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Anatomy

In humans, the kidneys are located in the posterior part of the abdominal cavity. There are two, one on each side of the spine; the right kidney sits just below the diaphragm and posterior to the liver, the left below the diaphragm and posterior to the spleen. Above each kidney is an adrenal gland (also called the suprarenal gland). The asymmetry within the abdominal cavity caused by the liver results in the right kidney being slightly lower than the left one while the left kidney is located slightly more medial. The bulk of water re-absorption in the vertebrate kidney takes place in the loop of Henle.

The kidneys are retroperitoneal and range from 9 to 13 cm in diameter; the left slightly larger than the right. They are approximately at the vertebral level T12 to L3. The upper parts of the kidneys are partially protected by the eleventh and twelfth ribs, and each whole kidney and adrenal gland are surrounded by two layers of fat (the perirenal and pararenal fat) and the renal fascia which help to cushion it. Congenital absence of one or both kidneys, known as unilateral (on one side) or bilateral (on both the sides) renal agenesis, can occur. Renal agenesis is also the base for the renal anal gland which helps the large intestine absorb water.

The kidneys receive unfiltered blood directly from the heart through the abdominal aorta which then branches to the left and right renal arteries. Filtered blood then returns by the left and right renal veins to the inferior vena cava and then the heart. Renal blood flow accounts for 20-25% of the cardiac output.[3]

Functions
Main article: Renal physiology

Blood Filtering
The renal corpuscle is the site of the nephron, where blood is "filtered".

The blood enters the kidney through the renal artery in the renal sinus. It branches into segmental arteries, which further divide into interlobar arteries penetrating the renal capsule and extending through the renal columns between the renal pyramids. The interlobar arteries then supply blood to the arcuate arteries that run through the boundary of the cortex and the medulla. Each arcuate artery supply a variety of additional interlobar arteries that feed into the afferent arterioles to be filtered through the nephrons. After filtration occurs the blood moves through a small network of venules that converge into interlobar veins. As with the arteriole distribution the veins follow the same pattern, the interlobar provide blood to the arcuate veins then back to the interlobar veins which come to form the renal vein exiting the kidney for transfusion for blood.

Blood filtering takes place in the (nephron), which is found in the kidney.

[edit] Excretion of waste products

The kidneys excrete a variety of waste products produced by metabolism, including the nitrogenous wastes: urea (from protein catabolism) and uric acid (from nucleic acid metabolism) and water.

Homeostasis

The kidney is one of the major organs involved in whole-body homeostasis. Among its homeostatic functions are acid-base balance, regulation of electrolyte concentrations, control of blood volume, and regulation of blood pressure. The kidneys accomplish these homeostatic functions independently and through coordination with other organs, particularly those of the endocrine system. The kidney communicates with these organs through hormones secreted into the bloodstream. .[4]

Acid-base balance

The kidneys regulate the pH of blood by adjusting H+ ion levels, referred as augmentation of mineral ion concentration, as well as water composition of the blood.
Blood pressure
Main article: Renin-angiotensin system

Sodium ions are controlled in a homeostatic process involving aldosterone which increases sodium ion reabsorption in the distal convoluted tubules.

Plasma volume

Any significant rise or drop in plasma osmolality is detected by the hypothalamus, which communicates directly with the posterior pituitary gland. A rise in osmolality causes the gland to secrete antidiuretic hormone, resulting in water reabsorption by the kidney and an increase in urine concentration. The two factors work together to return the plasma osmolality to its normal levels.

ADH binds to principal cells in the collecting duct that translocate aquaporins to the membrane allowing water to leave the normally impermeable membrane and be reabsorbed into the body by the vasa recta, thus increasing the plasma volume of the body.

There are two systems that create a hyperosmotic medulla and thus increase the body plasma volume: Urea recycling and the 'single effect.'

Urea is usually excreted as a waste product from the kidneys. However, when plasma blood volume is low and ADH is released the aquaporins that are opened are also permeable to urea. This allows urea to leave the collecting duct into the medulla creating a hyperosmotic solution that 'attracts' water. Urea can then re-enter the nephron and be excreted or recycled again depending on whether ADH is still present or not.

The 'Single effect' describes the fact that the ascending thick limb of the loop of Henle is not permeable to water but is permeable to NaCl. This means that a countercurrent system is created whereby the medulla becomes increasingly concentrated setting up a osmotic gradient for water to follow should the aquaporins of the collecting duct be opened by ADH.

Hormone secretion

The kidneys secrete a variety of hormones. Erythropoietin is released in response to low levels of O2 in the renal circulation. It stimulates erythrocyte production in red bone marrow. Renin is involved in the regulation of aldosterone secretion by the renin-angiotensin-aldosterone system. Calcitriol, the activated form of vitamin D, promotes the absorption of Ca2+ from the blood and the excretion of PO32-. They both help to increase Ca2+ levels[clarification needed].

Embryology
Main article: Development of the urinary and reproductive organs

The mammalian kidney develops from intermediate mesoderm. Kidney development, also called nephrogenesis, proceeds through a series of three successive phases, each marked by the development of a more advanced pair of kidneys: the pronephros, mesonephros, and metanephros.[5] (The plural forms of these terms end in -oi.)
Pronephros
Main article: Pronephros

During approximately day 22 of human gestation, the paired pronephroi appear towards the cranial end of the intermediate mesoderm. In this region, epithelial cells arrange themselves in a series of tubules called nephrotomes and join laterally with the pronephric duct, which does not reach the outside of the embryo. Thus the pronephros is considered nonfunctional in mammals because it cannot excrete waste from the embryo.

Mesonephros
Main article: Mesonephros

Each pronephric duct grows towards the tail of the embryo, and in doing so induces intermediate mesoderm in the thoracolumbar area to become epithelial tubules called mesonephric tubules. Each mesonephric tubule receives a blood supply from a branch of the aorta, ending in a capillary tuft analogous to the glomerulus of the definitive nephron. The mesonephric tubule forms a capsule around the capillary tuft, allowing for filtration of blood. This filtrate flows through the mesonephric tubule and is drained into the continuation of the pronephric duct, now called the mesonephric duct or Wolffian duct. The nephrotomes of the pronephros degenerate while the mesonephric duct extends towards the most caudal end of the embryo, ultimately attaching to the cloaca. The mammalian mesonephros is similar to the kidneys of aquatic amphibians and fishes.

Metanephros

During the fifth week of gestation, the mesonephric duct develops an outpouching, the ureteric bud, near its attachment to the cloaca. This bud, also called the metanephrogenic diverticulum, grows posteriorly and towards the head of the embryo. The elongated stalk of the ureteric bud, the metanephric duct, later forms the ureter. As the cranial end of the bud extends into the intermediate mesoderm, it undergoes a series of branchings to form the collecting duct system of the kidney. It also forms the major and minor calyces and the renal pelvis.

The portion of undifferentiated intermediate mesoderm in contact with the tips of the branching ureteric bud is known as the metanephrogenic blastema. Signals released from the ureteric bud induce the differentiation of the metanephrogenic blastema into the renal tubules. As the renal tubules grow, they come into contact and join with connecting tubules of the collecting duct system, forming a continuous passage for flow from the renal tubule to the collecting duct. Simultaneously, precursors of vascular endothelial cells begin to take their position at the tips of the renal tubules. These cells differentiate into the cells of the definitive glomerulus.
Terms
Microscopic photograph of the renal cortex.
Microscopic photograph of the renal medulla.

* renal capsule: The membranous covering of the kidney.
* cortex: The outer layer over the internal medulla. It contains blood vessels, glomeruli (which are the kidneys' "filters") and urine tubes and is supported by a fibrous matrix.
* hilum: The opening in the middle of the concave medial border for nerves and blood vessels to pass into the renal sinus.
* renal column: The structures which support the cortex. They consist of lines of blood vessels and urinary tubes and a fibrous material.
* renal sinus: The cavity which houses the renal pyramids.
* calyces: The recesses in the internal medulla which hold the pyramids. They are used to subdivide the sections of the kidney. (singular - calyx)
* papillae: The small conical projections along the wall of the renal sinus. They have openings through which urine passes into the calyces. (singular - papilla)
* renal pyramids: The conical segments within the internal medulla. They contain the secreting apparatus and tubules and are also called malpighian pyramids.
* renal artery: Two renal arteries come from the aorta, each connecting to a kidney. The artery divides into five branches, each of which leads to a ball of capillaries. The arteries supply (unfiltered) blood to the kidneys. The left kidney receives about 60% of the renal bloodflow.
* renal vein: The filtered blood returns to circulation through the renal veins which join into the inferior vena cava.
* renal pelvis: Basically just a funnel, the renal pelvis accepts the urine and channels it out of the hilus into the ureter.
* ureter: A narrow tube 40 cm long and 4 mm in diameter. Passing from the renal pelvis out of the hilus and down to the bladder. The ureter carries urine from the kidneys to the bladder by means of peristalsis.
* renal lobe: Each pyramid together with the associated overlying cortex forms a renal lobe