Systemic Circulation

Systemic circulation ensures the movement of blood between the heart and the various organ systems in the body.

The steps in systemic circulation can be summarized as:

  1. Oxygenated blood is pushed out of the left ventricle of the heart through semilunar valves and into the ascending aorta.

  2. This oxygenated blood travels through various portions of the aorta (ascending aorta, arch of aorta, thoracic aorta) while the aorta continuously gives off numerous branches.

  3. Those branches in turn branch repeatedly and become progressively smaller in diameter.Smaller arteries or arterioles have a greater proportion of smooth muscle.

    • The diameter of these muscular arteries is accurately monitored by sensors in their walls and controlled by the autonomic nervous system so as to regulate blood flow according to temperature, body activity, and other physiological variables.

  4. These arterioles then break up into capillaries which have walls that only allow the passage of one blood cell at a time.

    • Fenestrated capillaries have ‘windows’ or holes in their walls that allow the passage of quite large molecules; fenestrated capillaries are found in the kidneys, the intestinal walls, and in connective tissue underlying epithelia.

    • Sinusoids are capillaries with larger holes in their walls which allows for quick and efficient exchange of substances between blood and tissues. Sinusoids are found in the liver and tissues in the lymphatic system.

  5. At the capillaries, oxygen leaves the blood cells and moves into the cells of the organ systems. Simultaneously, carbon dioxide (and waste products) leave the cells of the organ systems and enter the blood cells to be removed from the body.

  6. Capillaries unite to form post-capillary venules that drain into a network of low-pressure, high-volume veins which return blood to the heart.

  7. Venules increase in diameter to form veins.

  8. Veins are larger than their corresponding arteries, have a lower rate of blood flow (veins are often surrounded by a dead space occupied by loose connective tissue into which they can expend when full of blood) and contain venous valves. These valves allow flow towards the heart, but fill with blood and occlude the lumen of the vessel when reverse flow starts.

  9. Venous return is aided by the action produced by the contraction and relaxation of the muscles around the vessels.

  10. The veins ultimately return deoxygenated blood from the body’s organ systems back to the right atrium of the heart through the superior vena cava and the inferior vena cava.

  11. This completes systemic circulation.