[image:] ===Nose & Mouth=== Mammals such as humans are terrestrial (live on land) and have evolved an internal gas exchange system in order to minimise water loss and prevent damage to the gas exchange surface. However, an internal gas exchange surface that is warm and moist is prone to bacterial infection and and the accumulation of dust/debris. Air enters through the nose and mouth and '''capillaries''' lining the nose and mouth help to warm air before it enters the lungs. '''Hairs''' lining the nose and '''Mucus''' help to trap dust and debris including bacteria. Mucus contains antibacterial components and may also contain white blood cells during an infection. Tiny hair-like structures called '''cilia''' line the airways. They sweep debris caught in the mucus to the top of the throat by moving in a wave-like motion (where it is then swallowed and destroyed in the stomach). Together, these features help to prevent materials damaging or sticking to the gas exchange surface which would reduce the surface area available for gas exchange. ===Trachea=== Air then passes down the '''trachea''' (windpipe), a tube lined with c-shaped bands of cartilage. These bands act like the rings on a vacuum cleaner hose, preventing the tube from collapsing under external forces due to gravity and body movements. This ensures the gas exchange system can be ventilated continuously (that a concentration gradient is maintained across the gas exchange surface). ===Bronchial Tree=== [image:] The trachea splits into two primary '''bronchi''' (singular bronchus). Each bronchus branches into secondary bronchi that continue to branch like a tree into many tiny '''bronchioles'''. Each bronchiole leads to a cluster of tiny sacks called '''alveoli'''. The alveoli are the site of gas exchange. This system of branching tubes is often refereed to as the bronchial tree and massively increases the surface area to volume ratio available for gas exchange. The lungs have a surface area many thousands of times larger than single single alveoli would even if it were the size of the lungs. Mammals such as humans are generally larger, warm-blooded animals that are higher in the food-chain. As a result mammals tend to have much higher energy demands than other animals. The large surface area of the lungs helps meet these demands. ===Alveoli=== [image:] Alveoli are tiny membrane sacks that are lined with capillaries. Gases are exchanged between the air inside the alveoli and the blood inside the capillaries lining them. Alveoli have very thin permeable walls, only one cell thick. This minimises the barrier / distance dissolved oxygen and carbon dioxide need to diffuse, ensuring that the rate of diffusion is fast enough to support rapid gas exchange. Alveoli also contain a surfactant (a chemical similar to a detergent) that reduces the surface tension of the water lining the alveoli. This prevents the alveoli walls from collapsing and sticking together ensuring the lungs can be ventilated. ===Blood & Blood Vessels=== [image:] The blood vessels lining the alveoli are part of a transport system that delivers oxygen to respiring cells and returns with carbon dioxide to the lungs. Blood flow though the capillaries ensures that oxygen reaches distant cells more rapidly as diffusion would take far too long in such large organisms. Blood circulates through the body and oxygen poor blood returns to the lungs. Thus blood flow helps to maintain the concentration gradient needed for gas exchange by ensuring that blood in the lungs has a lower concentration of oxygen (and higher concentration of carbon dioxide). Mammals require a transport system because of their large size. It would take far too long for oxygen to diffuse from the gas exchange surface to the body tissues. ===Blood Cells & Haemoglobin=== [image:] The bi-concave shape of blood cells helps to increase their surface area to volume ratio. Red blood cells are often wider than capillaries and they have to squeeze through, further increasing their surface area to volume ratio and reducing the distance that oxygen has to diffuse. This ensures gas exchange is efficient and rapid. Oxygen is carried by a protein called haemoglobin (inside each red blood cell). Each haemoglobin molecule can reversibly bind four oxygen molecules. The binding of oxygen to haemoglobin increases the affinity of the remaining binding sites for oxygen. Essentially this means that when oxygen is plentiful haemoglobin binds it more tightly. When oxygen becomes more scarce it binds oxygen more loosely. This aids oxygen binding in the lungs and oxygen offloading in the respiring tissues.
Credit: Ben Himme