The gas exchange system

All organisms take in gases from their environment and release gases to the environment. Animals take in O₂ for aerobic respiration and release CO₂. Plants also respire, but during daylight hours they photosynthesise at a greater rate than they respire, and so take in CO₂ and release O₂.

The body surface across which these gases diffuse into and out of the body is called the gas exchange surface. In mammals, including humans, the gas exchange surface is the surface of the alveoli in the lungs.

The human gas exchange system

The gas exchange surface in the lungs is extensive, very thin, well supplied with blood and well ventilated. The trachea and bronchi provide little resistance to the movement of air to and from the alveoli.

- The gross structure of the human gas exchange system

- Plan diagrams of the structure of the walls of the trachea, bronchi,  bronchioles 

Cartilage in the walls of the trachea and bronchi provides support and prevents the

tubes collapsing when the air pressure inside them is low.

Cillated epithelium is found lining the trachea, bronchi and some bronchioles. It

is a single layer of cells whose outer surfaces are covered with many thin extensions

(cilia) which are able to move. They sweep mucus upwards towards the mouth,

helping to prevent dust particles and bacteria reaching the lungs.

Goblet cells are also found in the ciliated epithelium. They secrete mucus, which

traps dust particles and bacteria.

Smooth muscle cells are found in the walls of the trachea, bronchi and bronchioles.

This type of muscle can contract slowly but for long periods without tiring. When it

contracts, it reduces the diameter of the tubes. During exercise it relaxes, widening

the tubes so more air can reach the lungs.

Elasticc fibres are found in the walls of all tubes and between the alveoli. When

breathing in, these fibres stretch to allow the alveoli and airways to expand. When

breathing out, they recoil, helping to reduce the volume of alveoli and expel air out

of the lungs.

Gas exchange at the alveolar surface

The air inside an alveolus contains a higher concentration of O₂, and a lower concentration of CO₂, than the blood in the capillaries. This blood has been brought to the lungs in the pulmonary artery, which carries deoxygenated blood from the heart. O₂therefore diffuses from the alveolus into the blood capillary, through the thin walls of the alveolus and the capillary. CO₂ diffuses from the capillary into the blood.

The diffusion gradients for these gases are maintained by:

• breathing movements, which draw air from outside the body into the lungs, and then push it out again; this maintains a relatively high concentration of O₂ and low concentration of CO₂ in the alveoli;
• blood flow past the alveolus, which brings deoxygenated blood and carries away
oxygenated blood.

Tidal volume and vital capacity

Air moves by mass flow into and out of the lungs during breathing. This is caused by the contraction and relaxation of external intercostal muscles and muscles in the diaphragm. When these contract, they increase the volume of the thoracic cavity and draw air down through the trachea and into the bronchi and bronchioles. When they relax, the thoracic volume decreases and air flows out, down a pressure gradient.

The volume of air that is moved into or out of the lungs during one breath is called the tidal volume. It is generally about 0,5 dm³. The maximum amount of air that can be moved in or out during the deepest possible breath is called the vital capacity. It is generally somewhere between 3 dm³ and 5 dm³.

Syllabus 2015 (a) [PA] describe the structure of the human gas exchange system, including the microscopic structure of the walls of the trachea, bronchioles and alveoli with their associated blood vessels; (b) [PA] describe the distribution of cartilage, ciliated epithelium, goblet cells and smooth muscle in the trachea, bronchi and bronchioles; (c) describe the functions of cartilage, cilia, goblet cells, mucous glands, smooth muscle and elastic fibres in the gas exchange system; (d) describe the process of gas exchange between air in the alveoli and the blood;

Syllabus:  The gas exchange system is responsible for the uptake of oxygen  into the blood and excreting carbon dioxide. An understanding of this system shows how cells, tissues and organs function  together to exchange these gases between the blood and the environment. The health  of this system and of the cardiovascular system is put at risk by smoking. 9.1    The gas exchange system The gas exchange surface in the lungs is extensive, very thin, well supplied with blood and well ventilated. The trachea and bronchi provide little resistance to the movement of air to and from the alveoli. a)   describe the gross structure of the human gas exchange system b)   observe and draw plan diagrams of the structure of the walls of the trachea, bronchi,  bronchioles and alveoli indicating the distribution of cartilage,  ciliated epithelium, goblet  cells, smooth muscle, squamous epithelium and blood vessels c)   describe the functions of cartilage,  cilia, goblet  cells, mucous glands,  smooth muscle and elastic  fibres and recognise these cells and tissues in prepared slides, photomicrographs and electron micrographs of the gas exchange system d)   describe the process of gas exchange between air in the alveoli and the blood