Low Blood Pressure (cont.)
John P. Cunha, DO, FACOEP
John P. Cunha, DO, FACOEP
John P. Cunha, DO, is a U.S. board-certified Emergency Medicine Physician. Dr. Cunha's educational background includes a BS in Biology from Rutgers, the State University of New Jersey, and a DO from the Kansas City University of Medicine and Biosciences in Kansas City, MO. He completed residency training in Emergency Medicine at Newark Beth Israel Medical Center in Newark, New Jersey.
In this Article
How is blood pressure generated?
During relaxation of the heart (diastole) the left ventricle of the heart fills with blood returning from the lungs. The left ventricle then contracts and pumps blood into the arteries (systole). The blood pressure in the arteries during contraction of the ventricle (systolic pressure) is higher because blood is being actively ejected into the arteries. It is lower during relaxation of the ventricle (diastolic pressure) when no blood is being ejected into the arteries. The pulse that we can feel when we place our fingers over an artery is caused by the contraction of the left ventricle and the ejection of blood.
Blood pressure is determined by two factors: 1) The amount of blood pumped by the left ventricle of the heart into the arteries, and 2) the resistance to the flow of blood caused by the walls of the arterioles (smaller arteries).
Generally, blood pressure tends to be higher if more blood is pumped into the arteries or if the arterioles are narrow and/or stiff. (Narrow and/or stiff arterioles, by resisting the flow of blood, increase blood pressure.) Arterioles may become narrower when the muscles surrounding them contract. Arterioles may become stiff and narrow when older patients develop atherosclerosis.
Blood pressure tends to be lower if less blood is being pumped into the arteries or if the arterioles are larger and more flexible and, therefore, have less resistance to the flow of blood.
How does the body maintain normal blood pressure?
The body has mechanisms to alter or maintain blood pressure and blood flow. There are sensors that sense blood pressure in the walls of the arteries and send signals to the heart, the arterioles, the veins, and the kidneys that cause them to make changes that lower or increase blood pressure. There are several ways in which blood pressure can be adjusted - by adjusting the amount of blood pumped by the heart into the arteries (cardiac output), the amount of blood contained in the veins, the arteriolar resistance, and the volume of blood.
For example, low blood volume due to bleeding (such as a bleeding ulcer in the stomach or from a severe laceration from an injury) can cause low blood pressure. The body quickly responds to the low blood volume and pressure by the following adjustments which all increase blood pressure:
These adaptive responses will keep the blood pressure in the normal range unless blood loss becomes so severe that the responses are overwhelmed.
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