Figure 1: Diagram of different body positions
Background reading: Sherwood, Chapter 10.
Objectives
1) Identify the factors that affect blood pressure and explain their influence.
2) Understand why there is differential pressure within arteries and how it can be measured.
3) Define systolic, diastolic, mean arterial pressure, cardiac output, stroke volume and heart rate.
4) Examine the effect of exercise on heart rate and blood pressure.
Blood pressure is the force exerted by blood on vessel walls. Pressure is therefore dependent on several factors: volume of blood and the compliance of the vessel walls. Blood is pumped through the circulatory system by the heart, which, for every cardiac cycle, results in large changes in blood volume from the left ventricle of the heart into the aorta. This means then that arterial blood pressure changes with the beating of the heart. Blood pressure reaches its peak when the ventricle contracts, pushing blood into the aorta, increasing the blood volume, this is called systolic blood pressure. Blood pressure is at its lowest between beats, while the ventricle is relaxing and blood is flowing through the arteries, decreasing the volume of blood just pumped into the aorta from the ventricle, (see figure 10-6, Sherwood). This pressure is diastolic blood pressure.
The systolic and diastolic blood pressures indicate the highest and lowest pressures seen in the circulatory system and these numbers are of clinical value. An average blood pressure (in a non-diseased individual) is about 120 mmHg /80mmHg. Elevated blood pressures indicate the possibility of arterial disease because blood pressure increases as compliance of the blood vessels decrease. Chronically elevated blood pressure can lead not only to advancing arterial disease but eventually to cardiac disease.
Mean arterial pressure, (MAP) is calculated according to the following equation, see also figure 10-7, Sherwood:
MAP= diastolic pressure (in mmHg) + 1/3 pulse pressure (measured as the difference between systolic pressure and diastolic pressure, in mmHg)
Mean arterial pressure, describes an average pressure in the arteries during the cardiac cycle. Mean arterial pressure is the force that drives the flow of blood through the circulatory system.
In order to ensure circulation of blood to body tissues, the circulatory system must branch into smaller and smaller vessels (decreasing the radius). The cardiac output is distributed throughout the body according to the needs of organ systems. In other words, distribution of blood can be regulated in various physiological states (i.e. after digestion or after exercise). For this reason, we now consider blood flow through the circulatory system. Blood flow is described by Pouseille's law, which takes factors into consideration such as blood viscosity and length of the tubes, as well as changes in pressure and radius (along that length of tube). We observe mainly that, blood flow is proportional to the change in pressure from one end of an artery to the other and, as well, blood flow is proportional to the radius of the artery.
Blood pressure is highest in the arteries and lowest in the veins, and this ensures that arterial blood flows away from the heart (figure 10-9, Sherwood). While flow is decreased when considering one capillary (see figure 10-16 purple panel--the velocity of flow (mm/sec), (thus allowing nutrient and waste exchange), when considering the entire capillary bed associated with the supply arteriole, the net flow of blood continues to flow away from the heart (see figure 10-16 red panel).
The radius of the tubes which carry blood varies throughout the circulatory system and therefore also affect blood flow. Arteries, blood vessels with the largest diameter, have the highest flow rate and because of their thick, elastic walls which act as high pressure reservoirs, keeping the driving force of blood flow. When an artery reaches an organ, the vessel branches into arterioles, which have a much smaller radius. Therefore, blood pressure in the arterioles drops, this further encourages blood flow through the cardiovascular system. In addition, control of blood flow is regulated by the diameter of the arterioles. Therefore, when exercise causes an increased demand for blood flow to the skeletal muscles, arterioles serving the skeletal muscles vasodilate (increase in diameter) and arterioles serving the digestive tract and kidneys vasoconstrict (decrease in diameter). Blood flow is slowest through capillaries. Because capillaries are the site of nutrient exchange, a slower blood flow optimizes the exchange of nutrients and waste products between tissue cells and blood.
Blood pressure can be determined directly. For a direct measurement, a transparent tube can be attached to a needle, which, in turn, is inserted into an artery. The height of the column of blood in the tube is a measure of pressure in mm of blood. The indirect method is non-invasive, and is therefore less likely to affect its own measurement.
The principle upon which the indirect measurement is based is the fact that sound is produced when blood flows past a constriction. When a cuff encircling an artery is inflated, the artery collapses when the cuff pressure exceeds arterial pressure. Since blood flow is stopped, no sound can be heard distal to the cuff. If the cuff pressure is slowly lowered, eventually it will equal systolic pressure. At this time, blood will flow past the constriction when it reaches its highest pressure and will thus produce sound (Sounds of Korotkoff). Eventually cuff pressure will only be able to narrow the artery when arterial pressure has its lowest value (during diastole). If cuff pressure drops any lower, sound disappears. Thus, the cuff pressure when sound first appears is equal to systolic pressure and when the sound last occurs, it is equal to diastolic. However, most people listen for a distinct change in tone rather than total disappearance of sound for diastolic pressure. See especially Figure 10-8 (page 350, Sherwood )
Blood Pressure Determination.
1. Securely wrap a blood pressure cuff around your subject's upper arm with arrow labeled "artery" over the brachial artery. Palpate for the artery on the medial aspect of the arm deep to the biceps brachii muscle. There is a "thin spot" where the underlying humerus is close to the skin.
2. Insert the ends of the Stethoscope into the ear with the tips angled forward.
3. Close the valve on the rubber bulb and inflate the cuff to 160 mm Hg (or until the needle on the sphygmomanometer stops jiggling but don't exceed 200 mm Hg).
4. Hold the stethoscope bell over the radial artery at the anterior surface of the arm where it bends at the elbow.
5. Slightly open the valve and slowly deflate the cuff (take about 10 sec) while listening.
6. Note the pressure reading when you first hear the sound of blood flowing and again when the tone becomes muffled. The first reading is systolic pressure and the second is diastolic. Record as a ratio S/D. NOTE: If you miss the reading, completely deflate the cuff and start again. The cuff cuts off blood flow.
7. Practice until you feel comfortable with the procedure.
Pulse Rate Determination.
1. Place your middle finger over the radial artery on the thumb side of the anterior surface of your forearm just proximal to the wrist. NOTE: Don't use your thumb, or you may feel your own pulse.
2. When the second hand of a watch reaches some reference point, start counting each pulse, beginning with "0".
3. Terminate the count after some reasonable interval, (e.g. 15, 30, or 60 sec) and correct to one minute. For example if you count for 15 sec, multiply the count by 4. No correction is necessary for a 60 sec count, and accuracy is greater with a longer time. The result is pulse rate in beats/min.
Effect of Posture.
Determine your subject's heart rate and blood pressure every two minutes until successive blood pressure readings are nearly identical in the following positions: 1) sitting; 2) standing; and 3) reclining.
Figure 1: Diagram of different body positions
Effect of Exercise.
1. This test is a group effort and requires 4 people.
a) Subject - Select a subject with no history of heart or other problems that would cause an adverse reaction from strenuous exercise.
b) Blood Pressure Taker
c) Heart Rate Taker
d) Recorder who writes down all values for the entire group.
2. Measure your subject's resting pulse rate and blood pressure (BP).
3. With the pressure cuff still attached, have the subject exercise vigorously for 5 min (step up and down on a stool at a rate of one step/sec, or carefully run up and down stairs, or go outside and run around campus).
4. As soon as exercise stops, immediately record pulse rate and BP. Repeat at 30 sec intervals for the first 3 min, at 30 sec intervals for the next 2 min, and at 60 sec intervals until you reach a steady state (i.e. blood pressure measures level off). NOTE: If you can't hear through the stethoscope, use the jiggling of the sphygmomanometer needle to indicate blood flow.
Link to information about the effect of exercise on blood pressure