The cardiovascular system plays a crucial role in maintaining the body’s homeostasis. With each beat, the heart pumps blood through the arterial system, distributing oxygen and nutrients to all the different tissues and organs. Waste products and deoxygenated blood are then moved from these distal organs back through the veins into the heart so it can be pumped back into the lungs for re-oxygenation. Both the distribution and return of blood are equally critical for the system to work.
In the thick walled arteries, the pressure from the heart is strong enough to push the blood to the distant capillaries for diffusion. However, once beyond these, the pressure in the much thinner walled veins is no longer enough to move the blood from the distal tissues back to the heart. Because of this, your body depends on two key components: skeletal muscles and respiration activity.
Veins are made of thin walls, easily bent by any external force (unlike the thick walled arteries). There are one way valves found throughout each vein, particularly in the distal appendages such as the arms and legs, which prevent blood from flowing backwards. As the skeletal muscles surrounding the veins contract (which happens in daily activities) the veins are compressed, forcing the blood to move up through the one way valves to a region where surrounding muscles are relaxed. The blood remains in the new proximal segment until those muscles then contract. In this way, the deoxygenated blood makes its way through the venous system back to the heart.
Respiratory activity also plays a role in venous return to the heart via pressure gradients in the cardiovascular system. The thoracic space is the region above the diaphragm between the chest wall and the organs within it (the lungs, heart, and vena cava to name a few). During inspiration, negative pressure is formed by the downward contraction of the diaphragm, causing not only the decrease in pressure within the lungs, but also a decreased pressure in the right atrium, right ventricle, and the thoracic inferior and superior vena cava. This creates a pressure gradient causing venous return to the right atria to increase. As exhalation occurs, the pressure gradient for venous return to the right ventricle then returns. The faster and more shallow the respirations, the less this pressure gradient helps blood flow and blood movement becomes more dependent on the action of the heart and the skeletal muscles. Conversely, as breathing slows and deepens, the thoracic pumping action caused by pressure changes plays a significant role in the force of blood flow, taking more of the effort away from the heart.
Both skeletal muscles and respiration activity play crucial roles in maintaining a well-functioning cardiovascular system. When disrupted, such as with decreased skeletal muscle movements and quick shallow breathing, blood does not move as well through the body. Blood can begin to pool in distant veins, sometimes breaking the one-way valves as seen in varicose veins. The heart also has to work harder to move nutrients and oxygen to the rest of the organs causing various other complications if this demand is sustained.