The circulatory system or cardiovascular system is the organ system which circulates blood around the body of most animals.
The Human Cardiovascular System
The human cardiovascular system comprises the blood, the heart, and a dual-circuit system of blood vessels that serve as conduits between the heart, the lungs, and the peripheral tissues of the body. The respiratory system and renal system are intricately interrelated to cardiovascular function; it is generally held that these systems must be understood in relation to each other, if they are to be understood at all.
The blood consists of a yellow fluid called 'plasma,' in which are suspended erythrocytes (a.k.a., red blood cells), leukocytes (a.k.a., white blood cells), and thrombocytes (a.k.a., platelets). Blood is the medium by which oxygen and nutrients are delivered to the peripheral tissues of the body, and by which carbon dioxide and other metabolic wastes are removed. Furthermore, the blood is a chief delivery mechanism for the endocrine and immune systems. The blood also serves as a chemical buffer, maintaining the body's overall pH within a desirable range.
The heart is the muscular organ which pumps the blood via its inherent contractile activity. The heart can be viewed as two separate pumps--the right-sided pump serving the pulmonary circulation (see below) and the left-sided pump serving the systemic circulation (see below). Simply stated, each side of the heart has a receiving portion (an atrium) and a pumping portion (a ventricle).
The vascular system is made up of arteries, veins, and capillaries. Arteries are blood vessels that carry blood away from the heart. Veins are blood vessels that return blood to the heart. Capillaries are the smallest blood vessels, and are the locus of nutrient and gas exchange between the blood plasma and the peripheral tissues of the body.
The Cardiac Cycle and Cardiovascular Circuitry
The cardiac cycle is classically divided into seven discrete phases, but will be radically simplified here. For a more detailed description, please refer to the cardiac cycle.
The cardiac cycle consists of alternating periods of relaxed filling and active ejection of blood. These actions are attributable to the inherent contractile activity of the heart (which is, after all, made of specialized muscle fibers). Cardiac contraction is coordinated by a specialized pacemaker-and-conduction system (see the heart for more details). During the diastolic phase of the cardiac cycle, the ventricles are filling with blood. During the systolic phase, the right and left ventricles are contracting, forcing their collected blood into the pulmonary and systemic circuits, respectively. Although it is a gross oversimplification, the atria can be thought of as continually filling.
The pulmonary and systemic circuits are routes-in-sequence. Blood is pumped from the right heart to the lungs, returns to the left heart, then is pumped to the periphery, then returns to the right heart to begin the sequence anew. This enables a perpetual cycle of oxygenation, systemic delivery, metabolic deoxygenation, and return.
The primary purpose of the pulmonary circulation is to oxygenate the blood. Deoxygenated blood returning from the peripheral tissues of the body collects in the right atrium of the heart, fills the right ventricle during diastole, and is ejected into the pulmonary arteries during systole. These arteries carry the blood to the lungs, where it passes through a capillary network close to air-filled alveoli. This enables the release of carbon dioxide and the uptake of oxygen from the air.
The now oxygenated blood returns to the left atrium via the pulmonary vein, fills the left ventricle during diastole, and is ejected into the aorta, the major artery which supplies blood to the body via its numerous branches. This is the starting point for the systemic circulation, which consists of the intricate network of arteries, arterioles, capillary beds, venules, and veins that service the peripheral tissues of the body (which include the brain and other organs, the skeletal muscles, etc.
The major functions of the renal system are to filter metabolic wastes from the blood passing through the peripheral circulation and to mediate systemic blood pressure via the renin-angiotensin-II-aldosterone system. Compromise of renal function, as occurs in widely-varying conditions, inherently disposes the patient to toxic states and/or cardiovascular pathologies.
Common Cardiovascular Disorders
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