The kidney is the most important organ of excretion in vertebrates. It is here that the body is able to selectively release toxins and other substances that exist in overabundance. At the cellular level, the functional component of the kidney is the nephron. These cells use a complex set of physical and chemical processes to selectively releases substances from the body.

The first stage of kidney function occurs at the Bowman’s capsule. Here, blood in the glomerulus, a capillary bed, is forced by pressure filtration into said capsule. The blood in the glomerulus comes from the afferent arteriole, which in turn connects to the renal artery. The kidneys process 180 litres of blood per day, making them both vital and busy. This blood leaves the glomerulus through the efferent arteriole. For the remainder of the length of the nephron, it is surrounded by the peritubular capillaries. Material is exchanged between them and the nephron through the processes of active transport, diffusion, and osmosis – which is a particular kind of diffusion involving only water.

Pressure filtration basically draws from the blood everything except the blood cells, red and white, and the blood proteins, like albumin. Thus, water, amino acids, glucose, vitamins, nitrogenous wastes, proteins, and other small molecules are introduced into the nephron.

The majority of these substances are subsequently returned to the blood at the proximal convoluted tubule. Here, the process of selective re-absorption occurs and all molecules desired by the body are pumped into the peritubular capillaries by active transport. Examples of molecules that the body re-absorbs are glucose and vitamins.

From here, the nephon contents enter the Loop of Henle. The most important function of this segment of the nephron is the removal of excess water. This is accomplished by a mechanism called the countercurrent effect. Because water cannot be moved by active transport, the nephron moves salts out of itself and into the area around it. This decreases the concentration of water outside the nephron and therefore draws water out, into the renal cortex, through osmosis.

Having had much of its water content removed, the contents of the nephron enter the distal convoluted tubule. Here, active transport works to move unwanted substances out of the peritubular capillaries and into the nephron. Excess salts and nitrogenous wastes, among a great many other things, enter the nephron thus.

Finally, the contents of the nephron enter the collecting duct. This conveys them to the renal pelvis where, by way of the ureter, they enter the urinary bladder. When the sphincter here relaxes, the urine flows out the urethra and exits the body.

It should be noted that the countercurrent effect also occurs in the collecting duct. Here, the hormone ADH (antidiuretic hormone) controls the active transport of urea into the area surrounding the collecting duct, as well as increasing its permeability to water. This allows osmosis to remove an additional quantity of water from the contents of the nephron, which can now be called urine.

Kid"ney (?), n.; pl. Kidneys (#). [OE. kidnei, kidnere, from Icel. koir belly, womb (akin to Goth. gipus, AS. cwip womb) + OE. nere kidney; akin to D. nier, G. niere, OHG. nioro, Icel. nra, Dan. nyre, Sw. njure, and probably to Gr. () Cf. Kite belly.]

1. Anat.

A glandular organ which excretes urea and other waste products from the animal body; a urinary gland.

⇒ In man and in other mammals there are two kidneys, one each side of vertebral column in the back part of the abdomen, each kidney being connected with the bladder by a long tube, the ureter, through which the urine is constantly excreted into the bladder to be periodically discharged.

2.

Habit; disposition; sort; kind.

Shak.

There are in later other decrees, made by popes of another kidney. Barrow.

Millions in the world of this man's kidney. L'Estrange.

Your poets, spendthrifts, and other fools of that kidney, pretend, forsooth, to crack their jokes on prudence. Burns.

⇒ This use of the word perhaps arose from the fact that the kidneys and the fat about them are an easy test of the condition of an animal as to fatness. "Think of that, -- a man of my kidney; -- . . . as subject to heat as butter."

Shak.

3.

A waiter

. [Old Cant]

Tatler.

Floating kidney. See Wandering kidney, under Wandering. -- Kidney bean Bot., a sort of bean; -- so named from its shape. It is of the genus Phaseolus (P. vulgaris). See under Bean. -- Kidney ore Min., a variety of hematite or iron sesquioxide, occurring in compact kidney-shaped masses. -- Kidney stone. Min. See Nephrite, and Jade. -- Kidney vetch Bot., a leguminous herb of Europe and Asia (Anthyllis vulneraria), with cloverlike heads of red or yellow flowers, once used as a remedy for renal disorders, and also to stop the flow of blood from wounds; lady's-fingers.

 

© Webster 1913.

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