Final answer:
The aortic pressure is around 120 mm Hg, and the vena cava pressure nears zero as blood returns to the heart. This pressure difference is essential for blood circulation and affected by factors such as vessel compliance and blood viscosity. Position and the total cross-sectional area in the venous system also influence pressure.
Step-by-step explanation:
The pressure difference between the aortic pressure and vena cava pressure is significant as blood moves through the circulatory system due to the mechanics of blood circulation and cardiac function. When blood is pumped from the heart, it leaves at about 120 mm Hg and this high pressure is necessary to overcome the resistance of the systemic circulation. As blood travels through arteries, arterioles, capillaries, and into the veins, its pressure gradually decreases, reaching almost 0 mm Hg by the time it arrives at the vena cava before returning to the heart.
The pressure in the arteries can be affected by various factors including blood viscosity, blood vessel length and diameter, and the compliance of the vessel walls. An increase in arterial pressure without a corresponding increase in cardiac function can lead to decreased blood flow. Conversely, the venous system exhibits the opposite relationship, where an increase in venous pressure can actually lead to increased flow, as the pressure in the veins is normally low. For efficient return of blood to the heart, especially during the atrial diastole, the pressure in the atria has to be even lower than in the veins, approaching zero.
It is important to note that the position of a person, such as standing up, can alter the pressure due to the weight of the blood column. Moreover, the total cross-sectional area of the venous system decreases as we move from smaller venules to the larger veins and vena cava, which also impacts the pressure relationships within the venous system. As for gas exchange, oxygen and carbon dioxide move down their own pressure gradients, independent of each other, and this pressure difference drives their diffusion during capillary-tissue exchange.