MAVO Year 4 · Gas exchange and excretion · Section 14.7

The liver and kidneys

The liver: metabolism and regulation

The liver helps regulate blood composition. After a meal, it can store glucose as glycogen. When blood glucose falls, it can release glucose by breaking down glycogen and making glucose from other raw materials, under hormonal control.

The liver processes nutrients and synthesises many plasma proteins, including fibrinogen, using amino acids. It also makes bile, which reaches the duodenum through bile ducts. Bile salts emulsify fats, increasing the surface area available to digestive enzymes.

Waste processing and detoxification

Components from old red blood cells are processed, producing bile pigments such as bilirubin. Products are excreted in bile and leave mainly through faeces after further changes.

When excess amino acids are broken down, nitrogen-containing groups are converted into urea, which is less toxic than ammonia. Urea enters the blood and is excreted mainly by the kidneys.

The liver also alters substances such as alcohol and medicines. This can make some substances easier to remove, but processing does not always make them harmless; harmful products and liver damage can occur.

Hepatitis

Hepatitis means liver inflammation. It can result from viruses and other causes. Hepatitis B can be transmitted through infected blood, sexual contact or from parent to baby around birth. It may become chronic and increase the risk of cirrhosis and liver cancer. Not everyone has obvious symptoms.

Kidney function

The kidneys lie towards the back of the upper abdomen, behind the lining of the abdominal cavity. Renal arteries bring blood, and renal veins carry it away. The kidneys filter plasma, selectively reabsorb needed substances and secrete certain substances into the forming urine. They regulate water and mineral balance and remove wastes including urea.

Each kidney has an outer cortex, inner medulla and central renal pelvis. Filtration and subsequent urine formation involve microscopic units, nephrons, extending through cortex and medulla. Urine collects in the pelvis and travels through a ureter to the bladder. It leaves through the urethra during urination.

Urine composition

Urine contains water, urea, mineral ions and variable amounts of other substances. When more water must be conserved, urine generally becomes smaller in volume and more concentrated. With excess water intake, it is often more dilute. Colour can also be affected by foods, medicines or illness, so colour alone is not a complete measure of hydration. These adjustments help maintain a stable internal environment.