Final answer:
The pressure difference driving blood through the systemic capillaries is between the arterial blood pressure and the venous pressure. This pressure gradient facilitates the exchange of oxygen and carbon dioxide between the blood and tissue cells. Additional systemic circulation elements like the skeletal muscle and respiratory pumps aid venous return to the heart.
Step-by-step explanation:
The approximate pressure difference between the venous pressure and the arterial blood pressure is the pressure driving blood through the systemic capillaries. Arterial blood pressure at the point where the blood leaves the heart is about 120 mm Hg, which then decreases through the arterial system. Meanwhile, venous pressure, at the point where the blood reaches the venous end of the capillaries, is around 18 mm Hg. This gradient in pressure from the arterial system to the venous system drives the blood through the capillaries, facilitating the exchange of gases, nutrients, and waste products between the blood and the tissues.
In the systemic capillaries, oxygen (Po₂) is about 100 mm Hg, but in the tissue cells, Po₂ is 40 mm Hg. This pressure gradient drives the diffusion of oxygen out of the capillaries and into the tissue cells. Similarly, blood carbon dioxide pressure (Pco₂) is 40 mm Hg, and systemic tissue Pco₂ is 45 mm Hg, thus driving CO₂ out of tissue cells and into the capillaries.
The combined effects of blood flow and the systemic venous return mechanisms, such as the skeletal muscle pump and the respiratory pump, contribute to the flow of blood back toward the heart. The blood ultimately returns to the lungs for gas exchange before being pumped back into the arterial system.