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Human Excretory System

The organs of excretion of nitrogenous waste products like urea are a pair of kidneys in mammals. The excretory system of mammals includes
  1. a pair of kidneys,
  2. a pair of ureters,
  3. a urinary bladder, and urethra.

Human Excretory System

These are pair of dark-red bean-shaped organs. They are situated one on either side of the median line in the anterior part of the abdomen. They are attached to the dorsal body wall. Ventrally they are covered with a fold of the peritoneum. The right kidney lies a little anterior to the left one. A renal artery supplies blood to each kidney and a renal vein collects blood from it. Each kidney is convex on the outer side, and concave on the inner side. On the concave side is a notch called the hilus. From the hilus arises a narrow urinary duct called the ureter.

Internal structure
Each kidney is enclosed in a thin, outer capsule of connective tissue. Inner to this, the kidney is differentiated into an outer cortex and an inner medulla. In a longitudinal section, the cortex shows a dotted appearance and the medulla shows a striated appearance.

The kidney is composed of a number of uriniferous tubules or nephrons which are interwoven with blood capillaries and are held together by connective tissue. Each nephron consists of two parts (i) malpighian body, and (ii) renal tubule.

Longitudinal Section of Mammalian Kidney

A Mammalian Nephron

The Malpighian body is the swollen, invaginated end of the uriniferous tubule. It lies in the cortex of the kidney. The Malpighian body consists of the Bowman's capsule and a glomerulus. The Bowman's capsule is in the form of a double walled cup. It is formed by the invagination of the blind end of the tubule. Its walls are lined by flattened epithelial cells.
Inside the kidney the renal artery branches into a number of renal arterioles. A branch from a renal arteriole enters the Bowman's capsule, and is called the efferent vessel. It breaks up into a network of capillaries which reunite to form an efferent vessel. The network of capillaries in the Bowman's capsule form a knot-like structure called the glomerulus. The efferent vessel comes out of the Bowman's capsule.

The remaining part of the nephron is behind the Malpighian body and is called the renal tubule. The renal tubule is differentiated into three regions namely the proximal convoluted tubule, which makes several coils in the cortex. It is lined by thin, cubical, glandular epithelial cells having a ciliated border. The proximal convoluted tubule narrows and moves towards the medulla as a descending limb. It then curves back and runs forward towards the cortex as an ascending limp. This loop formed by the ascending and descending limb is called the loop of Henle. The ascending limb is lined by cuboidal epithelium and the descending limb is lined by the flattened epithelia cells. The ascending limb is continued into the distal convoluted tubule which forms several coils in the cortex. The distal convoluted tubule is lined by cuboidal epithelium. It joins the collecting tubule.

The collecting tubules lie in the medulla. Each collecting tubule receives many nephrons. Several such tubules unite to form a large collecting duct. The collecting ducts are held together and converge to form a pyramid. The pyramid opens into the pelvis which leads into the ureter.

From the hilus of each kidney arises a narrow, thin, threadlike tube called the ureter. The ureters of both sides run back towards the posterior and open into the dorsal wall of the urinary bladder.

Urinary Bladder
It is a thin walled, transparent muscular sac with a distensible, wall and is ventral to the rectum. The narrow posterior part of the urinary bladder is the neck of the urinary bladder. It is provided with a sphincter muscle at its mouth.

The neck of the urinary bladder unites with a tube known as urethra which opens outside. In the male, urethra unites with the genital duct and opens outside as a common duct, through the penis. In the female, the urinary and genital ducts open outside separately.

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