Basic Course on the Human Cardiac Circulatory System

The heart and vascular system is a vast, closed system within the human body. Blood circulates within this system, carrying nutrients to all parts of the body to maintain various vital functions. The left and right sides of the heart are not directly connected; each atrium is connected to a ventricle, but valves act as gates, ensuring that blood can only flow from the atria to the ventricles and not back. The left ventricle connects to the aorta, which branches out to tissues and organs throughout the body, down to the smallest capillaries. The right ventricle connects to the pulmonary artery, which branches out to form pulmonary capillaries. Oxygen- and nutrient-rich arterial blood departs from the left ventricle, first entering the arms and brain, then the middle and lower parts of the body, supplying nutrients to organs and tissues along the way. It then becomes venous blood, gradually flowing into the large veins, and finally into the superior and inferior vena cava, returning to the right atrium. This process is called "systemic circulation" and takes approximately 40 seconds. Blood flows from the right atrium through the valves into the right ventricle, then from the right ventricle through the pulmonary artery to the lungs. In the pulmonary capillaries, venous blood becomes arterial blood and returns to the left atrium via the pulmonary veins. This process is called "pulmonary circulation" and takes about 20 seconds. In the human body, systemic and pulmonary circulation occur simultaneously and continuously. The left and right ventricles contract and relax simultaneously, and the amount of blood pumped by systemic and pulmonary circulation is the same. The heart is the driving force propelling blood. Blood pressure generally refers to systemic arterial blood pressure, which is simply the pressure exerted by blood on the walls of blood vessels as it flows through them; it is the driving force propelling blood flow. When the ventricles contract, blood flows from the ventricles into the arteries, causing a sharp rise in aortic pressure. The highest value reached is called systolic pressure, also known as high pressure. When the ventricles relax, the arteries elastically recoil, and blood continues to flow slowly forward, but aortic pressure decreases. The lowest value reached is called diastolic pressure, also known as low pressure. The difference between systolic and diastolic pressure is called pulse pressure. Generally, arterial blood pressure refers to aortic blood pressure. Because blood pressure drops very little in larger arteries, in practice, the brachial artery blood pressure measured in the upper arm is often used to represent the aortic blood pressure. For a long time, mercury manometers have been used to measure blood pressure, and therefore, the height of the mercury column is conventionally used to express blood pressure values, i.e., millimeters of mercury (mmHg). Arterial blood pressure is an important indicator of cardiovascular function and an important indicator of overall functional status. Sufficient circulating blood volume, normal cardiac function, and peripheral resistance are the three factors that maintain stable overall blood pressure. Stable blood pressure is one of the important conditions for promoting blood circulation and ensuring sufficient blood perfusion to various tissues and organs. Only when all tissues and organs receive sufficient blood perfusion can their biochemical metabolism and physiological functions proceed normally. The various blood vessels that circulate during blood circulation have different functional characteristics. Blood vessels are mainly divided into elastic reservoir arteries, muscular distributing arteries, arterioles, and arterioles. Large arteries include the aorta, the main pulmonary artery, and its major branches, such as the common carotid artery. These blood vessels are characterized by thick walls, rich in elastic fibers, and have significant elasticity and dilatability, which are important for maintaining normal blood pressure. When the left ventricle pumps blood into the aorta, it causes an increase in aortic pressure. This increased pressure propels the blood forward and dilates the aorta, increasing its volume. A portion of the blood ejected by the heart is stored in this dilated aorta. When the left ventricle finishes pumping and begins to diastole, the valve at the junction of the aorta and left ventricle closes, temporarily stopping blood flow into the aorta. At this time, the dilated aorta, due to reduced pressure, contracts, pushing the stored blood further to the periphery. This is the elastic reservoir function of the aorta, which buffers systolic pressure and maintains diastolic pressure. Although the heart pumps blood intermittently, the blood in the peripheral vessels flows continuously. If atherosclerosis occurs in the aorta, the elastic reservoir function is significantly impaired, and blood pressure becomes highly unstable. Arterioles and capillaries have small diameters and high resistance to blood flow, forming a major part of the peripheral resistance in blood circulation. Blood flows through capillaries, whose walls are very thin and highly permeable; these are where substances are exchanged between the fluids inside and outside the blood vessels. Compared to their corresponding arteries, large veins are more numerous, have larger diameters, and thinner walls, thus holding more blood and exhibiting better dilatation; even small pressure changes can cause significant changes in their volume. At rest, the entire venous system holds 60%–70% of the body's circulating blood volume, acting as a blood reservoir.