is denied oxygen, reabsorption stops, although filtration proceeds normally.

c. Tubular secretion:
· Some substances are added from the capillary blood to the filtrate by tubular secretion. These include creatinine, ammonia, hydrogen and potassium ions and various drugs.
· The highly concentrated urine, resulting from the tubular secretory exchange, contains water, urea, uric acid, creatinine, ammonia, etc.
· It also contains urochrome a pigment derived from the breakdown of haemoglobin, which imparts a yellow colour to urine.
· From nephrons, it passes into collecting tubules discharging into the pelvis and taken by the ureter to the urinary bladder for storage before discharge.

2. Homeostasis
· Kidney is not only an excretory organ.
· It also works as a homeostatic organ by regulating body fluids, such as blood and water.
· The term homeostasis means the tendency to regulate the stability of the normal conditions in the internal environment of a living organism.

A. Regulation of water (Osmoreg­ulation)
· If the animal drinks a great deal of water, the excessive water is allowed to escape by producing much dilute urine.
· Conversely, if the body has less water due to scarce drinking or loss in sweat, then scanty and more concentrated urine is produced after reclaiming a maximum amount of water from it.
· The volume of urine or water is controlled by an antidiuretic hormone (ADH) secreted by the posterior lobe of the pituitary.

B. Regulation of Blood
· By regulating the amount of water in the body, the kidney controls the osmotic pressure of the blood and tissue fluids.
· It helps in maintaining the right amount of salts in the blood. Similarly, it helps in preserving a constant level of pH in blood by allowing it to pass or retain certain acid and basic substances.
· Many types of medicines, such as antibiotics, aspirin, vitamins and many others, are removed from the body via the kidney.
· Blood volume is also regulated by the kidney. During the loss of blood, the blood pressure drops slowing urine production and retaining body fluids.

Previous Post Next Post

Main Tags