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 arteriole 2: glomerulus 3: glomerular capsule 4: afferent arteriole 5: branch of renal artery 6: capillary 7: branch of renal vein 8: ascending limb of loop of Henle 9: loop of Henle 10: collecting duct 11: descending limb of loop of Henle 12: distal convoluted tubule 13: proximal convoluted tubule

The mammalian nephron

Nephron function There are three main processes involved in the formation of urine Filtration 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 include Renal 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 contact Rest 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.