Kidney!
The kidneys are paired organs of the excretory system that have many homeostatic functions, including regulation of electrolytes, acid-base balance, and blood pressure; excretion of wastes such as urea and ammonium; reabsorption of glucose and amino acids; and production of hormones including vitamin D and erythropoietin.
Located behind the abdominal cavity in the retroperitoneum, the kidneys receive blood from the paired renal arteries, and drain into the paired renal veins. Each kidney excretes its waste product—urine—into a ureter, itself a paired structure that empties into the urinary bladder.
The medical field that studies the kidneys and diseases of the kidney is called nephrology.[1] The prefix nephro- meaning kidney is from the Ancient Greek word nephros (νεφρός); the adjective renal meaning related to the kidney is from Latin rēnēs, meaning kidneys.[2]
Anatomy
Location
In humans, the kidneys are bean-shaped organs located behind the abdominal cavity, in a space called the retroperitoneum. There are two, one on each side of the spine; they are approximately at the vertebral level T12 to L3.[3] 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 typically results in the right kidney being slightly lower than the left, and left kidney being located slightly more medial than the right.[citation needed] The upper (cranial) 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. Each adult kidney weighs between 125 and 170 g in males and between 115 and 155 g in females.[3] The left kidney is typically slightly larger than the right.[citation needed]
Blood supply
The kidneys receive blood from the renal arteries, left and right, which branch directly from the abdominal aorta. Despite their relatively small size, the kidneys receive approximately 20% of the cardiac output.[3]
Each renal artery branches into segmental arteries, dividing further into interlobar arteries which penetrate the renal capsule and extend 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 supplies several interlobular arteries that feed into the afferent arterioles that supply the glomeruli.
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.
Functions
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]
The majority of the kidney's basic functions are accomplished by relatively simple mechanisms of secretion and reabsorption, both of which take place in the nephron. Secretion is the process by which molecules are transported from the blood into the urine. Reabsorption is the reverse process, in which molecules are transported in the opposite direction, from the urine into the blood.
Excretion of wastes
The kidneys excrete a variety of waste products produced by metabolism. These include the nitrogenous wastes urea (from protein catabolism) and uric acid (from nucleic acid metabolism), and water.
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. Renal production of bicarbonate (HCO3–) ions buffer pH by reducing hydrogen ion concentrations in plasma; the bicarbonate serves as a proton acceptor.
Plasma volume maintenance
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 (ADH), 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.
Blood pressure regulation
Sodium ions are controlled in a homeostatic process involving aldosterone which increases sodium ion reabsorption in the distal convoluted tubules.
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. Calcitriol, the activated form of vitamin D, promotes the absorption of Ca2+ from the gut and the excretion of PO32-. They both help to increase Ca2+ levels[clarification needed]. The kidneys also secrete Renin, an enzyme involved in the regulation of aldosterone secretion by the renin-angiotensin-aldosterone system.
Embryology
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
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
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.
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