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| Excretory System in Man |
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| The excretory system in man is formed of one pair of kidneys, a pair of ureters, a urinary bladder and a urethra. |
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| Kidneys |
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| The kidneys are dark-red, slightly flattened, bean shaped organs about 10 cm long, 5 cm wide and 4 cm thick each weighing about 27 gms. They are placed against the back wall of the abdominal cavity just below the diaphragm, one on either side of the vertebral column, between the last thoracic vertebra and the third lumbar vertebra. They are protected by the last 2 ribs. The kidneys are covered by the peritoneum in the front and on the side and they rest against the abdominal muscles. Their position is slightly asymmetrical, the left kidney being a little higher than the right. |
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| Position of Kidneys in the body |
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| Each kidney is bean shaped. The outer surface is convex and the inner surface is concave. The inner surface has a deep notch called hilus. The ureters, renal artery, renal vein and the nerves enter the kidney through the hilus. |
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| Kidney of Man |
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| Each kidney is composed of numerous microscopic coiled tubules called nephron or renal tubules or uriniferous tubules. The nephrons are held together by connective tissue, and enveloped by a capsule of white fibrous tissue. Outside the capsule is a layer of fat called the adipose capsule, followed by a membrane called renal fascia. The adipose capsule forms a shock - absorbing cushion and the renal fascia fixes the kidney to the abdominal walls. |
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| The kidney is divided into 2 regions, an outer region called renal cortex and the inner region termed renal medulla. The medulla is subdivided into conical masses, the renal pyramids, each having a broad base towards the cortex and a narrow end called renal papilla towards the pelvis. The renal papilla projects into the wide funnel - like structure called the pelvis. The pelvis leads into the ureter between the pyramids. The cortex extends into the medulla as renal column of Bertin. |
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| Section through Kidney to show distribution of Glomeruli |
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| Ureters |
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| These are about 28 cm long. They are a pair of narrow, muscular, tubular structures which arise from the hilus of the kidney, run backward along the dorsal body wall and open on the dorsal wall of the urinary bladder. Their wall is made up of a layer of transitional epithelium surrounded by a layer of muscle fibres, followed by a layer of fibrous connective tissue. |
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| These conduct urine from the kidneys to the urinary bladder. |
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| Urinary Bladder |
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| It is a pear shaped sac situated in the pelvic region of the abdominal cavity. It has thick muscular distensible wall lined by transitional epithelium that allows expansion. It can store about 0.5 to 1 litre of urine. It receives the ureters through the lower part of its back wall. The lower part or neck of the bladder is guarded by 2 rings of muscle fibres called sphincters. Both the sphincters must relax to let urine pass out from the bladder. The act of voiding of urine is called micturition. |
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| Urethra |
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| The urethra starts from the neck of the urinary bladder and leads to the exterior. In females it is about 2 - 3 cm long and carries only urine. It opens by the urethral orifice or urinary aperture in the vulva in front of the vaginal or genital aperture. |
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| In male, urethra is about 20 cm long and carries urine as well as the spermatic fluid. It passes through the penis and opens out at the tip of the penis by a urinogenital aperture. |
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