Nephron Health Dictionary

A substance that increases the flow of urine, either by increasing permeability of the kidneys’ nephrons, increasing blood supply into the nephrons, or increasing the blood into each kidney by renal artery vasodilation.... diuretic

Either of the 2 organs that filter the blood and excrete waste products and excess water as urine. The kidneys are situated at the back of the abdominal cavity, on either side of the spine. Each kidney is surrounded by a fibrous capsule and is made up of an outer cortex and an inner medulla.

The cortex contains specialized capillaries called glomeruli, which, together with a series of tubules, make up the nephrons, the filtering units of the kidney. The nephrons filter blood under pressure and then selectively reabsorb water and certain other substances back into the blood. Urine is formed from substances that are not reabsorbed. The urine is conducted through tubules to the renal pelvis (the central collecting area of the kidney) and then through tubes called ureters to the bladder.

The kidneys also regulate the body’s fluid balance.

To do this, the kidneys excrete excess water, and when water is lost from the body (for example as a result of sweating), they conserve it (see ADH).

In addition, the kidneys control the body’s acid–base balance by adjusting urine acidity.

The kidneys are also involved in hormonal regulation of red blood cell production and blood pressure.... kidney

That part of the nephron (see KIDNEYS) between the proximal and distal convoluted tubules. It extends into the renal medulla as a hairpin-shaped loop. The ascending link of the loop actively transports sodium from the lumen of the tube to the interstitium, and this, combined with the ‘counter-current’ ?ow of ?uid through the two limbs of the loop, plays a part in concentrating the urine.... henle, loop of

6.5 cm wide, 5 cm thick, and weighs around 140 grams.

Adult kidneys have a smooth exterior, enveloped by a tough ?brous coat that is bound to the kidney only by loose ?brous tissue and by a few blood vessels that pass between it and the kidney. The outer margin of the kidney is convex; the inner is concave with a deep depression, known as the hilum, where the vessels enter. The URETER, which conveys URINE to the URINARY BLADDER, is also joined at this point. The ureter is spread out into an expanded, funnel-like end, known as the pelvis, which further divides up into little funnels known as the calyces. A vertical section through a kidney (see diagram) shows two distinct layers: an outer one, about 4 mm thick, known as the cortex; and an inner one, the medulla, lying closer to the hilum. The medulla consists of around a dozen pyramids arranged side by side, with their base on the cortex and their apex projecting into the calyces of the ureter. The apex of each pyramid is studded with tiny holes, which are the openings of the microscopic uriniferous tubes.

In e?ect, each pyramid, taken together with the portion of cortex lying along its base, is an independent mini-kidney. About 20 small tubes are on the surface of each pyramid; these, if traced up into its substance, repeatedly subdivide so as to form bundles of convoluted tubules, known as medullary rays, passing up towards the cortex. One of these may be traced further back, ending, after a tortuous course, in a small rounded body: the Malpighian corpuscle or glomerulus (see diagram). Each glomerulus and its convoluted tubule is known as a nephron, which constitutes the functional unit of the kidney. Each kidney contains around a million nephrons.

After entering the kidney, the renal artery divides into branches, forming arches where the cortex and medulla join. Small vessels come o? these arches and run up through the cortex, giving o? small branches in each direction. These end in a tuft of capillaries, enclosed in Bowman’s capsule, which forms the end of the uriniferous tubules just described; capillaries with capsule constitute a glomerulus.

After circulating in the glomerulus, the blood leaves by a small vein, which again divides into capillaries on the walls of the uriniferous tubules. From these it is ?nally collected into the renal veins and then leaves the kidney. This double circulation (?rst through the glomerulus and then around the tubule) allows a large volume of ?uid to be removed from the blood in the glomerulus, the concentrated blood passing on to the uriniferous tubule for removal of parts of its solid contents. Other arteries come straight from the arches and supply the medulla direct; the blood from these passes through another set of capillaries and ?nally into the renal veins. This circulation is con?ned purely to the kidney, although small connections by both arteries and veins exist which pass through the capsule and, joining the lumbar vessels, communicate directly with the aorta.

Function The kidneys work to separate ?uid and certain solids from the blood. The glomeruli ?lter from the blood the non-protein portion of the plasma – around 150–200 litres in 24 hours, 99 per cent of which is reabsorbed on passing through the convoluted tubules.

Three main groups of substances are classi?ed according to their extent of uptake by the tubules:

(1) SUBSTANCES ACTIVELY REABSORBED These include amino acids, glucose, sodium, potassium, calcium, magnesium and chlorine (for more information, see under separate entries).

(2) SUBSTANCES DIFFUSING THROUGH THE TUBULAR EPITHELIUM when their concentration in the ?ltrate exceeds that in the PLASMA, such as UREA, URIC ACID and phosphates.

(3) SUBSTANCES NOT RETURNED TO THE BLOOD from the tubular ?uid, such as CREATINE, accumulate in kidney failure, resulting in general ‘poisoning’ known as URAEMIA.... structure each kidney is about 10 cm long,

A small tube. There are several named tubules in the body: examples include convoluted tubules in the NEPHRON of the kidney (see KIDNEYS) and the seminiferous tubules in the testes (see TESTICLE).... tubule

the cup-shaped end of a *nephron, which encloses a knot of blood capillaries (glomerulus). It is the site of primary filtration of the blood into the kidney tubule. [Sir W. P. Bowman (1816–92), British physician]... bowman’s capsule

(GFR) the rate at which substances are filtered from the blood of the glomeruli into the Bowman’s capsules of the *nephrons. It is calculated by measuring the *clearance of specific substances (e.g. creatinine) and is an index of renal function. See eGFR.... glomerular filtration rate

n. (pl. glomeruli) 1. the network of blood capillaries contained within the cuplike end (Bowman’s capsule) of a *nephron. It is the site of primary filtration of waste products from the blood into the kidney tubule. 2. any other small rounded mass.... glomerulus

the part of a *nephron comprising the blood capillaries of the glomerulus and its surrounding Bowman’s capsule. [M. Malpighi (1628–94), Italian anatomist]... malpighian body

(uriniferous tubule) the fine tubular part of a *nephron, through which water and certain dissolved substances are reabsorbed back into the blood.... renal tubule

Inflammation of the glomeruli (see glomerulus), affecting both kidneys. Damage to the glomeruli hampers the removal of waste products, salt, and water from the bloodstream, which may cause serious complications.

Some types of glomerulonephritis are caused by immune complexes (components of the immune system produced in response to infection) becoming trapped in the glomeruli. The condition occurs in some autoimmune disorders. Infectious diseases such as malaria and schistosomiasis are important causes of glomerulonephritis in tropical countries.

Mild glomerulonephritis may produce no symptoms. Some sufferers experience a dull ache over the kidneys. The urine may become bloodstained. Loss of protein into the urine may cause oedema (see nephrotic syndrome). Hypertension is a potentially serious complication. Long-term glomerulonephritis is a common cause of chronic kidney failure.

Diagnosis involves kidney function tests, urinalysis, and kidney biopsy. Treatment depends on the cause and severity of the disease. Children with nephrotic syndrome usually respond to corticosteroid drugs. In adults, kidney failure can sometimes be prevented or delayed by drug treatment and dietary control to reduce the work of the kidneys.glomerulosclerosis Scarring caused by damage to the glomeruli (see glomerulus). Mild glomerulosclerosis occurs normally with age. Glomerulosclerosis may occur in some severe types of glomerulonephritis. It is also sometimes associated with diabetes mellitus, hypertension, AIDS, or intravenous drug abuse. glomerulus A filtering unit of the kidney that consists of a cluster of capillaries enclosed in a capsule and supplied with blood from the renal artery. Each glomerulus is a part of a larger filtering unit called a nephron. Filtered blood eventually leaves the kidney via the renal vein.

(See also glomerulonephritis.)... glomerulonephritis

(JGA) a microscopic structure within the kidney that is important in regulating blood pressure, body fluid, and electrolytes. It is situated in each nephron, between the afferent arteriole of the glomerulus and the returning distal convoluted tubule of the same nephron. The JGA consists of specialized cells within the distal tubule (the macula densa), which detect the amount of sodium chloride passing through the tubule and can secrete locally acting vasoconstrictor substances that act on the associated afferent arteriole to induce a reduction in filtration pressure (tubuloglomerular feedback). Modified cells within the afferent arterioles secrete *renin in response to a fall in perfusion pressure or feedback from the macula densa and form a central role in the renin-*angiotensin-aldosterone axis. Mesangial cells support and connect the macula densa and the specialized cells in the afferent arteriole and have sympathetic innervation, facilitating the renin response to sympathetic nervous stimulation.... juxtaglomerular apparatus

a male accessory sex gland that opens into the urethra just below the bladder and vas deferens (see illustration). The prostate is divided into different anatomical regions called McNeal’s zones (transition, central, peripheral, and anterior fibromuscular zones). During ejaculation it secretes an alkaline fluid that forms part of the *semen. The prostate may become enlarged in elderly men (benign prostatic hyperplasia; BPH). This may result in obstruction of the neck of the bladder, impairing urination. The bladder dilates and the increased pressure is transmitted through the ureters to the kidney nephrons, leading to damage and impaired function of the kidneys. Treatment is by transurethral *resection of the prostate (see also prostatectomy) or by means of drugs (e.g. *finasteride, alpha blockers).... prostate gland

(RTA) metabolic acidosis due to failure of the kidney to excrete acid into the urine. Three types of RTA are recognized. Type 1 (distal RTA) results from a reduction in net acid secretion in the distal convoluted tubule (see nephron) and an inability to acidify the urine. Hypokalaemia is often present and may be severe. The condition can be either genetically determined or, more commonly, the result of systemic disease (e.g. autoimmune disorders) or drugs (e.g. amphotericin). Type II (proximal RTA) is due to a lowered threshold for bicarbonate reabsorption; eventually a steady state is established with a low serum bicarbonate but capacity to acidify the urine. Hypokalaemia is present due to *aldosteronism caused by the increased delivery of sodium to the distal tubule. Proximal RTA usually occurs as part of more widespread proximal tubule dysfunction with the *Fanconi syndrome. Type IV RTA results from impaired excretion of both acid and potassium and results in acidosis with hyperkalaemia. It is most commonly seen with aldosterone deficiency. This may be isolated, especially in diabetics, or it may be induced by drugs (angiotensin II antagonists or ACE inhibitors).... renal tubular acidosis