Kidney Function & Structure PDF

# The structure of the nephron - On these pages you will learn to: - Describe the detailed structure of the nephron with its associated blood vessels using photomicrographs and electron micrographs - Describe the gross structure of the kidney ## Description of the nephron The nephron is the...

# The structure of the nephron - On these pages you will learn to: - Describe the detailed structure of the nephron with its associated blood vessels using photomicrographs and electron micrographs - Describe the gross structure of the kidney ## Description of the nephron The nephron is the functional unit of the kidney. It is a narrow tube, closed at one end, with two twisted regions separated by a long hairpin loop. Each nephron is made up of: - **Renal (Bowman's) capsule** - the closed end at the start of the nephron. It is cup-shaped and contains within it a mass of blood capillaries known as the glomerulus. Its inner layer is made up of specialised cells called podocytes. - **Proximal (first) convoluted tubule** - a series of loops surrounded by blood capillaries. Its walls are made of cuboidal epithelial cells with microvilli. - **Loop of Henlé** - a long, hairpin loop that extends from the cortex into the medulla of the kidney and back again. It is surrounded by blood capillaries. - **Distal (second) convoluted tubule** - a series of loops surrounded by blood capillaries. Its walls are made of cuboidal cpithelial cells, but it is surrounded by fewer capillaries than the proximal tubule. - **Collecting duct** - a tube into which a number of distal convoluted tubules empty. It is lined by cuboidal epithelial cells and becomes increasingly wide as it empties into the pelvis of the kidney. ## Blood vessels associated with the nephron - **Afferent arteriole:** a tiny vessel that is a branch of the renal artery and supplies the nephron with blood. The afferent arteriole enters the renal capsule of the nephron where it forms the glomerulus. - **Glomerulus:** a many-branched knot of capillaries from which fluid is forced out of the blood. - **Efferent arteriole:** a tiny vessel that leaves the renal capsule. It has a smaller diameter than the afferent arteriole, which causes an increase in blood pressure within the glomerulus. The efferent arteriole carries blood away from the renal capsule and later branches to form the peritubular capillaries. - **Peritubular capillaries:** a concentrated network of capillaries that surrounds the proximal convoluted tubule, the loop of Henlé and the distal convoluted tubule and from where they reabsorb mineral salts, glucose and water. The peritubular capillaries merge together into venules (tiny veins) that in turn merge together to form the renal vein. # Kidney function – ultrafiltration and selective reabsorption - On these pages you will learn to: - Describe how the processes of ultrafiltration and selective reabsorption are involved with the formation of urine in the nephron ## Functions of the kidney - Regulating the composition of the blood and maintaining a constant water potential by: - Maintaining a constant volume of water - Removing wastes such as urea - Maintaining the concentration of mineral ions and other substances constant - Regulating blood pressure - Maintaining the body's calcium level - Stimulating the production of red blood cells ## Ultrafiltration - Blood enters the kidney through the renal artery, which branches frequently to give around one million tiny arterioles, each of which enters a renal (Bowman's) capsule of a nephron. This is called the afferent arteriole and it divides to give a complex of capillaries known as the glomerulus. - The glomerular capillaries later merge to form the efferent arteriole, which then sub-divides again into capillaries (the peritubular capillaries), which wind their way around the various tubules of the nephron before combining to form the renal vein. - The walls of the glomerular capillaries are made up of endothelial cells with pores between them. As the diameter of the afferent arteriole is greater than that of the efferent arteriole, there is a build up of hydrostatic pressure within the glomerulus. - As a result, water, glucose, mineral ions and other substances up to a relative molecular mass of up to 68000 are squeezed out of the capillary to form the glomerular filtrate. The movement of this filtrate out of the glomerulus is resisted by the: - capillary endothelium - basement membrane of the epithelial layer of the renal (Bowman's) capsule - epithelial cells of the renal (Bowman's) capsule - the hydrostatic pressure of the fluid in the renal capsule space the intracapsular pressure - the low water potential of the blood in the glomerulus. ## Selective reabsorption - In the proximal convoluted tubule nearly 85% of the filtrate is reabsorbed back into the blood. - Why then, you may ask, allow it to leave the blood in the first place? Ultrafiltration operates on the basis of size of molecule: all below 68000 relative molecular mass are removed. Some are wastes, but most are useful. - About 180 dm³ of water enters the nephrons each day. Of this volume, only about 1 dm³ leaves the body as urine. - Eighty-five per cent of the reabsorption of water occurs in the proximal convoluted tubule. # The cells of the proximal convoluted tubules - The cells of the proximal convoluted tubules are adapted to reabsorb substances into the blood by having microvilli which give them a large surface area and many mitochondria to provide ATP for active transport of sodium ions. The process is as follows: - Sodium ions are actively transported out of the cells lining the proximal convoluted tubule into blood capillaries which carry them away. This is by the action of a membrane carrier protein, the sodium-potassium pump. The sodium ion concentration of these cells is therefore lowered. - Sodium ions now diffuse along a concentration gradient from the lumen of the proximal convoluted tubule into the lining cells but only through special carrier proteins. These carrier proteins are of different types, each of which carries another molecule (glucose or amino acids or chloride ions, etc.) along with the sodium ions. This is known as co-transport. Water follows osmotically down the water potential gradient that is created. - The molecules which have been co-transported into the cells of the proximal convoluted tubule then diffuse into the blood. As a result, all the glucose, amino acids, chloride ions and most other valuable molecules are reabsorbed as well as water.