Why Kidneys is the Principal Excretory Organs of Mammals?

The two kidneys are not situa­ted at the same level. The right kidney is slightly on a lower level than the left one.

It is due to the fact that the right side of the abdominal cavity is occupied by the liver. They are said to be retroperitoneal because they lie behind the peritoneal lining of the abdominal cavity.

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They are held in place by the padding of fat which surrounds them and by pressure from the other organs.

Each kidney is enclosed in a thin, tough, fibrous, whitish cap­sule. The outer susface of each kidney is convex, while the inner one is concave. In the concave depression there is an opening called the hilus renalis through which the blood vessels, nerves and lymphatics enter or leave the kidney.

Inside the kidney the hilus expands into a central cavity called the renal sinus which contains the major and minor renal calyces, the renal pelvis, nerves and blood vessels.

If we study a longitudinally cut section of the kidney, we find two distinct regions of the kidney, an outer firm region containing the uriniferous tubules or nephrons which manufacture the urine, the cor­tex and an inner region, the medulla, containing conical projections, the renal pyramids which contain the collecting and discharging tub­ules which carry the urine from the nephrons in the cortex to the pelvis of the kidney.

The base of each pyramid is in contact with the cortex and the apex which is called the papilla, projects into the sac­like cavity of the kidney called the pelvis.

The pelvis is the expan­ded beginning of the ureter and consists of several major cap-like structures called calyces.

The calyces branch out to form smaller ones, pach of which fits over the apex or papilla of a pyramid.

The pelvis, in turn, leads into a long, narrow tube, the ureter. The ureters (one for each kidney) empty into a urinary bladder. Thus, the urine which is secreted in the nephrons flows through the collecting tubules and

openings of the papilla into the calyx of the pelvis and thence through the ureter to the urinary bladder which on contraction removes the urine outside the body time to time.

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