Mammals – gas exchange electricity electricity schoolhouse rock

####

Most mammals live on land, so they get their oxygen from air. They are at risk of desiccation, so their gas exchange system (the lungs) is inside their bodies to reduce water loss. Air has a relatively high oxygen concentration, and is easy to ventilate.

Air enters the lungs through the trachea, which splits into two bronchi and then smaller bronchioles. All of these tubes are held open by rings of cartilage. On the bronchioles are alveoli, where gas exchange takes place. These alveoli greatly increase the surface area:volume ratio, increasing the efficiency of gas exchange and more particles can diffuse at once. The alveoli are surrounded by capillaries so gases can diffuse between the air and blood. Mammals have a circulatory system, so their size isn’t limited by their gas exchange system. Because mammals breathe air, unwanted particles sometime get into the gas exchange system. There is mucus in the trachea and bronchioles to keep them clean and moist. The alveoli must stay moist so oxygen can dissolve and then diffuse into the blood. The lungs are also kept moist by being deep inside the body making it harder for moisture to evaporate. The lungs are ventilated by the diaphragm contracting to draw air in, and then relaxing to push it out. The air goes in and out the same way, which is called tidal ventilation. This isn’t very efficient because not all of the air that is breathed in with each breath is breathed out again, meaning some of the air in each breath has already lost its oxygen to the blood. Also, some air from each breath doesn’t make it as far as the gas exchange surface in the alveoli.

Marine mammals also breathe air, and must come up to the surface of the water to breathe. They have small lungs in relation to their body size compared to humans to reduce problems of nitrogen building up in the blood caused by diving. They have to make sure no water gets into the gas exchange system, and gas exchange must occur efficiently. Marine mammals have to be able to hold their breaths for long periods of time while they are diving.

An advantage of the gas exchange system in mammals is that they have a circulatory system. Oxygen is absorbed by the blood to be transported to every cell in the body. This means the size of mammals isn’t limited by their gas exchange system. No matter how big the mammal gets, every cell will have a capillary delivering oxygen.

Another advantage is the large surface area:volume ratio of the gas exchange surface. The alveoli maximise the surface area of the lungs, allowing more oxygen to diffuse at once, and increasing the efficiency of gas exchange. This is important because it allows mammals to get plenty of oxygen to their cells for respiration.

A limitation is tidal ventilation. Not all of the air breathed in makes it to the gas exchange surface, so some air breathed out still has oxygen in it. Also, not all air makes it out of the body, so some air breathed in doesn’t have oxygen in it any more. This isn’t a very efficient way of ventilating the lungs, as not all of the oxygen coming into the body is absorbed by the blood.

Usually mammals can only live on land because they can only breathe air. This is another limitation of their gas exchange system. Some mammals have adapted to live in water, but they still have to come up to the surface to breathe air. They can only hold their breaths for a limited time, so must come up to the surface often to breathe.