The cardiovascular system lies at the heart of human movement. In this unit, learners explore the structure and function of the heart and blood vessels, the dynamics of blood flow, and the mechanisms that regulate circulation both at rest and during exercise. The new IB SEHS specification requires students to understand the anatomy of the heart, the cardiac cycle, stroke volume, heart rate, cardiac output, blood pressure, and the redistribution of blood during exercise. In addition, learners examine intrinsic, neural and hormonal control of cardiac function, as well as long-term adaptations to regular physical activity. This unit builds a foundation for understanding oxygen transport, endurance performance, and cardiovascular health.

At its core, the cardiovascular system has one primary task: to transport oxygen, nutrients, hormones, and heat around the body while removing waste products such as carbon dioxide and lactic acid. It consists of the heart (the pump), blood vessels (arteries, veins, capillaries), and the blood itself. Together, these components maintain homeostasis and support the body's increased demands during exercise.

The heart is a double pump, with the right side sending deoxygenated blood to the lungs via the pulmonary circulation and the left side pumping oxygenated blood to the rest of the body via systemic circulation. A cardiac cycle comprises systole (contraction) and diastole (relaxation), which together produce the familiar "lub-dub" heart sounds and propel the movement of blood through the chambers.

Key variables include:

Stroke volume (SV): the amount of blood ejected by the left ventricle per beat.

Heart rate (HR): the number of beats per minute.

Cardiac output (Q): the total volume of blood pumped per minute (Q = HR × SV).

During exercise, all three variables increase to meet the rising oxygen demands of working muscles. The increase in cardiac output is particularly crucial, as it determines the rate at which oxygen and nutrients are delivered.

Control of heart rate is multifactorial. Intrinsic regulation involves the sinoatrial node (SA node), which acts as the heart’s natural pacemaker. Neural control via the autonomic nervous system adjusts heart rate through sympathetic stimulation (increasing HR) or parasympathetic activity (slowing it). Hormonal regulation includes the release of adrenaline and noradrenaline during exercise, which accelerates the heart rate and enhances cardiac output.

Blood flow is redistributed during exercise through a process called vascular shunting, prioritising muscles over internal organs. This is achieved through vasodilation (widening) of arterioles leading to active tissues and vasoconstriction of vessels supplying less active areas.

With regular training, the cardiovascular system undergoes significant long-term adaptations. These include a lower resting heart rate, increased stroke volume, greater cardiac output during exercise, and improved capillarisation of muscle tissue. These changes enhance endurance capacity, recovery, and overall cardiovascular efficiency.