Systemic Circulation
The systemic circulation describes the extensive collection of arteries that distribute oxygenated blood to the tissue of the body and return deoxygenated blood to the heart.
Once returned to the right atrium of the heart, deoxygenated venous blood is pumped from the right ventricle into the pulmonary circulation where it is oxygenated in the alveoli of the lungs before being returned to the left atrium of the heart. This freshly oxygenated blood then moves into the right ventricle of the heart where a forceful muscular contraction of the heart's cardiac muscle expels the oxygenated blood into the aorta.
Because the coronary circulation derives from coronary arteries that almost immediately branch off the aorta, the coronary circulation can be described as a specialized part of the systemic circulation.
Blood in the aorta blood passes upward (superiorly) through the artic arch. From this arch, the aorta gives rives to a number of important arterial groups, including the carotid system of arteries (internal and external carotids) that ultimately branch to form smaller arteries that supply oxygenated blood to the head and neck, and the subclavian arteries that supply blood to the shoulder and upper limbs. After turning downward, the arch of the aorta continues as the thoracic aorta and then the abdominal aorta, before ultimately dividing into the iliac arterial system that serves the lower limb. All along its course, the aorta gives off branches of arteries that supply particular structures, organs, and tissues.
Each branch of the aorta gives off increasing smaller arteries or arterioles before multiplying (ramifying) into a capillary bed that most intimately services the target tissue or organ. It is within the capillaries, with membranes as thin as one cell, that gas and nutrient exchange takes place between blood, tissue, and interstitial fluids.
Ultimately, deoxygenated blood and metabolic waste products are removed from target tissues through a venous network. Smaller venules fuse together to create larger vessels. Small valves help keep the blood flowing toward the heart and venous pools help assure constant blood supply to the heart. The larger veins are classified by the region they drain, and the particular vessel through which they return blood to the heart.
Veins that drain blood from the head, neck, thorax, and upper limbs ultimately direct that blood into the right atrium of the heart through the superior vena cava. Blood from the abdomen, pelvis, and lower limbs returns to the right atrium of the heart through the inferior vena cava. A coronary sinus collects blood from cardiac veins and enters the right atrium near a point where the inferior vena cava enters the chamber.
The flow of blood within the systemic circulatory system flows a rhythmic pattern of higher and lower pressure associated with systole and diastole in the cardiac cycle. Accordingly, the flow of blood in the arterial system is pulsating rather than smooth and continuous.
At any given time, the venous portion of the systemic circulatory systems contains approximately 65% of the total volume of blood in the body. Blood in the systemic arterial system accounts for about 15 to 12.5% of the total blood volume.
Regulation of the systemic circulatory system is mostly achieved through the autonomic nervous system. Accordingly, sympathetic and parasympathetic stimulation can greater change vessel size and the flow of blood to particular organs, muscles groups, and tissues. Circulating hormones also play a regulatory role in altering systemic circulation and blood pressure. The proper maintenance of blood pressure within physiological limits is important. Low pressures reduce the availability of blood to tissues (tissue perfusion) and pressures that are too high can lead to stroke or embolism.
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