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Good day everyone, Today we are going to discuss fully on kidneys;

Learning outcome

After studying this section, you should be able to:

  • identify the organs associated with the kidneys
  • outline the gross structure of the kidneys
  • describe the structure of a nephron
  • explain the processes involved in the formation of urine
  • explain how body water and electro balance is maintained.

The kidneys lie on the posterior abdominal wall, one on each side of the vertebral column, behind the peritoneum and below the diaphragm. They extend from the level of the 12th thoracic vertebra to the 3rd lumbar vertebra, receiving some protection from the lower rib cage. The right kidney is usually slightly lower than the left, probably because of the considerable space occupied by the liver. Kidneys are bean-shaped organs, about 11 cm long 6 cm wide, 3 cm thick and weigh 150 g. They are embedded in, and held in position by, a mass of fat. A sheath of fibrous connective tissue, the renal fascia, encloses the kidney and the renal fat.

A diagram of the kidneys
The kidney

Organs associated with the kidneys

As the kidneys lie on either side of the vertebral column, each is associated with different structures.

Right kidney

Superiorly Anteriorly – the right adrenal gland

Anteriorly – the right lobe of the liver, the duodenum and the hepatic flexure of the colon

Posteriorly – the diaphragm, and muscles of the posterior abdominal wall.

Left kidney

Superiorly – the left adrenal gland

Anteriorly – the spleen, stomach, pancreas, jejunum and splenic flexure of the colon

Posteriorly – the diaphragm and muscles of the posterior abdominal wall.

Also read: Tissues

Gross structure of the kidney

There are three areas of tissue that can be distinguished when a longitudinal section of the kidney is viewed with the naked eye:

  • an outer fibrous capsule, surrounding the kidney
  • the cortex, a reddish-brown layer of tissue immediately below the capsule and outside the renal pyramids
  • the medulla, the innermost layer, consisting of pale conical-shaped striations, the renal pyramids.

The hilum is the concave medial border of the kidney where the renal blood and lymph vessels, the ureter and nerves enter.

Urine formed within the kidney passes through a renal papilla at the apex of a pyramid into a minor calyx several minor calyces merge into a major calyx and two or three major calyces combine forming the renal pis, a funnel shaped structure that narrows when it leaves the kidney as the ureter. The walls of the calyces and renal pelvis are lined with transitional epithelium and contain smooth muscle. Peristalsis, intrinsic contraction of smooth muscle, propels urine through the calyces, nal pelvis and ureters to the bladder.

Microscopic structure of the kidney

The kidney contains about 1-2 million functional units, the nephrons, and a much smaller number of collecting ducts. The collecting ducts transport urine through the pyramids to the calyces, giving the pyramids their striped appearance. The collecting ducts are supported by connective tissue, containing blood vessels, nerves and lymph vessels.

The nephron

The nephron is essentially a tubule closed at one end that joins a collecting duct at the other end. The closed or blind end is indented to form the cup-shaped glomerular capsule (Howman’s capsule), which almost completely encloses a network of tiny arterial capillaries, the glomerulus. Continuing from the glomerular capsule, the remainder of the nephron is about 3 cm long and described in three parts:

  • the proximal convoluted tubule
  • the medullary loop (loop of Henle)
  • the distal convoluted tubule, leading into a collecting duct.

The collecting ducts unite, forming larger ducts that empty into the minor calyces.

Also read: the pulse

The kidneys receive about 20% of the cardiac output. Alter entering the kidney at the hilum, the renal artery divices into smaller arteries and arterioles. In the cortex atriole, the afferent arteriole, enters each glomerular capsule and then subdivides into a cluster of tiny arterial Capillaries, forming the glomerulus. Between these capillary loops are connective tissue phagocytic mesangial ee which are part of the monocyte-macrophage defence system. The blood vessel leading away from the glomerulus is the efferent arteriole. The afferent arteriole has a larger diameter than the efferent arteriole, which Increases pressure inside the glomerulus and drives filtra tion across the glomerular capillary walls. The efferent arteriole divides into a second peritubular ning around tubules’) capillary network, which waps around the remainder of the tubule, allowing exchange between the fluid in the tubule and the blood stream.

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