Cardiovascular System Overview, Animation

(USMLE topics, cardiology) Functions of the circulatory system, anatomy and basic physiology of the heart, components of blood and structure of blood vessels.

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The cardiovascular system consists of the heart, blood, and blood vessels. The best-known function of the circulatory system is perhaps the transport of inhaled oxygen from the lungs to body’s tissues, and removal of carbon dioxide in the opposite direction to be exhaled. Basically, oxygen-poor blood from the body returns to the right side of the heart, where it is pumped to the lungs. In the lungs, blood picks up oxygen and releases carbon dioxide. Oxygen-rich blood then returns to the left side of the heart. This part of the system is called the pulmonary circuit. The left side of the heart pumps oxygen-rich blood to body’s tissues, where it unloads oxygen and picks up carbon dioxide. The resulting deoxygenated blood again returns to the heart’s right side to complete the cycle. This part is the systemic circuit.
The heart is enclosed in a double-walled protective sac called the pericardium. The pericardial cavity contains a fluid which serves as lubricant and allows the heart to contract and relax with minimum friction. The heart wall has 3 layers: epicardium, endocardium, and myocardium.
The contraction of the heart muscle is initiated by electrical impulses, known as action potentials. The impulses start from a small group of cells called the pacemaker cells, which constitute the cardiac conduction system. The primary pacemaker is the SA node, it initiates all heartbeats and controls heart rate. Apart from transporting gases, the blood also supplies body’s tissues with nutrients and removes metabolic wastes. It receives nutrients from the digestive system, wastes are filtered from the blood in the kidneys and removed in urine.
The blood also carries hormones from endocrine glands to target organs, and plays an important role in the body’s immune defense.
The blood has two main components: a clear extracellular fluid called plasma, and the so-called formed elements which include red blood cells, white blood cells and platelets. Arteries and veins essentially serve to conduct blood, their walls consist of 3 layers: loose connective tissue, a middle layer of mostly smooth muscles, and an inner layer consisted of thin squamous endothelium.
In general, larger vessels have more connective tissue and smooth muscle. In addition, arteries have more muscles than veins because they carry blood away from the heart and must withstand higher pressures generated by the beating of the heart.
The walls of capillaries, whose function is to exchange substances between the blood and surrounding tissue, consist solely of a thin endothelium with its basement membrane, thus permitting easy diffusion of blood solutes.