MAVO Year 4 · Gas exchange and excretion · Section 14.3

The human respiratory system

What is gas exchange?

Aerobic respiration in cells uses oxygen and produces carbon dioxide. In the lungs, gas exchange transfers oxygen from air into blood and carbon dioxide from blood into air. This occurs mainly across the surfaces of the alveoli, the tiny air sacs.

The route taken by air

Air enters through the nose or mouth, passes through the pharynx and larynx, then enters the trachea. The trachea divides into two main bronchi, one entering each lung. These branch into smaller airways and bronchioles, leading to the gas-exchange regions.

Protection in the nose

Nasal hairs trap larger particles. Mucus catches smaller particles and some microbes, while cilia move mucus towards the throat. Blood vessels warm incoming air, and moist surfaces humidify it. Smell receptors can detect some warning odours, but not all harmful gases have a detectable smell.

Swallowing

The throat carries both air and food along different routes. During swallowing, the soft palate and uvula help close the passage towards the nose. Movement of the larynx and epiglottis helps protect the airway while food enters the oesophagus. If material enters the airway, coughing helps remove it.

Keeping airways open and clean

Cartilage supports the trachea and larger bronchi. The trachea has C-shaped cartilage rings; cartilage in bronchi is arranged differently. Smaller bronchioles lack cartilage and contain smooth muscle that can alter their diameter. Mucus and cilia line much of the conducting airway, carrying trapped material towards the throat. Coughing provides an additional clearance mechanism.

Alveolar gas exchange

Alveoli are surrounded by pulmonary capillaries. The thin alveolar and capillary walls, moist exchange surface, large total area and maintained blood and air flow support rapid diffusion. Oxygen moves towards blood and carbon dioxide towards alveolar air according to their partial-pressure gradients. The many alveoli provide a total surface area of many tens of square metres.

Inhaled and exhaled air

Inhaled air is mainly nitrogen, with about 21% oxygen and a small amount of carbon dioxide. Exhaled air still contains nitrogen and considerable oxygen, but usually less oxygen and more carbon dioxide and water vapour. Only some of the inhaled oxygen is taken up during each breath. Much exhaled water vapour comes from moist respiratory surfaces.