MAVO Year 4 · Gas exchange and excretion · Section 14.1

A stable internal environment

Internal and external environments

The internal environment surrounding body cells consists mainly of extracellular fluid, including blood plasma and tissue fluid. The external environment includes the surroundings and spaces continuous with them, such as the inside of the digestive tract and air in the lungs. A swallowed nutrient has not entered the internal environment until it crosses the relevant body lining.

Skin and mucous membranes provide barriers, while secretions such as stomach acid help protect against harmful agents. These defences are selective rather than perfectly impenetrable.

Uptake, storage and excretion

The body keeps internal conditions within suitable ranges through coordinated processes.

• Uptake: digested nutrients cross the intestinal lining, and oxygen crosses the lung gas-exchange surface.
• Storage: liver and muscle store glycogen, adipose tissue stores fat and bone stores mineral reserves. Yellow marrow in long bones also contains fat. Some vitamins can be stored, while others have more limited reserves.
• Excretion: metabolic wastes and excess substances leave through routes including the lungs, kidneys and liver-associated processes.

Homeostasis

Maintaining a relatively stable internal environment is called homeostasis. Conditions are not perfectly constant: they fluctuate within regulated ranges. Hormones, nerves and organs coordinate these adjustments. For example, insulin and glucagon help regulate blood glucose, while kidneys adjust water and salt excretion. Stable conditions support normal enzyme activity and cellular function.