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Anatomy and Physiology of Kidneys

The urinary tract is composed of the following four structures

  • Kidneys
  • Ureters
  • Bladder
  • Urethra

The kidneys balance the urinary excretion of substances against the accumulation within the body through ingestion or production. Consequently, they are a major controller of fluid and electrolyte homeostasis. The kidneys also have several non-excretory metabolic and endocrine functions, including blood pressure regulation, erythropoietin production, insulin degradation, prostaglandin synthesis, calcium and phosphorus regulation, and vitamin D metabolism.

Filtration at the renal glomerulus is the first step in urine formation. Noramlly, a volume equal to plasma volume is filtered every 45 minutes, and a volume equal to total body water is filtered every 6 hours.

Glomerular filtrate is similar to plasma but lacks cell and large-malecular-weight proteins. The glomerular filtrate is modified by active transport, diffusion and osmosis as it passes through the renal tubules.

Reabsorption of filtrate components, enhances conservation of glucose, peptides, electrolytes and water.

Secretion of plasma components enhances elimination of organic acids, and bases (and some drugs). The remnants of the glomerular filtrate exit the kidney through the Ureters.

The Ureters conduct urine from the kidneys to the bladder by the peristaltic contraction.

The Bladder is a distensible chamber that stores urine until it is eliminated.

The Urethra is the exit passageway from the bladder and it carries urine for elimination from the body.

Kidneys

The kidneys are a pair of bean-shaped organs, which is found along the posterior wall of the abdominal cavity. The left kidney is located slightly higher than the right kidney because the right side of the liver is situated  The kidneys are located in the abdominal cavity, posterior to the peritoneum on each side of the vertebral column, at about the level of the twelfth rib and touch the muscles of the back. Adult kidneys average 11ch in length, 5 to 7.5 cm in width and 2.5 cm in thickness.

Affixing the kidneys in position behind the parietal peritoneum is a mass of perirenal fat (Adipose capsul) and connective tissue called Gerota’s (subserosa) fascia. A fibrous capsule,(renal capsule) forms the external covering of the kidney itself, except the hilum. The kidney is further protected by layers of muscle of the back, flank and abdomen as well as by layers of fat, subcutaneous tissue and skin.

Each kidney is divided into three major areas…

  • Cortex
  • Medulla
  • Pelvis

Cortex

The cortex of the kidney lies just under the fibrous capsule and portions of it extend down into the medullary layer to form the renal columns (columns of Bertin) or cortical tissue that separates the pyramids.

Medulla

The medulla is divided into 8 to 18 cone-shaped masses of collecting ducts called renal pyramids. The bases of the pyramids are positioned on the corticomedullary boundary. Their apices extend toward the renal pelvis, forming papillae, Each papilla has 10 to 25 openings on the surface, through which the urine empties into the renal pelvis. Eight or more groups of papillae are present in each pyramid; each empties into a minor calyx and several minor calices join to form a major calyx, The two to three major calices are outpouchings of the renal pelvis. They channel the urine from the pyramids to the renal pelvis.

Pelvis

The renal pelvis cavity is lined with transitional epithelium. The combined volume of the pelvis and calices is about 8 ml. Volumes in excess of this amount damage the renal parenchymal tissue. The renal pelvis narrows as it reaches the hilus and becomes the proximal end of the ureter.

Within the cortex lies the nephron, the functional unit of the kidney, which consists of both vascular and tubular elements. each kidney contains about 1.3 million of them

The nephron is the unit of the kidney responsible for ultrafiltration of the blood and reabsorption or excretion of products in the subsequent filtrate. Each nephron is made up of…

A filtering unit

  • The glomurulus 125ml/min of filtrate is formed by the kidneys as blood is filtered through this sieve-like structure. This filtration is uncontrolled.
  • The Proximal Convoluted Tubule Controlled absorption of glucose, sodium, and other solutes goes on in this region.
  • The Loop of Henle, this region is responsible for concentration and dilution of urine by utilising a counter-current multiplying mechanism- basically,water is impermeable but can pump sodium out, which in turn affects the osmolarity of the surrounding tissues and will affect the subsequent movement of water in or out of the water-permeable collecting duct.

 The distal convoluted tubule, This region is responsible, along with the collecting duct that it joins, for absorbing water back into the body- simple maths is that the kidney doesn’t produce 125ml of urine every minute. 99% of the water is normally reabsorbed, leaving highly concentrated urine to flow into the collecting duct

Each nephron consists ofglomerulus and renl tubule. Each nephron is capable of forming urine. Urine is formed by three basic processes…

  • Filtration
  • Reabsorption
  • Secretion

Filtration

Glomerulus is a tiny flter. It contains tuft of glomerular capillaries. This capillary tuft is present in a double layered Bowman’s capsule. Capillaries are thin walled and pressure in them is high as compared to capillaries present at other sites in the body. This allows fluid and water products to be filtered into the Bowman’s capsule. Bowman’s capsule continues as renal tubule. So glomerular filtration is 125 ml/minute (1.80 liters/day) The amount of urine formed is approximately 1 to 1.5 mi/minute this meansthat large portion of filtrate is absorbed as it passes through the tubule of the nephron.

Reabsorption

It is the important function of the tubule. It prevents loss of fluid, salts and other important substances such as glucose, amino acids, protein. Waste products are not absorbed therefore they are allowed to retain in the tubule for excretion.

Secretion

Certain substances are added to the filtrate while it is passing through the tubule so these substances are excreted.

The blood supply to the kidney directly through from the aorta via the renal arteries; Renal veins take blood away from the kidneys into the inferior vena cava.