Biology Basics: Systemic Circulation

Systemic circulation brings oxygenated blood around to all your body’s cells. Here is a description of how blood moves through this pathway (see the figure):

1. The pulmonary veins push oxygenated blood into the left atrium.

   When the left atrium relaxes, the oxygenated blood drains into the left ventricle through the left AV valve.

2. As the left ventricle contracts, the oxygenated blood is pumped into the main artery of the body — the aorta.

   To get to the aorta, blood passes through the aortic semilunar valve, which serves to keep blood in the aorta from flowing back into the left ventricle.

3. The aorta branches into other arteries, which then branch into smaller arterioles, carrying oxygenated blood all around your body.

   Throughout your body, arterioles meet up with capillaries where oxygen is exchanged for carbon dioxide.

4. Through capillary exchange, oxygen leaves red blood cells in the bloodstream and enters all the other cells of the body.

   Capillary exchange also allows nutrients to diffuse out of the bloodstream and into other cells. At the same time, the other cells expel waste products, including carbon dioxide, that then enter the capillaries.

   Your capillaries are only as thick as one cell, so the contents within them can easily exit by diffusing through the capillaries’ membranes. And because the capillaries’ membranes touch the membranes of other cells all over the body, the capillaries’ contents can easily continue moving through adjacent cells’ membranes.

5. The deoxygenated blood moves into the smallest veins, called venules, and then into bigger veins until it reaches the vena cava.

   The two branches of the vena cava enter the right atrium, which is where pulmonary circulation begins.

Answer the following practice questions about systemic circulation before you go on.

1. Using a different color ink than you used to mark your pulmonary circulatory pathway, shade in your systemic circulatory pathway in the figure.

2. Place the following terms in order, ranking them from vessels that carry blood with the least oxygen to those that carry blood with the most oxygen.

   1. Venules

   2. Capillaries

   3. Arteries

   4. Veins

   5. Arterioles

3. Imagine that you’re a blood cell in the kidneys. In sequential order, name all the chambers and valves of the heart you’ll travel through between the time you leave the kidney and the time you return.

   For questions 4–7, use the following terms to label the structures of the systemic pathway of your circulatory system in the figure.

   1. Aorta

   2. Vein

   3. Artery

   4. Capillaries

      

4. You should have shaded the entire rest of the circulatory system (except for the loop from the heart to the lungs and back to the heart).

5. The answer is Veins→Venules→Capillaries→Arterioles→Arteries.

6. The answer is Kidneys→Capillary→Venule→Vein→Inferior Vena Cava→Right atrium→Right AV valve→Right ventricle→Pulmonary semilunar valve→Pulmonary artery→Capillaries in lungs→Pulmonary vein→Left atrium→Left AV valve→Left ventricle→Aortic semilunar valve→Aorta→Artery→Arteriole→Capillaries in kidneys.

   The following is how the figure should be labeled:

7. 1. Aorta

8. 2. Artery

9. 4. Capillaries

10. 2. Vein.