04-23-07, Urinary system

Bowman's capsule, glomerulus, glucose, amino acids, distal tubule, proximal tubule, …

The Bowman's capsule surrounds the glomerulus. The proximal tubule is where there's the water-hydrogen exchange going on. The distal tubule comes after the proxima ltubule. The aldosterone is in the distal tubule. The collecting tubule is in the loop of henle (the big orange part in the diagram).

This all takes place somewhere in the kidney. This is the renal tubule, which is where we have the Bowman's capsule around the glomerulus, which goes to the proximal tubule, and then to the disdal tubule, to the collecting tubule in the loop of henle, where there's some exchanges between water and other portions etc.

Stuff is filtered at the glomerulus, and then you have a long and winding road. The longer the tube, the more chances to get water and sodium cations out of the renal tubule. Materials from the blood are forced out of the glomerulus and into the bowman's capsule during the process of filtration. The materials in the blood are under pressure, and there's also the laws of diffusion. The glomerulus is the physical barrier that keeps some portions of the blood in the blood, and some portions are going to get through the semipermeable wall of the glomerulus. The materials that are filtered are now called filtrate.

The filtrate contains: water, urea, glucose, salts, amino acids, vitamins. Most of this stuff is dissolved in water. Otherwise, urinating would be a lot more painful. Urea is where you have your nitrogen waste-products.

Plasma proteins, cells, thrombocytes (platelets) are too large to pass through the membrane; they remain in the blood. So the kidney is only getting water, urea, vitamins, salts, amino acids, and glucose, not the cells or proteins found in the blood.

Approximately 180 liters of filtrate pass from the blood into the collecting tubules each day. Not all of this 180 liters of filtrate in the collecting tubules are excreted—some are reabsorbed.

Most of the materials removed from the blood at bowman's capsules make their way back into the blood by a process known as reabsorption. So the bowman's capsule is the site of reabsorption where materials find their way back into the blood.

You have to keep the concentration of the blood the same, so if you're going to take all of the water out, the composition of the blood is changed, so lots of these nutrients and particles and water molecules have to be regulated back into the blood stream, so that there's no abrupt change in the blood composition.

Approximately 99% of the water that is filtered into the bowman's capsule is reabsorbed into the blood. If you have 5 L of blood, then your kidney has maybe 36 cycles per day.

Most reabsorption occurs in the proximal tubule (PCT). In this region, about 75% of the water in the filtrate returns to the capillaries by osmosis. The proximal tubule is close to the midline.

Glucose and minerals are returned to the blood by active transport. This means that there is an energy requirement in order to remove glucose and minerals from filtrate. Some additional reabsorption occurs in the distal convoluted tubules.

When the filtrate reaches the distal convoluted tubule (DCT), some substances pass from the blood into the filtrate through a process called secretion. These substance include wastes and toxic materials.

The pH of the blood is adjusted by H+ ions (hydronium ions, H₃O⁺) that are secreted from the blood into the filtration. The material that remains in the distal convoluted tubule is called urine, which consists of excess salts, water, and urea (nitrogen-based waste product).

Urine becomes concentrated in the loop of Henle. This is where a majority of further-waters is going to be taken out, and you have about 75% of water that is taken out by the distal convoluted tubule and then there's the concentration of the urea products in the loop of Henle. This area helps to conserve water and minimize the volume of urine. Urine from the collecting ducts flows through the renal pelvis and into a narrow tube called the ureter.

Ureter -> urinary bladder -> urethra -> two sphincters (internal and external sphincters). One is involuntary, and the other is voluntary. The external sphincter is voluntary.

A normal adult eliminates from 1.5 L (or 1.6 quarters) to 2.3 (or 2.4 quarts) of urine/day. That's about one-half gallon per day. Depending on the amount of water taken in and the amount of water lost through respiration and perspiration. Purified blood is returned to the circulatory system through the renal vein.

There are two important things controlled by the kidneys: (1) concentration of water in blood and (2) the level of salt in the blood to maintain pH.

Drink too much liquid and the kidneys will then decrease the rate of reabsorption, excess water is sent to the urinary bladder to be excreted. If you eat salty foods then the kidneys will be respond by returning less salt to the blood through reabsorption. Then the salt excess is excreted in the urine.