The Cardiovascular System
Introduction
The cardiovascular system, consisting of the heart and the blood vessels, plays a critical role in the maintenance of homeostasis and health. The primary function of the cardiovascular system is to supply the various body tissues with adequate quantity of oxygen and nutrients, and remove carbon dioxide and other waste products of metabolism. In addition, the cardiovascular system transports hormones to the target organs that play important role in the regulation of the body’s functions. The requirements of different tissues for oxygen and nutrients vary according to the level of activity, e.g. during exercise the skeletal muscle metabolic rate increases progressively with intensity. Furthermore, the circulatory homeostasis is profoundly affected by stressors such as orthostasis and exercise. It is therefore critical that the cardiovascular system is regulated on a moment-to-moment basis. The cardiovascular system comprises the heart and the blood vessels consisting of arteries, veins and capillaries.
The Heart
The heart that acts as a “pump”, consisting of two atria and two ventricles, is divided into a left and a right side by a septum that runs along its longitudinal axis. The main function of the atria is to optimise ventricular filling, while that of the ventricles is to generate sufficient force to overcome the vascular resistance and propel blood in a pulsatile manner. The right side of the heart propels blood around the pulmonary circulation, while the left side propels blood to the systemic circulation. The contractions (systole) and relaxations (diastole) of the cardiac muscle (the myocardium) occur in a coordinated and rhythmical fashion, allowing an efficient and constant blood delivery around the body. The heart rate (HR) is dictated by the depolarization of cells in the sinus node. The sinus node is constantly under the influence of the autonomic nervous system, by the action of the cardiac vagal and sympathetic traffic that regulate cardiac activity, even though the sinus node cells may undergo self-depolarisation, termed intrinsic HR.
The blood Vessels – Arteries, Capillaries and Veins
Blood flows through a network of specialized vessels that can be broadly categorized in arteries, capillaries, and veins. The walls of arteries and veins contain three layers, the tunica externa, tunica media, and tunica interna. The tunica externa forms a connective tissue sheath that stabilizes the position of the vessel. The tunica media contains smooth muscle and connective tissue fibres such as elastin. The tunica interna includes the endothelium and its underlying elastic membranes. The arterial system includes elastic arteries, muscular arteries, and arterioles. Close to the heart the arteries are large, while towards the periphery the diameter of the vessels decreases and their walls become relatively thin as their number increases.
Capillaries are the smallest blood vessels, measuring 5-10 μm, which connect arterioles and venules, and are the main sites of substance exchange between blood and tissue cells. Diffusion across capillary walls depends on the organization of the endothelium, the size of the diffusing molecule, and its lipid solubility. Individual capillaries are usually part of an organized capillary plexus, or capillary bed. Pre-capillary sphincters determine the relative volume of flow through each of the capillaries. Blood flow through a capillary plexus changes as vasomotion (change in diameter of blood vessels) occurs. The entire network may be bypassed by blood flow through arteriovenous anastomoses or via preferred channels within the capillary plexus.
Venules are small veins that collect the blood leaving a capillary network. They merge into medium-sized veins; these convey the blood to large veins including the superior and inferior vena cavae that empty in the heart. The arterial blood pressure (BP) is highest in the aorta, and it decreases progressively in smaller arteries, reaching the lowest values in the large veins. Given the low BP in the veins, special valves are necessary to prevent the backflow of blood. Furthermore, dynamic muscular contractions (the muscle pump) rhythmically compress the blood vessels and contribute significantly to venous return to the heart. In addition, the pressure changes in the pleural cavity associated with breathing also assist in moving blood towards the heart.