The excretory system in human beings consists of:
i. Kidneys:
The kidneys are reddish-brown, paired structures which lie along the posterior abdominal wall on either side of the vertebral column. They are protected by the last two pairs of ribs.
Each kidney is a bean-shaped organ, about 10 cm long, 6 cm wide and 4 cm thick. The right kidney is positioned slightly lower than the left kidney as the right side of the abdominal cavity is occupied by the liver. They are held in position by padding. On the inner concave side of the kidney is present a notch called hilum. It is the place from where the ureter and the renal vein come out and the renal artery enters the kidney.
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ii. Ureters:
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The ureters arise from within the renal sinus of the kidneys at hilum. The anterior part of each ureter is a funnel-shaped structure, the pelvis. The ureters transport urine from the kidneys to the urinary bladder. Each ureter opens obliquely in the urinary bladder by a slit-like aperture to prevent back flow of the urine.
iii. Urinary Bladder:
The urinary bladder acts as a muscular reservoir for storage of urine. It lies in the pelvic cavity of the abdomen. The neck of the urinary bladder is surrounded by sphincters (which act like a valve) which remain closed until the time of micturition (urination). In an adult male, the average capacity of the urinary bladder varies from 150 ml to 300 ml.
iv. Urethra:
The urethra is a membranous tube that arises from the neck of urinary bladder and extends up to the urethral orifice. It serves as a common passage for urine and sperms.
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Urethra in females serves as a passage for urine only.
Role of the Urinary Bladder in Urination:
Besides functioning as a temporary reservoir of urine, the bladder also evacuates the urine at suitable intervals. The act of voiding or discharging urine is called micturition.
When enough urine gets accumulated in the bladder to raise its pressure sufficiently, a spontaneous nervous activity (reflex) is initiated. This causes the smooth muscles on the bladder wall to contract and the urethral sphincter to relax.
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Urine flows to the outside. As the ureters enter obliquely through the bladder wall, their openings get shut due to compression by the contracting bladder muscles and backward flow of urine is prevented.