General characteristics of the circulatory system
Humans have a four-chambered heart, consisting of two atria and two ventricles, and two circulatory loops — systemic and pulmonary. The systemic circuit supplies blood to the entire body, while the pulmonary circuit ensures gas exchange in the lungs. The circulatory system includes three types of vessels: arteries, capillaries, and veins, along with the blood circulating through them.
Types of blood vessels
Arteries carry blood away from the heart. Their walls are three-layered, elastic, and able to withstand high pressure. Arteries exhibit the highest blood flow velocity and the greatest blood pressure. Rhythmic contractions of arterial walls form the pulse. Typically, arteries transport oxygenated blood, but the pulmonary arteries of the pulmonary circuit carry deoxygenated blood.
Veins carry blood to the heart. The wall also consists of three layers, but it is capable of stretching; the blood pressure and flow velocity are lower. A characteristic feature is the semilunar valves, which prevent the backflow of blood. Pulmonary veins are an exception: they carry arterial blood.
Capillaries are the smallest vessels with a single-layer epithelial wall and no muscle fibers. It is in the capillaries that oxygenated blood turns into deoxygenated blood (and vice versa in the pulmonary circuit). Blood flow velocity in capillaries is minimal, and pressure is moderate.
Blood composition and functions
Blood is a liquid connective tissue consisting of plasma and formed elements. Plasma contains water, mineral salts, and organic substances (including the protein fibrinogen).
- Transport — transport of gases, nutrients, metabolic waste products, hormones;
- Protective — immunity, coagulation, cessation of bleeding;
- Regulatory — maintenance of body temperature and water-salt balance;
- Homeostatic — maintenance of pH and osmotic pressure.
Formed elements of blood
Erythrocytes (red blood cells) are anucleated biconcave cells containing the protein hemoglobin (a globulin with a heme group and an iron ion). They are formed in the red bone marrow, live for 120–130 days, and are destroyed in the spleen and liver. They transport oxygen and carbon dioxide and help maintain pH.
Leukocytes (white blood cells) are nucleated cells capable of independent movement and exiting the vascular bed. They live for 3–5 days. They carry out cellular and humoral immunity, and phagocytosis — the capture and digestion of foreign particles. An accumulation of dead leukocytes forms pus.
Platelets (thrombocytes) are anucleated blood cell fragments that live for 7–10 days. They are capable of adhesion (attaching to vessel walls) and aggregation (clumping together). They participate in blood clotting (coagulation) and tissue regeneration.
Hemolysis and blood coagulation
Hemolysis is the destruction of red blood cells under the action of mechanical damage, chemical agents, hypotonic solutions, electric current, freezing, or heating.
Stages of blood coagulation:
- Vascular wall injury;
- Movement of platelets to the site of injury and their destruction with the release of thromboplastin;
- Conversion of prothrombin into thrombin with the participation of calcium ions;
- Conversion of soluble fibrinogen into insoluble fibrin;
- Formation of a network of fibrin fibers in which blood cells are trapped — forming a blood clot (thrombus).
Heart structure
The heart is surrounded by a pericardial sac — the pericardium, consisting of fibrous and serous layers; pericardial fluid reduces friction. The heart wall includes three layers: epicardium (outer connective tissue layer), myocardium (middle muscular layer), and endocardium (inner epithelial layer, forming the valves).
Between the right atrium and right ventricle lies the tricuspid valve, and between the left atrium and left ventricle is the bicuspid (mitral) valve. Semilunar valves are located at the exit of the ventricles. The valves ensure the unidirectional flow of blood. The myocardium of the left ventricle is significantly thicker than that of the right because it initiates the systemic circulation, which requires a powerful contraction.
Cardiac cycle and automatism
Cardiac automatism is the ability of the myocardium to independently generate nerve impulses through specialized cells of the conduction system. A full cardiac cycle lasts 0.8 seconds and includes three phases:
- Atrial systole (0.1 s): cuspid valves are open, semilunar valves are closed, blood flows into the ventricles;
- Ventricular systole (0.3 s): cuspid valves close, semilunar valves open, blood is ejected into the aorta and pulmonary trunk;
- General diastole (0.4 s): muscles relax, atria fill with blood, which passively flows into the ventricles.
Circulatory loops
Systemic circulation begins in the left ventricle and ends in the right atrium: left ventricle → aorta → arteries → arterioles → tissue capillaries (oxygenated blood becomes deoxygenated) → venules → veins → superior and inferior venae cavae → right atrium.
Pulmonary circulation begins in the right ventricle and ends in the left atrium: right ventricle → pulmonary trunk → pulmonary arteries (carrying deoxygenated blood) → arterioles → lung capillaries (deoxygenated blood becomes oxygenated) → pulmonary veins → left atrium.
Key points
- The heart is four-chambered; septa and valves prevent the mixing of oxygenated and deoxygenated blood.
- In the pulmonary circuit, arteries carry deoxygenated blood, and veins carry oxygenated blood.
- The highest pressure and blood flow velocity are in the arteries; the lowest velocity is in the capillaries; the lowest pressure is in the veins.
- All formed elements are formed in the red bone marrow and destroyed in the spleen and liver.
- Hemoglobin is a globulin-class protein with an iron ion that transports oxygen and carbon dioxide.
- Blood coagulation is a cascade of reactions: thromboplastin → thrombin → fibrin → blood clot.
- The cardiac cycle (0.8 s) includes atrial systole, ventricular systole, and general diastole.
- The systemic circuit runs from the left ventricle to the right atrium, while the pulmonary circuit runs from the right ventricle to the left atrium.