The systems of the human body are usually studied between November and December. In Year 4 of Italian high school, especially in tracks such as the scienze umane (human sciences) high school where biology is a subject with written and oral tests, the first test that carries real weight falls four to eight weeks from now. The format is almost always the same: a multiple-choice test and two or three open questions such as “describe the path of the blood” or “explain how gas exchange happens”. Now is the right moment to build your outline, before the systems turn into a list of names to learn by heart all at once.
In this post I put in order the two systems that tests pair most often, the circulatory and the respiratory, because they explain each other. At the end you will find four typical questions with a model answer. For the more cellular level, that is, how molecules cross membranes, I refer you to plasma membrane, diffusion and osmosis: the diffusion we talk about here is the same.
The heart: four chambers and four valves
The heart is a hollow muscle divided into four chambers: two atria at the top, which receive the blood, and two ventricles at the bottom, which push it out. A wall, the septum, separates the right side from the left side: the blood on the right and the blood on the left never mix.
Between atria and ventricles, and between ventricles and arteries, there are four valves that let the blood pass in one direction only:
| Valve | Where it sits | Side |
|---|---|---|
| Tricuspid | between atrium and ventricle | right |
| Pulmonary | between ventricle and pulmonary artery | right |
| Mitral (bicuspid) | between atrium and ventricle | left |
| Aortic | between ventricle and aorta | left |
The left ventricle has the thickest wall of all, because it has to push the blood around the whole body. The right one sends it only to the lungs, which are close by, and needs less force.
The cardiac cycle
The heart contracts and relaxes at a regular rhythm. The contraction is called systole, the relaxation diastole. A complete beat follows three stages:
- the atria contract (atrial systole) and fill the ventricles;
- the ventricles contract (ventricular systole): the valves between atria and ventricles close, those towards the arteries open and the blood goes out;
- the whole heart relaxes (diastole) and the atria fill again.
At rest an adult has about 60-100 beats per minute. The sound of the heartbeat, the “lub-dub”, is the closing of the valves.
Pulmonary and systemic circulation, step by step
This is the question that comes up most often, and it is best learnt as a path, not as two lists. You start from the right atrium, where the oxygen-poor blood arrives from the body:
- the venae cavae pour the blood into the right atrium;
- through the tricuspid valve it passes into the right ventricle;
- the pulmonary valve opens and the blood enters the pulmonary artery, which divides into two branches, one for each lung;
- in the capillaries of the lungs it gives up carbon dioxide and takes up oxygen;
- the pulmonary veins bring the blood, now oxygenated, back to the left atrium;
- through the mitral valve it passes into the left ventricle;
- the aortic valve opens and the blood enters the aorta, which distributes it to the whole body;
- in the capillaries of the tissues it gives up oxygen and collects carbon dioxide, then the veins bring it back to the venae cavae.
Steps 1 to 5 are the pulmonary circulation (the “small circuit”); from step 5 back to the venae cavae is the systemic circulation (the “large circuit”). The heart sits in the middle and makes them work in series: the blood passes through the heart twice on every complete round.
The vessels: arteries, veins, capillaries
- Arteries: carry the blood away from the heart. They have thick, elastic walls, because they withstand the pressure of every systole.
- Veins: carry the blood towards the heart. They have thinner walls and pocket-shaped valves that prevent backflow.
- Capillaries: very thin vessels, with a wall made of a single layer of cells. This is where exchanges with the tissues happen, and the fact that the wall is so thin is exactly what makes them possible.
The definition to remember is based on direction, not on content: artery = leaves the heart, vein = enters the heart. Oxygen and carbon dioxide depend on which stretch of the circulation you are in.
The blood and blood pressure
An adult has about 5 litres of blood. It is a tissue made of a liquid part, the plasma (water with salts, proteins, nutrients), and a cellular part:
- red blood cells, which carry oxygen and have no nucleus;
- white blood cells, which defend the body;
- platelets, fragments of cells that are needed for clotting.
Blood groups (A, B, AB, O) depend on the molecules present on the surface of red blood cells; this is why compatibility has to be checked in transfusions.
Blood pressure is the force that the blood exerts on the walls of the arteries. It is measured in mmHg with two values: the systolic (during systole) and the diastolic (during diastole). A normal value in a healthy adult is around 120/80 mmHg.
The respiratory system: from the airways to the alveoli
The air follows a path: nose (it makes the air warmer, moister and cleaner), pharynx, larynx, trachea, two bronchi, which branch into ever smaller bronchioles and end in the alveoli. The alveoli are tiny sacs with a very thin wall, surrounded by a network of capillaries: they are the only places where oxygen really passes into the blood. There are very many of them, and this is what gives the lungs an enormous exchange surface.
How we breathe: the mechanics
The lungs have no muscles of their own: they inflate and deflate because the volume of the chest cavity changes.
- Inhalation: the diaphragm contracts and moves down, the ribs lift, the volume of the rib cage increases, the internal pressure falls below the external one and the air comes in.
- Exhalation (at rest): the muscles relax, the volume decreases, the pressure rises above the external one and the air goes out.
Air always moves from where the pressure is higher to where it is lower. If you understand this, you do not have to learn by heart which muscle does what.
Gas exchange: diffusion
In the alveoli and in the tissues, gases are exchanged by diffusion, that is, they pass from where their partial pressure is higher to where it is lower, without spending energy. With approximate values:
| Gas | Air in the alveoli | Blood arriving at the lungs | Direction |
|---|---|---|---|
| Oxygen | about 100 mmHg | about 40 mmHg | alveolus → blood |
| Carbon dioxide | about 40 mmHg | about 46 mmHg | blood → alveolus |
In the tissues the opposite happens: the blood arrives with plenty of oxygen, the cells have used some up and have little, so the oxygen leaves the capillary and the carbon dioxide enters it. The direction is always the same: from more to less.
Transport with haemoglobin
Oxygen dissolves poorly in the plasma, so almost all of it travels bound to haemoglobin, a protein of the red blood cells. Each haemoglobin molecule contains four haem groups, and each one binds one molecule of oxygen: four O molecules in total. Where oxygen is abundant, as in the lungs, haemoglobin binds it; where it is scarce, as in the tissues, it releases it. Carbon dioxide travels partly dissolved in the plasma, partly bound to haemoglobin and largely in the form of bicarbonate ions.
How they work together
The two systems are a single chain: the respiratory system brings oxygen from the air to the blood and removes carbon dioxide; the circulatory system distributes it to all the cells and carries the waste back. If the heart stops, the lungs are of no use; if the lungs do not work, the heart pumps oxygen-poor blood. This is why, during exertion, heart rate and breathing rate rise together: the cells use more oxygen and produce more carbon dioxide. The link with cellular metabolism, that is, where the oxygen is used, is in glycolysis, the Krebs cycle and oxidative phosphorylation, while for the energy side of the whole body there is basal metabolism.
Four typical questions with a model answer
1. Describe the path of the blood from the vena cava to the aorta. From the venae cavae the blood enters the right atrium, passes through the tricuspid valve into the right ventricle and, through the pulmonary valve, into the pulmonary artery, which carries it to the lungs. Here, in the capillaries, it is oxygenated; the pulmonary veins carry it to the left atrium, then through the mitral valve to the left ventricle and, through the aortic valve, to the aorta.
2. Why is the pulmonary artery an artery even though it carries oxygen-poor blood? Because arteries are defined by direction, not by content: they carry the blood away from the heart. The pulmonary artery leaves the right ventricle and carries blood rich in carbon dioxide to the lungs.
3. Explain why oxygen passes from the alveoli into the blood. By diffusion: the partial pressure of oxygen in the alveolar air (about 100 mmHg) is greater than that in the blood arriving at the lungs (about 40 mmHg), so the oxygen moves towards the blood without using energy. Carbon dioxide follows the opposite difference and goes the other way.
4. What happens to the diaphragm during inhalation and why does the air come in? The diaphragm contracts and moves down, the volume of the chest cavity increases and the pressure in the lungs falls below atmospheric pressure; air moves from the higher pressure to the lower, so it comes in.
The mistakes that keep coming up
- “Artery = oxygenated blood, vein = oxygen-poor blood”: true almost everywhere, false in the pulmonary circulation. It is the classic trick question.
- Right and left the wrong way round: the diagram of the heart is drawn as if the body were facing you, so the “right” atrium sits on the left of the page. Repeat it with the path, not with the drawing.
- Confusing the systemic and pulmonary circulation: the pulmonary one goes from the right ventricle to the lungs and returns to the left atrium; the systemic one goes from the left ventricle to the body and returns to the right atrium.
- Saying that the lungs “suck in” the air: it is the rib cage that moves, and the lungs follow it.
Practice and next step
There is no dedicated practice test for biology: the practice tests for high school only cover maths and physics, so I am not suggesting one for this topic. The method I recommend works all the same: draw the path of the blood by hand starting from a blank page, then repeat the four questions above aloud without looking; if you get stuck at some point, that is where you need to revise. For the science subjects in general, the page on science subjects explains how I work.
If you would like me to look at where you lose marks in your biology tests, write to me — the first introductory call, in which we work out together where to restart from, is free.
