Circulatory System — REET Notes

Structure of the heart

The human heart is a muscular organ with four chambers — two upper chambers called atria (left and right atrium), and two lower chambers called ventricles (left and right ventricle). The two atria are the chambers that receive blood returning to the heart, while the two ventricles are the chambers that pump blood back out of the heart.

The right and left sides of the heart are completely separated by a wall called the septum, which stops oxygenated and deoxygenated blood from mixing. This separation matters because the right side of the heart only ever handles deoxygenated blood on its way to the lungs, while the left side only ever handles oxygenated blood on its way to the rest of the body — the septum is what keeps these two separate flows from mixing inside the same organ.

The ventricle walls are thicker than the atrial walls because the ventricles have to pump blood out to the whole body or the lungs, which needs more force than simply passing blood down from an atrium into the ventricle just below it. Of the two ventricles, the left one has the harder job, since it has to push blood all the way around the body through the aorta — which is why its wall is the thickest of all four chambers.

Heart valves and their location

All these valves let blood flow in only one direction, which keeps circulation efficient. Each one works like a one-way gate: it opens as blood moves forward and closes the moment blood tries to flow backward, so once blood has left a chamber it cannot slip back into it.

Double circulation — pulmonary and systemic circulation

In humans, blood passes through the heart twice, which is why it's called double circulation. In pulmonary circulation, the right ventricle sends deoxygenated blood to the lungs via the pulmonary artery, where it becomes oxygenated and returns to the left atrium via the pulmonary vein. After that, in systemic circulation, the left ventricle sends this oxygenated blood to the whole body via the aorta, and deoxygenated blood returning from the body reaches the right atrium via the venae cavae. Put simply, one loop carries blood between the heart and the lungs to pick up oxygen, and the second loop carries that oxygenated blood between the heart and the rest of the body to deliver it — together, these two loops make up one complete double circulation.

There's one exception in this sequence worth remembering — the pulmonary artery carries deoxygenated blood (whereas arteries normally carry oxygenated blood), and the pulmonary vein carries oxygenated blood (whereas veins normally carry deoxygenated blood). Both loops begin and end at the heart, but it is specifically at the pulmonary loop's outward and return vessels — the pulmonary artery and the pulmonary vein — that this naming exception occurs. This is a point that comes up again and again in REET.

Types of blood vessels

It's precisely the thin walls of capillaries that make the exchange of substances between tissue and blood possible. Arteries, by contrast, need thick and elastic walls simply to withstand the force of blood being pumped into them, while veins can have thinner walls because the blood inside them is under much lower pressure and relies on valves, rather than wall thickness, to keep moving in one direction.

Blood components and other key facts

Plasma

The liquid part of blood, in which nutrients, hormones, and waste products stay dissolved as they are carried around the body

Red blood cells (RBCs)

Carry oxygen via haemoglobin; mature human RBCs have no nucleus, which is what lets them carry more haemoglobin and more oxygen

White blood cells (WBCs)

Protect the body from infection (immunity) as part of the immune system; unlike RBCs, they have a nucleus

Platelets

Also called thrombocytes; help blood clot at the site of an injury, which stops excess bleeding

Heart-rate regulator

The SA node (sino-atrial node), located in the right atrium, is the heart's natural pacemaker structure and regulates the rhythm of the heartbeat

Heart rate, pulse rate, and blood groups

A healthy adult's normal resting heart rate is roughly 72 to 80 beats per minute, which can be felt as a pulse by placing a finger on the wrist or neck. This rate can rise temporarily during physical activity, stress, or fever, while regular exercise can improve resting heart rate over time. This same resting range, and the fact that it changes with activity, stress, or illness, is exactly what the classroom pulse-counting activity described below is meant to help students notice for themselves.

Blood groups were discovered by Karl Landsteiner, who classified blood into four main groups under the ABO system — A, B, AB, and O. Knowing blood groups is extremely important for blood donation and blood transfusion, since mixing incompatible blood groups can cause a serious reaction in the body. This ABO classification is exactly why a donor's and a recipient's blood group are always matched before a transfusion is carried out.

How to teach this to students

Practice questions

Frequently asked questions

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