0610 · 11.1Gas exchange in humans

Key processes and airway protection

0:00Walk quickly upstairs and you will probably notice your breathing becoming faster and deeper. But your lungs are not muscles that simply suck air in, and breathing is not the same thing as respiration. So what actually moves the air, and why does exercise change how you breathe? In this video, we will connect the structure of your breathing system to gas exchange, ventilation and the changes you can investigate.

0:20First, separate three connected processes. Ventilation means moving air into and out of the lungs, while gas exchange means oxygen and carbon dioxide moving between air and blood. Respiration is the chemical reactions in cells that break down nutrient molecules and release energy for metabolism. For this topic, remember that aerobic respiration uses oxygen and produces carbon dioxide; the reactions themselves belong in the video on aerobic respiration. We also need diffusion, which is the net movement of particles from a region of higher concentration to a region of lower concentration, as a result of their random movement.

0:55Oxygen diffuses into blood when its concentration is higher in the air inside the lungs. Carbon dioxide diffuses in the opposite direction when its concentration is higher in the blood. Breathing and blood flow maintain these concentration differences. Follow the route taken by incoming air. It passes through the larynx, or voice box, and down the trachea, the main airway running towards the chest. The trachea divides into two bronchi, with one bronchus entering each lung.

1:20Inside the lungs, these branch repeatedly into smaller bronchioles, which lead to tiny air sacs called alveoli. Around the lungs are the ribs, with intercostal muscles between neighbouring ribs. Below the lungs is the diaphragm, a sheet of muscle separating the thorax from the abdomen. The thorax is the chest region containing the lungs. In diagrams, identify structures by their position and connections, rather than relying on a particular colour or drawing style.

1:45The trachea must remain open while air passes through it. Cartilage supports its wall and keeps the trachea open, preventing it from collapsing. Think of it as reinforcement around a flexible tube, rather than something that pumps air. Its supporting rings are a useful identifying feature in diagrams of the trachea. Incoming air can contain particles and pathogens, which are organisms that cause disease. Goblet cells in the airway lining make mucus, and this mucus traps pathogens and particles.

2:11Nearby ciliated cells have tiny hair-like structures called cilia on their surface. The cilia beat to move the mucus up towards the throat, where it can be swallowed. The mucus and cilia therefore do different jobs. Mucus traps material, while the beating cilia transport that material away from the lungs. This helps prevent particles and pathogens reaching the delicate gas exchange surfaces. Cilia do not filter air by themselves, and goblet cells do not move the mucus.

Alveoli and efficient diffusion

2:36Now zoom in to an alveolus, one of the tiny air sacs. Its wall lies very close to the wall of an associated capillary, a small blood vessel. Oxygen diffuses from the air in the alveolus into the blood, while carbon dioxide diffuses from the blood into the alveolus. The gases cross the thin walls separating these two spaces. An effective gas exchange surface needs a large surface area and a thin surface.

3:00The many alveoli together provide a large surface area, allowing more gas to diffuse at the same time. The alveolar and capillary walls are each only one cell thick, creating a short diffusion distance. Imagine many small doorways with a very short passage through each one. The other two important features are a good blood supply and good ventilation with air. Blood flow brings carbon dioxide to the alveoli and carries oxygen away, helping maintain concentration differences.

3:26Ventilation replaces air in the alveoli, bringing in oxygen and removing carbon dioxide. Without this renewal on both sides, the concentration differences would decrease and net diffusion would slow.

Mechanics of breathing

3:37How does ventilation happen if the lungs do not pull themselves open? The diaphragm and intercostal muscles change the volume of the thorax, and these changes alter air pressure. Air moves from higher pressure to lower pressure. The external intercostal muscles lie in the outer layer between the ribs, while the internal intercostal muscles lie further inward. During inspiration, the external intercostal muscles contract and the internal intercostal muscles relax.

4:02The ribs move upwards and outwards, while the diaphragm contracts and flattens. These movements increase the volume of the thorax, and the lungs expand with it. The important connection is that muscle contraction changes the space available, rather than directly dragging air through the trachea. As the lungs expand, the air inside occupies a larger volume and its pressure falls below atmospheric pressure. Atmospheric pressure is the pressure of the air outside your body.

4:29Air therefore moves into the lungs down this pressure difference. The sequence is muscle action, increased thorax volume, decreased pressure, then inward airflow. During quiet expiration, the external intercostal muscles and diaphragm relax. The diaphragm returns to its dome shape, and the ribs move downwards and inwards as the chest and lungs recoil. Thorax volume decreases, so pressure inside the lungs rises above atmospheric pressure. Air then moves out of the lungs.

4:56Forced expiration, such as deliberately blowing air out strongly, involves contraction of the internal intercostal muscles. They pull the ribs downwards and inwards, helping reduce thorax volume. Do not assume that these muscles actively contract during every normal outward breath, because quiet expiration mainly results from relaxation and recoil. Also notice that a relaxed diaphragm is domed, not flattened.

Inspired and expired air

5:18Inspired air is the air you breathe in, and expired air is the air you breathe out. Inspired air contains about twenty-one per cent oxygen and about zero point zero four per cent carbon dioxide. Expired air contains about sixteen per cent oxygen and about four per cent carbon dioxide. These are approximate values, not fixed figures for every breath. Expired air has less oxygen because oxygen diffuses into the blood and is used by cells in aerobic respiration.

5:44It has more carbon dioxide because cells produce carbon dioxide during respiration, and blood carries it to the lungs. The water vapour content of inspired air changes with the weather, but expired air is always saturated with water vapour. Water evaporates from the moist surfaces of the breathing system into the passing air. This comparison also corrects a common misconception: expired air is not just carbon dioxide. It still contains a substantial amount of oxygen, because not all the oxygen entering the lungs diffuses into the blood.

6:13Nor do the lungs simply change oxygen into carbon dioxide. The composition changes because gases are exchanged with blood and water evaporates into the air. You can investigate the carbon dioxide difference using limewater. Carbon dioxide turns limewater milky, or cloudy. Compare separate samples by passing room air through one tube of limewater and expired air through another, using teacher-approved apparatus. Use equal volumes of fresh limewater and pass equal volumes of air through them to make the comparison fair.

6:41Under these conditions, expired air should produce cloudiness sooner or greater cloudiness, because it contains more carbon dioxide. Room air also contains carbon dioxide, so it may eventually turn limewater cloudy too. The test demonstrates a difference in carbon dioxide content; it does not directly measure oxygen. For safety, never suck through the apparatus, inhale through limewater or share mouthpieces.

Exercise and breathing changes

7:04Now investigate what physical activity does to breathing. Let a student rest first, then count their breaths over a fixed time, treating one complete inspiration and expiration as one breath. Repeat after a standard period of suitable exercise, measuring immediately afterwards because breathing changes during recovery. Repeat the investigation and keep the exercise duration and measurement period consistent. Rate tells you how often someone breathes, but depth tells you how much air moves in each breath.

7:31You can observe whether chest movements become larger, although this is only a rough indicator of depth. Suitable breathing-volume apparatus, used under supervision, can measure the air volume more directly. Physical activity normally increases both the rate and the depth of breathing. Let us work through an illustrative set of results, not results from a real experiment. Suppose a student takes eight breaths in thirty seconds at rest and sixteen breaths in thirty seconds after exercise.

7:56To express each rate per minute, we need to account for the fact that one minute contains two periods of thirty seconds. We therefore multiply each breath count by two. The resting rate is sixteen breaths per minute, and the rate after exercise is thirty-two breaths per minute. In this example, the rate has doubled. However, counting breaths alone cannot tell us whether breathing depth has doubled, or even give its exact value.

8:18That needs a separate observation or measurement of the volume moved per breath. Why does exercise produce these changes? Working muscles carry out more respiration, producing more carbon dioxide, so the carbon dioxide concentration in the blood increases. This increase is detected by the brain, which leads to an increased rate and greater depth of breathing. Greater ventilation removes carbon dioxide more rapidly and also helps supply the additional oxygen needed by the working muscles.

8:46For an explanation question, saying that you breathe faster because you need more oxygen is incomplete. The required control link is increased carbon dioxide concentration in the blood, detection by the brain, and increased breathing rate and depth. Rate and depth are separate changes, so include both. Faster breathing means more breaths per minute, whereas deeper breathing means more air moved per breath.

Exam answers and recap

9:08Let us finish with an exam-style question: explain how inspiration occurs. Start with the muscles, then follow the changes in volume and pressure before describing airflow. This creates a causal explanation, rather than a disconnected list of structures. A precise answer can be built in the following sequence. The external intercostal muscles contract and the internal intercostal muscles relax. The ribs move upwards and outwards. The diaphragm contracts and flattens.

9:35The volume of the thorax increases. The pressure inside the lungs falls below atmospheric pressure. Air moves into the lungs down the pressure difference. Each sentence gives a distinct, creditworthy point in the explanation. Avoid saying that air enters first and makes the chest expand, because that reverses the sequence being tested. For diagram questions, follow the branching airway from trachea to bronchi, bronchioles and alveoli, and identify capillaries around the alveoli.

10:01For practical questions, distinguish an observation, such as limewater turning cloudy, from its explanation, such as a higher carbon dioxide content. When asked to describe a difference, state what changes. When asked to explain it, connect that change to the biological process responsible. Here is the whole picture. Muscles change thorax volume and pressure to ventilate the lungs, while diffusion exchanges gases across the thin alveolar and capillary walls. Large surface area, a thin surface, good blood supply and good ventilation make this exchange effective.

10:32Goblet cells, mucus and ciliated cells help protect the airways on the journey to these surfaces. Expired air contains less oxygen, more carbon dioxide and more water vapour than typical inspired air. During physical activity, increased blood carbon dioxide is detected by the brain, leading to faster and deeper breathing. Keep ventilation, gas exchange and respiration separate, but understand how they support one another. That connection is what turns a collection of facts into a working breathing system.