Structure Outer part of each kidney – CORTEX Inner part of each kidney – MEDULLA Blood enters the kidney through the RENAL ARTERY and leaves through the RENAL VEIN The capillaries form knots called a GLOMERULUS This is surrounded by a RENAL CAPSULE The RENAL CAPSULE leads into a NEPHRON TUBULE The NEPHRON TUBLULES are divided into four parts: the PRIMAL CONVOLUTED TUBE (nearest the renal capsule), the DISTAL CONVOLUTED TUBE (furthest from the renal capsule), the LOOP OF HENLE and the COLLECTING DUCT

THE CAPILLARY NETWORK AND NEPHRON TUBULE

What happens in the kidney? 1.The RENAL ARTERY splits into arterioles 2.Each arteriole forms a GLOMERULUS 3.ULTRAFILTRATION takes place here, where substances are forced out of the arteriole, into the RENAL CAPSULE 4.This filtrate enters the NEPHRON TUBULE 5.Useful substances are REABSORBED into the blood by capillaries surrounding the NEPHRON TUBULE 6.Eventually URINE is formed and this flows into the COLLECTING DUCT and into the URETER.

ULTRAFILTRATION The blood in the glomerulus is under high pressure, because the AFFERENT arteriole is wider than the EFFERENT arteriole This forces fluid in the blood out into the renal capsule The fluid has to travel through 3 layers – the ENDOTHELIUM of the capillaries, the BASEMENT MEMBRANE and the PODOCYTES – EPITHELIAL cells of the renal capsule

ULTRAFILTRATION The capillary endothelium has narrow gaps that allow blood plasma and dissolved substances to pass through The BASEMENT MEMBRANE is a mesh of collagen fibres that acts as a filter, only allowing molecules with a molecular mass of below through The PODOCYTES have projections called MAJOR PROCESSES that make gaps between the EPITHELIAL cells of the renal capsule, allowing the substances to pass into the capsule Water, amino acids, glucose, urea are filtered out Large proteins and blood cells are retained

THE RENAL CAPSULE

SELECTIVE REABSORBTION 85% of the filtrate is reabsorbed into the blood in the PROXIMAL CONVOLUTED TUBE 1.Na + ions are actively transported across the BASAL MEMBRANE into the blood. 2.This reduces the concentration of Na + ions in the cells between the capillaries and nephron. 3.This causes Na + ions to diffuse into those cells (down a concentration gradient) through COTRANSPORTERS, bringing Cl - ions, glucose and vitamins with them. 4.Water will follow as they lower the water potential. 5.The glucose, Cl - ions and vitamins then diffuse into the blood. Small proteins are absorbed by ENDOCYTOSIS.

THE LOOP OF HENLE Helps the blood reabsorb water Made up of an ASCENDING and DESCENDING limb 1.At the top of the ASCENDING limb Na + & Cl - ions are pumped out (active transport) creating a low water potential in the MEDULLA, water doesn’t follow because the ascending limb is impermeable to water. 2.This causes water in the DESCENDING limb to move out by osmosis, increasing the ion concentration in the DESCENDING limb. 3.At the bottom of the ASCENDING limb Na + & Cl - ions DIFFUSE out into the medulla, further lowering the water potential. 4.The low water potential causes lots of water to move out of the collecting duct by osmosis, making the urine concentrated. This is an example of a HAIRPIN COUNTERCURRENT MULTIPLIER

THE LOOP OF HENLE

OSMOREGULATION The water potential of the blood is monitored by the OSMORECEPTOR cells in the HYPOTHALMUS in the brain. NEUROSECRETORY cells are specialised nerve cells that produce ADH (antidiuretic hormone) and store it in their TERMINAL BULBS in the POSTERIOR PITUITARY GLAND. When the OSMORECEPTOR cells lose water (blood has a low water potential) they stimulate the NEUROSECRETORY cells to release the ADH. ADH binds to receptors in the membranes of the cells in the collecting duct wall. This causes vesicles with water-permeable channels (AQUAPORINS) to fuse with the membrane next to the lumen of the COLLECTING DUCT. More water is absorbed by these cells and reabsorbed back into the blood. If the blood has a high water potential, less ADH is produced.

KIDNEY FAILURE Occurs for three main reasons Diabetes, infection and high blood pressure (HYPERTENSION) Infection damages the kidney cells causing them to become inflamed, this interferes with filtration and reabsorption Hypertension damages the capillaries in the glomerulus, affecting ultrafiltration

TREATMENT OF KIDNEY FAILURE DIALYSIS – where a machine filters patients blood. It consists of a PARTIALLY PERMEABLE MEMBRANE with DIALYSIS FLUID on one side. DIALYSIS FLUID contains the correct ion and salt concentrations. The patients blood flows on the other side of the membrane. Any excess salts or ions in the blood diffuse into the DIALYSIS FLUID. If the blood is lacking certain salts or ions, these diffuse into the blood from the DIALYSIS FLUID. The blood leaves the machine with the correct concentrations of ions and salts. Often called HAEMODIALYSIS.

TREATMENT OF KIDNEY FAILURE KIDNEY TRANSPLANT – when a new kidney is implanted in a patient to replace a damaged one. The new kidney has to be from a person with the same blood type. Patients sometimes have to be given IMMUNOSUPPRESSANT drugs to stop rejection issues. Cheaper in the long term, and more convenient than regular DIALYSIS sessions. However it is a risky, major operation and their immune system may reject the new kidney.

TESTING FOR PREGNANCY An embryo secretes the hormone hCG once it is implanted in the lining of the uterus. hCG has a molecular mass less than so can pass through the basement membrane into the nephrons and then into the urine. To test for hCG, a stick with an APPLICATION AREA which contains ANTIBODIES FOR hCG BOUND TO BLUE BEADS, is soaked in the urine. Any hCG in the urine will bind to the antibodies on the beads, which then move into the TEST STRIP. The TEST STRIP also contains antibodies for hCG but these are fixed in place. These IMMOBILISED ANTIBODIES bind to the hCG on the blue beads, concentrating them in that area so that the test strip turns blue.

TESTING FOR PREGNANCY

TESTING FOR STEROIDS Steroids have a molecular mass of less than so can pass into the nephron tubules and so the urine. Urine is tested for steroids by GAS CHROMATOGRAPHY. The urine sample is vaporised and passed through a column containing liquid. Different substances in the sample take different times to pass through the column. If one of the substances takes the same amount of time to pass through the column as a steroid, then the urine is deemed to contain steroids.