Capillaries are only one cell thick to facilitate efficient exchange of materials between the blood and surrounding tissues. Their thin walls, composed of a single layer of endothelial cells, minimise the distance over which oxygen, nutrients, and waste products like carbon dioxide and urea must diffuse. This structural simplicity allows for the rapid and direct transfer of essential substances to and from the bloodstream, ensuring that cells receive the necessary components for metabolism and waste products are promptly removed. The extensive network of capillaries throughout the body further maximises this exchange efficiency.

The body regulates blood flow to different organs through a process known as vascular autoregulation, involving the dilation and constriction of arterioles. When an organ requires more oxygen and nutrients, such as during exercise, the arterioles supplying that organ dilate. This vasodilation increases blood flow to the organ. Conversely, when less blood is needed, the arterioles constrict (vasoconstriction), reducing blood supply. This regulation is primarily achieved through local metabolic changes (like increased carbon dioxide or decreased oxygen levels), neural mechanisms, and hormonal influences. Additionally, the body can divert blood away from less critical areas to prioritise organs with immediate, vital needs, such as the brain and heart.

Valves in veins play a crucial role in maintaining unidirectional blood flow back to the heart, especially against gravity. Given that venous blood pressure is relatively low, these valves prevent backflow, ensuring a steady return of deoxygenated blood. They are particularly important in the legs, where blood must travel a considerable distance against gravity. In contrast, arteries do not have valves because the blood pressure in them is high, ensuring a continuous forward flow. The elastic walls of arteries also help maintain pressure and flow, reducing the need for valves.

During increased physical activity, the heart adapts through several mechanisms to meet the heightened demand for oxygen and nutrients. Firstly, the heart rate increases, a response primarily mediated by the sympathetic nervous system and the release of adrenaline. This results in more frequent contractions of the heart, enhancing blood flow. Secondly, stroke volume, the amount of blood pumped with each contraction, also increases, due to stronger contractions of the heart muscle. These adaptations result in a significant increase in cardiac output (heart rate x stroke volume), effectively supplying more oxygenated blood to the muscles and other tissues actively engaged in the physical activity.

The left ventricle's muscularity is greater than the right ventricle's due to its demanding role in the circulatory system. While the right ventricle only needs to pump deoxygenated blood to the nearby lungs, the left ventricle is tasked with pumping oxygenated blood throughout the entire body. This systemic circulation requires much higher pressure to overcome the greater resistance and distance involved in reaching all bodily tissues. The thick muscular walls of the left ventricle are therefore essential to generate this high pressure, ensuring efficient blood flow to maintain the metabolic activities of various organs and tissues.