Urine is composed of water, certain electrolytes, and various waste products filtered out of the blood system. As blood flows through the body, wastes resulting from the metabolism of foodstuffs in the body cells are deposited into the bloodstream.

Blood cells have a life of only about four months (120 days), and as they die, the cells are broken down. The yellow colour of urine comes from the breakdown of blood cells that is considered a waste to remove from the body.

About one-fifth of the total blood pumped by the heart each minute will enter the kidneys to undergo filtration. The rest of the blood (about 80%) does not go through the filtering portion of the kidney. The kidneys' two million or more nephrons (about a million in each kidney) form urine by three precisely regulated processes: filtration, reabsorption, and secretion.

Glomerular filtration moves water and solutes from blood plasma into the nephron. Blood enters the glomerulus through the afferent (incoming) arteriole where fluid enters the Bowman's capsule from the blood due to the high blood pressure in the glomerulus. This filtered fluid is referred to as filtrate. The glomerular filtrate consists primarily of water, excess salts (primarily Na ⁺ and K⁺ ), glucose, and a waste product of the body called urea. Urea is the most abundant of the waste products that must be excreted by the kidneys. The remaining unfiltered portion of blood leaves the glomerulus by way of the efferent (outgoing) arteriole.

The total rate of glomerular filtration for the whole body is normally about 125 mL per minute. That is about 180 litres per day!

Filtration occurs because of two factors. Firstly, the blood pressure in the glomerulus is four times greater than in other capillaries in the body. This creates a force against capillary walls that aids in moving water and solutes from the blood into the Bowman's capsule. Secondly, the glomerulus' capillaries are quite permeable. Thus, small molecules (including water, NaCl, glucose, urea, and certain amino acids) are forced into the Bowman's capsule. Larger proteins and red blood cells are left behind in the arteriole leading away from the glomerulus. If protein or blood is found in urine, this indicates a problem with the kidney function.

Read

Read page 311 and "Glomerular Filtration Filters Blood" on page 312 of your textbook.

This resource sheet on active and passive transport will be useful in this unit.