International A Level Biology / Control of the internal environment
Urea production and ultrafiltration
Edexcel International A Level Biology (YBI11), chapter 22: Control of the internal environment, specification point 7.19. Free lesson.
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The liver converts excess amino acids into urea, which is carried in the blood to the kidneys. In the kidney, high pressure in the glomerulus forces small molecules into Bowman's capsule while cells and large proteins remain in the blood.
How the liver makes urea
Your body cannot store excess amino acids, so the liver breaks them down. In deamination, the amino group is removed and becomes ammonia. Ammonia is very toxic, so the liver combines it with carbon dioxide to make the less toxic waste product urea. The urea travels in the blood to the kidneys. The rest of the amino acid can be respired or converted to carbohydrate or fat and stored. The detailed steps of the ornithine cycle are not required.
Why pressure is high in the glomerulus
Ultrafiltration is filtering under pressure. Blood enters the glomerulus through the afferent arteriole and leaves through the efferent arteriole. The afferent arteriole is wider than the efferent arteriole, so blood leaves through a narrower route. This creates a high hydrostatic pressure, meaning the blood pushes strongly on the capillary walls. The pressure forces fluid out of the glomerulus and into Bowman's capsule. This fluid is called the glomerular filtrate, and it flows towards the proximal tubule. To explain the high pressure, state that the afferent arteriole is wider…
The three layers of the filtration barrier
The filtrate passes from the blood in the glomerulus into Bowman's capsule through three layers. Each layer helps keep particular large components in the blood while allowing fluid and small substances through.
What is filtered and what stays in the blood
Ultrafiltration separates substances by size, not by whether the body needs them. The filtrate is basically blood plasma without the large proteins. Small molecules and ions pass through all three layers, while blood cells and large proteins stay in the blood. The filtrate contains water, glucose, amino acids, urea and mineral ions such as sodium ions. Blood cells and platelets are too large to pass through the pores in the endothelium. Large plasma proteins are held back by the basement membrane. Glucose is filtered because it is small, even though the body needs it. Normally, all of it…