Homeostasis

The rate at which the fluid filters from the blood in

the glomerular capillaries into the Bowman’s capsule

is called the glomerular filtration rate.

This is determined by the differences in water potential between the plasma in glomerular capillaries and the filtrate in the Bowman’s capsule.water moves from a region of higher water potential to a region of lower water potential, down a water potential gradient. Water potential is lowered by the presence of solutes, and raised by high pressures.Inside the capillaries in the glomerulus, the blood pressure is relatively high, because the diameter of the afferent arteriole is wider than that of the efferent arteriole, causing a head of pressure inside the glomerulus. This tends to raise the water potential of the blood plasma above the water potential of the contents of the Bowman’s capsule.However, the concentration of solutes in the blood plasma in the capillaries is higher than the concentration of solutes in the filtrate in the Bowman’s capsule. This is because, while most of the contents of the blood plasma filter through the basement membrane and into the capsule, the plasma protein molecules are too big to get through, and so stay in the blood. This difference in solute concentration tends to make the water potential in the blood capillaries lower than that of the filtrate in the Bowman’s capsule. Overall, though, the effect of differences in pressure outweighs the effect of the differences in solute concentration. Overall, the water potential of the blood plasma in the glomerulus is higher than the water potential of the filtrate in the capsule. So water continues to move down the water potential gradient from the blood intothe capsule.

Many of the substances in the glomerular filtrate need to be kept in the body, so they are reabsorbed into the blood as the fluid passes along the nephron. As only certain substances are reabsorbed, the process is called selective reabsorption. Most of the reabsorption takes place in the proximal convoluted tubule. The lining of this part of the nephron is made of a single layer of cuboidal epithelial cells.

•  microvilli to increase the surface area of the inner surface facing the lumen
• tight junctions that hold adjacent cells together so that fluid cannot pass between the cells (all substances that are reabsorbed must go through the cells)
•  many mitochondria to provide energy for sodium–potassium (Na+–K+) pump proteins in the outer
• membranes of the cells
•  co-transporter proteins in the membrane facing the lumen