Gas Exchange in Humans

Ventilation is not respiration

Get this distinction straight before anything else, because it is the mistake that costs the most marks in this topic. Ventilation (also called breathing) is the mechanical movement of air into and out of the lungs. Respiration is the chemical reaction inside cells that breaks down nutrient molecules such as glucose and releases the energy stored in them. Ventilation happens in the lungs and airways; respiration happens in every living cell.

The two are linked, but they are not the same process. Ventilation brings oxygen into the alveoli so that it can diffuse into the blood, and it removes the carbon dioxide that respiration has produced. Breathing serves respiration: it supplies the oxygen aerobic respiration needs and takes away the carbon dioxide it makes, but it releases no energy itself.

Watch the wording of a question. If it mentions energy, cells, glucose or metabolism, it wants respiration. If it mentions air, pressure, volume, the diaphragm or the lungs, it wants ventilation.

What makes a good gas exchange surface

Gas exchange means oxygen diffusing from the air into the blood, and carbon dioxide diffusing from the blood into the air. Any surface that does this efficiently has four features, and Cambridge expects all four to be named and understood.

The reason all four matter comes down to diffusion. Diffusion is faster when the surface is large, when the diffusion distance is short, and when the concentration gradient across the surface is steep. Each feature either shortens the distance or keeps a gradient steep.

• A large surface area - so that more oxygen and carbon dioxide can diffuse across at once, giving a faster rate of diffusion. The millions of alveoli provide this.
• A thin surface - the wall of an alveolus and the wall of its capillary are each only one cell thick, so the diffusion distance is short and gases cross quickly.
• A good blood supply - a dense network of capillaries carries oxygen away and brings carbon dioxide to the surface, keeping the concentration gradient steep.
• Good ventilation with air - moving fresh air in and stale air out keeps the concentration of oxygen high and carbon dioxide low in the alveoli, so the gradients stay steep and diffusion continues.

The breathing system and its parts

Air follows a single route: nose or mouth -> larynx (voice box) -> trachea (windpipe) -> bronchi -> bronchioles -> alveoli. The lungs are the two organs that contain this branching network, and they sit inside the thorax, protected by the ribs.

The trachea is held open by rings of cartilage. Cartilage is strong but flexible, and its function is to keep the trachea open so that it does not collapse when the pressure inside the thorax changes during breathing. The rings are incomplete at the back, which allows the oesophagus behind it to expand when you swallow.

The trachea divides into two bronchi, one to each lung. Each bronchus branches again and again into smaller bronchioles, which end in clusters of alveoli. The alveoli are the gas exchange surfaces, and each one is wrapped in a network of capillaries so that gases can diffuse between the air and the blood.

How ventilation works: volume, pressure and air movement

Air is drawn into the lungs because the volume of the thorax is increased, which lowers the pressure inside so that it is below atmospheric pressure. Air is pushed out when the volume is reduced and the pressure rises again. The muscles that cause these volume changes are the diaphragm and the intercostal muscles between the ribs.

Inhalation (breathing in): the external intercostal muscles contract, pulling the ribs up and out, and the diaphragm contracts and flattens, moving down. The volume of the thorax increases, the pressure inside falls below atmospheric pressure, and air flows in.

Exhalation (breathing out): the external intercostal muscles relax and the diaphragm relaxes and becomes domed again, moving up. The volume of the thorax decreases, the pressure inside rises above atmospheric pressure, and air is pushed out. At rest this is a passive process, but during forceful exhalation the internal intercostal muscles contract to pull the ribs down and in.

Inhaled and exhaled air compared

Exhaled air is not inhaled air with all the oxygen used up. Only part of the oxygen is absorbed, so exhaled air still contains plenty of oxygen - which is why rescue breathing can save a life.

The differences all come down to what the body does to the air as it passes through the lungs: oxygen is taken out, carbon dioxide and water vapour are added, and nitrogen is left alone.

• Limewater is the test for carbon dioxide: it turns milky (cloudy) when carbon dioxide is bubbled through it. Bubbling exhaled air through limewater turns it milky quickly; inhaled air contains only a trace of carbon dioxide, so it turns milky much more slowly.
• Compare the two results by bubbling the same volume of air through equal volumes of limewater for the same length of time, so that the only difference is the source of the air.

Exercise: the rate and depth of breathing

During physical activity the muscles contract more often, so they respire faster. They use more oxygen and produce more carbon dioxide, which passes into the blood.

The increased carbon dioxide concentration in the blood is detected by the brain. The brain responds by sending nerve impulses to the intercostal muscles and the diaphragm, so the rate of breathing (the number of breaths per minute) increases and the depth of each breath increases. This takes in more oxygen for aerobic respiration and removes the extra carbon dioxide faster.

Both rate and depth matter. A faster rate alone would not move enough air, and a deeper breath alone would not keep up with demand. The two together increase the volume of air moved per minute, which is the ventilation rate.

Keeping the airways clean, and what tobacco smoke does

The airways are lined with goblet cells and ciliated cells, which protect the breathing system from pathogens and particles. Goblet cells secrete mucus, a sticky fluid that traps dust, bacteria and other particles. Ciliated cells have tiny hair-like cilia that beat in a wave, sweeping the mucus upwards towards the throat, where it is swallowed. This is the mucus escalator.

Tobacco smoke damages these defences and harms the gas exchange system in several ways. Tar is a sticky mixture that coats the lining of the airways and contains carcinogens, chemicals that cause mutations in the DNA of the cells lining the airways. These mutations can lead to uncontrolled cell division and lung cancer. Tar also paralyses and destroys the cilia, so mucus and trapped particles are no longer swept away; the smoker has to cough to clear them, producing a persistent cough and the inflammation of the airways called bronchitis.

Nicotine is the substance in tobacco smoke that makes smoking addictive; it is a stimulant drug, and it also raises the heart rate. Carbon monoxide in the smoke combines with haemoglobin in red blood cells more readily than oxygen does, so it reduces the amount of oxygen the blood can carry. Less oxygen reaches the tissues, and the heart must work harder to deliver it.

In emphysema the walls of the alveoli are destroyed, so many small alveoli merge into fewer, larger air spaces. This reduces the total surface area for gas exchange, so less oxygen diffuses into the blood and the person becomes breathless, especially during exercise. The damage to the alveoli is permanent.