The nephron and kidney function

HBS3

The nephron

The nephron - structure

The base unit of the kidneys is the nephron. The main parts of the nephron are• Glomerular capsule• Proximal convoluted tubule• Distal convoluted tubule• Loop of Henle• Collecting tubule (or duct)

The blood vessels associated with the nephron are the• Renal artery and vein• Afferent arteriole• Efferent arteriole• Glomerulus• Capillary network

1: efferent arteriole2: glomerulus3: glomerular capsule4: afferent arteriole5: branch of renal artery6: capillary7: branch of renal vein8: ascending limb of loop of Henle9: loop of Henle10: collecting duct11: descending limb of loop of Henle12: distal convoluted tubule13: proximal convoluted tubule

The mammalian nephron

Nephron functionThere are three main processes involved in the formation of urineFiltration is separation of particles by passing them through small pores (sieving) It occurs in the renal corpuscle between the glomerulus and the glomerular capsule. Some substances filtered include anything small & soluble – water, nutrients (eg glucose, amino

acids, vitamins), ions, wastes (eg urea), drugs, hormones The resultant fluid is called filtrate

Reabsorption is the return of substances from the nephron back into the blood It occurs in the proximal convoluted tubule, loop of Henle & distal convoluted tubule - along the

length of the nephron

Some substances reabsorbed include urea, some ions, water glucose, amino acids, vitamins Water is reabsorbed either by facultative reabsorption, which is an active process under the

control of the hormone ADH, in which the amount of reabsorption can be controlled, or obligatory reabsorption, which is a passive process due to osmotic pressure (not controlled).

Secretion is removal of substances from the blood into the nephron It occurs in the distal convoluted tubule Some substances secreted include urea, creatinine, some ions, drugs

How the nephron works

Nephron adaptations

Some of the ways the nephron is related to its function includeRenal capsule:• High pressure differential between glomerulus & capsule due to short renal artery,

and afferent arteriole being wider than efferent arteriole• Large surface in the glomerulus due to large number of coils in glomerulus and

many nephrons• Extremely permeable (leaky) capillaries• Extremely thin membranes lining capillaries & capsule so very close contactRest of nephron• Large exchange area due to length of nephron and number of capillaries• favourable concentration gradients due to active transport of some substances

such as glucose and amino acids, and due to the flow of blood in capillaries being in the opposite direction to the flow of filtrate in the nephron (counter current).

Urine formation

The amount of urine produced each day is about 1 - 2 L. Of this, about 96% is water.

Urine normally contains no glucose and very little protein.

The rest of the urine is mainly made up of ions and nitrogenous wastes (eg urea, uric acid and creatinine).

Nitrogenous wastes are products from the breakdown of proteins or amino acids. The main one is urea, which is made in the liver from amino acids by the process of deamination.