Biology12-3.3C Blood Pressure

Overview

Normal blood pressures vary from species to species and from person to person. A giraffe has very high blood pressure to prevent it from getting light headed. For humans, average blood pressure changes as a person ages. Older people have an average blood pressure of 140/95. For young adults the average is 120/80 and for infants it is 80/45. About one in five North Americans has high blood pressure, which is a major risk factor for heart attack and stroke.

Systolic/Diastolic Pressure

Now let's consider the two numbers in a blood pressure measurement. The first number recorded is called the systolic pressure, and it occurs during the contraction of the ventricles of the heart. This pressure matches the pressure at which the ventricles begin to force blood in the brachial artery past the arm cuff. The second number is called the diastolic pressure, which occurs when the ventricles of the heart are relaxed. It matches the pressure at which the blood in the brachial artery can move past the arm cuff continuously.

Explain systolic blood pressure and diastolic blood pressure.

Systolic blood pressure is measured when the ventricles contract. Diastolic pressure is measured when the ventricles relax.

Describe how a sphygmomanometer and a stethoscope are used to determine a patient's blood pressure.

Whether blood pressure is measured by a doctor with a cuff and stethoscope or by you with a cuff and digital monitor, the basic principle is the same. In both cases, the cuff goes over the brachial artery on the inside of the upper arm. The cuff is filled with air to a pressure of about 170 to180 mmHg, which stops the flow of blood through the artery. Air is then released from the cuff. The larger or top number of the blood pressure reading matches the pressure at which the heart can force blood through the brachial artery past the cuff during ventricle contraction. The smaller or bottom number matches the pressure between ventricular contractions, when blood flows freely through the brachial artery and past the cuff. A doctor using a stethoscope listens for the movement of blood through the artery below the cuff.

Summary

demonstrate how to measure blood pressure understand what blood pressure measurements mean explain the differences between systolic and diastolic pressures define hypertension and hypotension, and list their causes

What are the names of the pulses located in your neck and wrist?

neck = carotid pulse, wrist = radial pulse

What is the name of the instrument used to measure blood pressure?

sphygmomanometer

What is a normal blood pressure reading for a young adult?

120/80 (Systolic pressure = 120 mmHg, diastolic pressure = 80 mmHg)

What is Normal Blood Pressure?

Blood pressure is a measure of the force exerted by blood on the walls of the blood vessels. Blood pressure in arteries is highest when the ventricles are contracting (called systole or systolic pressure) and lower when the ventricles are relaxed (called diastole or diastolic pressure). For humans, the average values for systolic and diastolic pressures are approximately 120 mmHg and 80 mmHg, respectively. This blood pressure reading is often written as 120 over 80 or 120/80. Blood pressure is greatest in the arteries at 120 mmHg during systole, and arteries have thick, elastic, muscular walls to accommodate this. Blood pressure is lowest in the vena cava (0 to 20 mmHg) because it is furthest from the heart. As blood moves away from the heart, the branching of arteries into arterioles and arterioles into capillaries reduces blood pressure and blood velocity. This reduction in pressure and velocity is due to the huge increase in cross-sectional area in the capillaries. The lower velocity in the capillaries allows the time necessary for effective capillary tissue-fluid exchange. Blood leaving the capillaries converges into venules and then into veins. This results in a decrease in cross-sectional area and an increase in the velocity of blood flow, even though the pressure remains low.

Measuring Blood Pressure

Blood pressure is the pressure exerted by blood on the walls of blood vessels. Blood pressure drops with increased distance from the heart due to the branching of blood vessels and the corresponding increase in cross-sectional area. The effect is much like a mighty river fanning out into a delta. Once the pressure in the capillary beds drops, it cannot be increased until the blood returns to the heart. The lower pressure in veins explains the importance of one-way valves and muscular movement to return blood to the heart. Blood pressure measurement using a digital blood pressure monitor is demonstrated in this lesson. You may have taken your blood pressure at the pharmacy with a similar unit. The technical name for the instrument that measures blood pressure is a sphygmomanometer. The basic principle behind blood pressure measurement is the same, whether done by a doctor with a cuff and stethoscope or by you with a cuff and digital monitor. In both cases, the cuff goes over the brachial artery on the inside of the upper arm. The cuff is then filled with air to a pressure of about 170 to 180 mmHg, which stops the flow of blood through the artery. Air is then released from the cuff. The larger or top number in the blood pressure matches the pressure at which the heart can force blood through the brachial artery past the cuff during ventricle contraction. The smaller or bottom number in the blood pressure matches the pressure at which blood begins to flow freely through the brachial artery and past the cuff in between ventricular contractions. A doctor using a stethoscope listens for the movement of blood through the artery below the cuff

Hypertension

Hypertension is a condition in which a person's blood pressure is above normal. High blood pressure exists when the systolic pressure exceeds 140 mmHg or the diastolic pressure exceeds 90 mmHg. Causes of hypertension include sympathetic nerve activity that constricts blood vessels, too much salt in the diet or reabsorption of sodium ions from the kidneys, and artherosclerosis. Artherosclerosis is caused by fat deposits in the walls of arteries. These deposits reduce the size of the lumen and a greater resistance to blood flow, which increases the blood pressure.

Hypotension

Hypotension is a condition characterized by below normal blood pressure. Hypotension can be caused by problems with the autonomic nervous system or by blood loss following a severe trauma. Patients with low blood pressure are prone to fainting. Fainting can be a benefit because it gets the head into a lower position, making it easier for it to receive blood from the heart.