Gas exchange [humans]

Ventilation is the process of air entering and leaving the lungs.

After air passes through the nose, it passes through the trachea, which is lined with cartilage for support. After, it moves through the two bronchii. These bronchii extend into smaller bronchioles which end in tiny, microscopic air sacs called alveoli. These are single celled and situated close to blood capillaries. The trachea and bronchi are lined with cilia and mucus which prevent dust and dirt from entering the lungs and causing infection.

The thorax is the name given to the whole respiratory system containing the lungs, trachea, bronchi, alveoli, intercostal muscles, diaphragm, bronchioles, ribs and pleural membranes.

The diaphragm is a muscular, fibrous dome-shaped tissue that separates the contents of the thorax from the rest of the abdomen.

When a person breathes in, the diaphragm contracts and the external intercostal muscles contract allowing the rib cage to expand outwards and upwards. The volume of the lungs therefore increases as well as the pressure inside decreasing. As a result, the air is drawn into the lungs. The pleural membranes prevent the friction between the lungs and other organs.

The opposite happens when a person exhales. The internal intercostal muscles contract whilst the diaphragm expands. The volume of the lungs decreases and the pressure increases. As a result, air is pushed out of the lungs.

The alveoli are adapted for gas exchange in four main ways:

1. Their walls are thin, making the diffusion of gases more efficient and occur at a greater rate.
2. Their walls are one cell thick and are situated close to blood capillaries which are also one cell thick. Therefore the diffusion distance is very small hence gas exchange can occur at a greater rate.
3. The alveoli have moist linings which allows gases to dissolve into.
4. Combined, the walls of the alveoli have a massive surface surface area. This allows gases to diffuse in and out at a great rate.

Smoking

Smoking is very destructive for the body. It primarily affects the circulatory system and respiratory system. There are a vast number of ways it can inhibit the function of various organs:

1. The carbon monoxide from the smoke binds irreversibly to haemoglobin thus preventing oxygen from being transported around the body.
2. Smoking destroys the cilia that lines the trachea and bronchi. This makes dust and dirt more prevalent in the lungs, making one more prone to infection.
3. It can cause excess mucus production. This inhibits the air that can be inhaled and taken into the lungs.
4. Tobacco smoke causes bad cholesterol to build up in veins and arteries. This decreases the space for blood to travel through as it narrows the lumen therefore increasing the blood pressure. High blood pressure means there is a greater chance of narrower blood vessels bursting.
5. Angina is the pain in the chest resulting from lack of oxygen. When carbon monoxide binds to haemoglobin it reduces the amount of oxygen in the blood. A lack of oxygen means cells respire anaerobically. This produces lactic acid. When the lactic acid is not removed and the 'oxygen debt' has not been paid, pain is caused.

An experiment investigating the effect of exercise on the breathing rate:

Count how many breaths a person takes when stationary for a minute.
Then make them run at 5mph for one minute.
As soon as they stop, count how many breaths they take for a minute.
Make them run at 5mph for two minutes.
Count how many breaths they take for a minute.
The increase in rate should be linear.