Blood pressure is the force of the blood through the vessels of the circulatory system. Blood pressure maintains circulatory homeostasis by pumping the blood from the left ventricle of the heart through the aortic semilunar valve into the aorta to the arteries, which propel the blood away from the heart and to the organs and tissues of the body. The blood is then pushed through the capillaries to the venules and into the veins, which carry the blood back to the heart via the venae cavae. The pressure in the circulatory system must run down a gradient scale from the aorta to the venae cavae, with the pressure being highest in the aortic arteries and lowest in the venae cavae, in order to maintain circulation.
As the blood flows through the arteries to the arterioles the pressure drops significantly, however the pressure does not deplete completely in the capillaries. The blood pressure produced by the heart continues to push the blood through the capillaries to the venules and into the veins; this pressure does not dissipate completely until it reaches the venae cavae and reenters the right atrium of the heart.
The valves within the veins are bicuspid valves with two flaps that help control the flow of blood through the veins as it moves back to the venae cavae. These valves help push the blood through the veins while also preventing any backflow. The arteries do not need valves; as the pressure from the pumping heart rapidly propels the blood outwards with high force, backflow is not a concern in the arteries. The decreased pressure in the veins however, cause the blood to move much slower than in the arteries and the valves are needed to keep the blood flowing in the proper direction and move the blood from the lower extremities back up to the heart. If the veins in our legs did not have valves, the blood would not be able to overcome gravity and flow back up to either to the liver or the heart. Valves are just one of the mechanisms the body uses to control venous blood flow; the contractions of the heart, the arterial blood pressure, contractions of skeletal muscle, and the pressure within the thoracic cavity created by breathing all play a role in maintaining central venous pressure and ensuring the proper flow of blood through the entire body.
Overall systemic blood pressure is affected by many factors; the strength of the heart’s contractions, the heart rate, blood volume, blood viscosity, and peripheral resistance to blood flow are all major factors in maintaining a healthy blood pressure.
About 5L of blood flows through a healthy heart every minute. Strong, healthy myocardium pushes more blood out of the left ventricle into the aorta with each contraction; weaker myocardium produces a lower stroke volume and reduces the cardiac output, resulting in decreased blood pressure. The rate of the heart’s contractions also influences blood pressure. An increased heart rate has the potential to both raise and lower blood pressure. If the stroke volume remains the same and the heart rate increases, blood will move through the circulatory system faster, increasing the cardiac output and raising the blood pressure; however, if the heart begins to beat too rapidly to allow the left ventricle to fill with blood, the stroke volume decreases, lowering the blood pressure.
The volume of blood in the arteries directly affects systemic blood pressure in a simple cause and effect manner: more blood in the vessels causes higher blood pressure, while less blood in the vessels decreases blood pressure. The blood’s viscosity is another important factor in regulating blood pressure. Thicker blood, weighed down by extraneous red blood cells, glucose, lipids and other substances, flows through the small arterioles and venules much slower than blood of normal viscosity. Thin blood, on the other hand, flows too quickly and decreases blood pressure.
Peripheral resistance refers to any influence that impedes blood flow through the vessels. Many different factors can create peripheral resistance. Tension in the muscles of the walls of the vessels constricts the pathway, slowing the blood flow and decreasing the blood pressure. Increased viscosity is also an example of peripheral resistance that can result in lowered blood pressure.
Blood pressure is regulated by a delicately interwoven system that works to maintain consistent circulatory homeostasis. Any change in this system can have a domino effect on other primary regulators. Blood volume is directly connected to the strength and rate of the heart’s contractions. Peripheral resistance can be caused by thicker blood viscosity and can cause backups in the vessels, which are unable to propel the volume of blood flowing through the circulatory system.
Thibodeau GA, Patton KT (2008). Structure and function of the body (13th Ed). St. Louis, MO: Mosby-Elsevier Inc.