How Is Blood Pressure Generated?
The left lower chamber of the heart (ventricle) receives oxygenated blood from the lungs and pumps it throughout the body. The heart fills with blood between heartbeats. This phase in the heart cycle is called diastole. When the heart pumps to push blood throughout the arteries, this phase is called systole. You can place your fingers on your neck or the inside of your wrist to detect your heartbeat. The pulse you feel is the contraction of the heart's left ventricle.
Several factors influence blood pressure. Blood volume and blood vessel wall behavior are two important determinants of blood pressure. The more blood pumped with each heartbeat, the higher the blood pressure. The presence of stiff or narrow artery walls that resist blood flow also increases blood pressure. Having lower blood volume and open, flexible arteries decreases blood pressure.
Baroreceptors are small nerve cells within arteries close to the heart that help regulate blood pressure. Baroreceptors communicate with the kidneys, arteries, veins, and heart to increase, decrease, or maintain blood pressure, as needed. The function of the baroreceptors is to ensure that sufficient blood reaches the organs and tissues of the body.
If blood pressure becomes too low, baroreceptors send signals to the heart telling it to beat faster and pump more blood per minute. The result is blood flow increases and blood pressure rises. If blood pressure becomes too high, baroreceptors send signals to the veins instructing them to expand and store more blood and return less blood to the heart. The result is blood flow decreases and blood pressure becomes lower. Conversely, veins can become narrower and return more blood to the heart, which increases blood pressure.
Baroreceptors communicate directly with arteries when blood pressure is too high or too low to bring it into a more appropriate level. Baroreceptors tell arteries to constrict when blood pressure is too low to help raise blood pressure. Baroreceptors tell arteries to relax when blood pressure is too high to help lower blood pressure.
Kidneys participate in blood pressure control by regulating urine production. When kidneys pull more water out of the blood, blood pressure decreases. When the kidneys decrease urine output, water remains in the blood and blood pressure increases. The action of the kidneys on blood pressure is slow -- acting over hours to days -- compared to baroreceptor control and other systems that influence blood pressure very quickly.
Lower blood pressure is a good thing as long as it doesn't cause symptoms that could damage organs and tissues of the body. Many people live with low blood pressure but they don't experience any symptoms. If that's the case, the low blood pressure is of no consequence. However, others who have low blood pressure experience dizziness and light headedness. This is an indication that insufficient blood flow is reaching the brain. This, in turn, can lead to weakness, nausea, confusion, and blurry vision. Low blood pressure can affect other organs leading to shortness of breath, fainting, blacking out, chest pain, and cool, clammy skin.
Low blood pressure may be induced by conditions that decrease tension in artery walls or decrease blood volume. Dehydration and bleeding are two examples of conditions that reduce blood volume. Conditions that reduce the amount of blood pumped by the heart -- such as cardiomyopathy and heart attack -- may be associated with lower blood pressure. Injuries to the spinal cord and side effects from certain medications can also reduce blood pressure.
Proper functioning of the central nervous system is necessary to maintain adequate blood pressure. The vagus nerve and adrenaline system of the body work together to affect blood pressure. When the vagus nerve is overstimulated, veins expand, insufficient blood returns to the heart, and blood pressure may decrease. Vasovagal syncope is a term for a type of fainting that occurs when the vagus nerve is overstimulated. Vasovagal syncope may happen to those who are sensitive to pain or cannot stand the sight of blood. The vagus nerve is overstimulated in these cases and fainting occurs. This type of fainting may even occur when straining to urinate or while having a bowel movement.
Conditions not associated with the neurological system may cause low blood pressure. Anything that causes a loss of fluids, including dehydration from diarrhea, vomiting, or bleeding may cause low blood pressure. Adrenal gland dysfunction, pregnancy, and blood loss may lower blood pressure as well. more