What are the three most important variables that affect blood pressure?


1.) Cardiac Rate- How fast the heart beats (heart rate). The pace at which the heart beats, or heart rate, is counted in heartbeats per minute. Generally, when heart rate increases, blood pressure rises. When heart rate decreases, blood pressure drops.

2.) Stroke Volume- (determined primarily by the blood volume) The amount of blood ejected from each ventricle at each heart beat. When you're resting, stroke volume is about the same as the amount of blood that veins carry back to the heart. But under stressful conditions, the nervous system can increase stroke volume by making the heart pump harder. Stroke volume can also be affected by certain hormones, drugs, and diseases, as well as increases or decreases in the amount of blood in the body, called blood volume.

3.) Peripheral Resistance- The resistance of blood flow through the arterial system. Peripheral resistance is largely a function of the radius of small arteries and arterioles. The resistance to blood flow is proportional to the fourth power of the radius of the vessel.
Blood traveling in narrower vessels encounters more resistance than blood traveling through a wider vessel. Depending on what a person is doing, the amount of blood the heart pumps varies enormously. Yet the blood pressure normally remains pretty stable. That's mainly because the body adjusts the resistance of the arteries, either widening or narrowing them as appropriate, to prevent the blood pressure from swinging wildly. This ability to regulate the width of the blood vessels is called the peripheral resistance. Most of the resistance to blood flow in the circulation occurs in the small-diameter arteries called arterioles. These arterioles are very important in the immediate regulation of blood pressure. That's because they contain specialized smooth muscle in their walls that can relax or contract, allowing the blood vessels to get wider or narrower.

Blood Pressure

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Two Reflexes that help maintain blood pressure within limits.

Baroreceptor Reflex: Baroreceptor reflex helps to maintain normal blood pressure on a beat-to-beat basis. In order for blood pressure to be maintained within limits, specialized receptors for pressure are needed. These baroreceptors are stretch receptors located in the aortic arch and in the carotid sinuses. They are constantly active, producing a baseline frequency of action potentials in their sensory neurons. When your blood pressure is increased, the walls of the aortic arch and carotid sinuses stretch, this produces an increased frequency of action potentials along the sensory nerve fibers.

Baroreceptor Reflex

Atrial Stretch Reflex: As venous return increases, the pressure in the superior and inferior vena cava increase. This results in an increase in the pressure of the right atrium, which stimulates the atrial stretch receptors. These receptors in turn signal the medullary control centers to decrease parasympathetic tone via the vagus nerve to the heart, leading to increased heart rate, known as tachycardia. Increasing the heart rate serves to decrease the pressure in the superior and inferior vena cavae by drawing more blood out of the right atrium. This results in a decrease in atrial pressure, which serves to bring in more blood from the vena cavae, resulting in a decrease in the venous pressure of the great veins. This continues until right atrial blood pressure returns to normal levels, upon which the heart rate decreases to its original level.

The affects on blood pressure serve greatly in the medical field. Every time a patient enters the doctor's office or the hospital, a blood pressure is needed. A nurse must know the normal ranges in blood pressure to determine if the patient has a normal blood pressure, high blood pressure, or low blood pressure. It is very important to be able to determine the blood pressure reading to help with maintaining a healthy balance for the body.

References:

Human Physiology Tenth Edition Stuart Ira Fox
You Tube- Blood Pressure

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