Excretion: the liver and the kidney

Last time

Before we start: last time, in Anaerobic respiration and oxygen debt, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Cleaning the blood

Your kidneys filter all the blood in your body about sixty times a day. Together with the liver, they remove waste products that would poison you if they built up. Today: how the liver makes urea, and how the kidney's nephrons filter the blood and take back what is useful.

Excretion

Excretion is the removal of waste products of metabolism and substances in excess of requirements. The lungs excrete carbon dioxide. The kidneys excrete urea, excess water and excess ions. If urea or carbon dioxide build up, they are toxic.

The liver

The body cannot store excess amino acids. In the liver, the nitrogen-containing part is removed from each amino acid. This is deamination. The nitrogen part is turned into urea. The rest is used for energy or stored. Urea leaves the liver in the blood and is carried to the kidneys.

The urinary system

Here is the urinary system. Two kidneys filter the blood. The ureters carry urine to the bladder, which stores it, and the urethra carries it out of the body. The renal artery brings blood in, and the renal vein takes cleaned blood away.

The nephron

Each kidney contains about a million tiny tubules called nephrons. Each nephron starts with a glomerulus, a knot of capillaries, inside a cup called the Bowman's capsule. A long tubule then leads to a collecting duct, which carries urine to the ureter.

Step 1: filtration

Blood enters the glomerulus at high pressure. Small molecules are forced out into the capsule: water, glucose, urea and salts. Large molecules, such as proteins, and blood cells are too big and stay in the blood.

Step 2: reabsorption

All of the glucose is reabsorbed back into the blood, in the first part of the tubule. Some salts are reabsorbed, as much as the body needs. Much of the water is reabsorbed, mainly from the collecting duct, depending on how much the body needs. What is left, water, urea and excess salts, flows on as urine.

What is in the fluid

Here is the table to learn. Glucose is filtered but none is in urine. Protein is not even filtered. Urea becomes more concentrated in urine, because water is reabsorbed.