how is water removed from the blood in the kidneys

Question

hi 

i really need to know how is water removed from the blood in the kidneys its for an a level question

Answer 1

If you mean simply "removed from the blood" as in removed from circulation to some other location, it is in the Glomerulus of the nephron. That is the knot of capillaries you see at the beginning of any image of a kidney nephron. Normal capillaries in your body allow water to diffuse out of them into surrounding tissues by moving through the squamous cells that form the wall of the capillary. That is relatively slow. To remove water in the kidneys it needs to be able to get out of the capillaries much faster. Blood flows through the glomerulus in seconds, so there is not enough time for osmosis to remove much water. Instead of being a relatively continuous tube like normal capillaries, the ones in the glomerulus are basically poked full of slits. There are called fenestrated capillaries. They have vessel walls that look like a spaghetti strainer. As the blood flows through these capillaries in the glomerulus, the pressure of the blood shoves water through the slits. This basically squeezes water out of the blood stream and it is captured in the sac called the glomerular capsule located around the capillaries. A large percentage of the water is squeezed from the blood along with almost all dissolved nutrients, salts, and proteins. About the only things not squeezed out are the blood cells, which are too big to fit through the slits (like spaghetti being trapped in a strainer but water passing through). So at that point, water has been "removed from the blood". If you just got rid of that water in your urine, you would lose WAY too much water along with nearly all nutrients, salts, etc. You would die very quickly. The rest of the nephron is a series of tubules that reabsorb most of that water, nutrients, salts, etc, leaving only some of the water by the end that becomes your urine. I am not sure if your question is asking for all those other steps, but if I take what you wrote literally, the water being squeezed through the fenestration slits in the glomerular capillaries and collecting in the glomerular capsule is the actual "removal of water" step. The rest are recapturing most of it along with all the other good stuff you don't want to lose in your urine.

Answer 2

There are two main processes that remove the water: (1) ultrafiltration in the glomerulus and (2) selective reabsorption in the proximal convoluted tubule (PCT) and the collecting duct.

Ultrafiltration happens when pressurized blood enters the glomerulus through a wide tube (afferent arteriole), and it is forced to exit through a narrower tube (efferent arteriole). This creates high hydrostatic pressure, and this "pushes" the blood through the glomerular membrane. Note that only small molecules like ions can pass through the glomerulus, but not hemoglobin. So what gets filtered out is mostly plasma, ions, glucose, and urea. Cells and proteins remain in the blood.

Then, selective reabsorption happens when Na+ ions are actively transported from the filtrate and into the blood. This creates a chemical gradient, and causes osmosis (diffusion of water) from the filtrate (where the Na+ conc is lower) to the blood (where Na+ conc is higher). ADH is a hormone released from the pituitary gland in the brain, when there is too little water in blood. In the collecting duct, ADH causes more aquaporins to be inserted, which causes more water to go from the filtrate, back into the blood. Conversely, when ADH levels are low, less water is reabsorbed and more dilute urine is produced.