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The very fact of the circulation of the blood shows that it is always under pressure which is constantly highest in the arteries and lowest in the veins. Blood pressure can be defined as “the pressure that the blood exerts against the wall of its containing vessels.” The force of the heart contraction is the cause of blood pressure. But this force alone would not furnish a sustained pressure if unaided by other factors. One of them is the pressure that arterial wall exerts upon their contents. The second important factor in sustaining arterial pressure is the resistance to the passage of blood through the small blood vessels, especially the arterioles, and capillaries. This factor is commonly called peripheral resistance. The cause of this resistance is the friction produced by the passage of the viscous blood through minute tubes. This resistance brings up liquid pressure in existence. The third factor, which is of course minor and which modifies the blood pressure is the breathing. During inspiration, the thorax enlarges and some suction is exerted upon all the blood within the thorax, especially upon the large veins near the heart. The veins are affected largely because veins possess comparatively less sturdy and elastic walls. Another modifying factor is gravity. This force tends to increase pressure in arteries at levels below that of the heart and to decrease pressure in arteries at levels above that of the heart.
Arterial Blood Pressure:
Arteries distribute blood through the body under certain pressure. As it exists in the arteries, it is called arterial blood pressure. At the height of ventricular contraction (ventricle systole) pressure in the arteries reaches a maximum. This is the systolic blood pressure. During the relaxation of the ventricle (ventricular diastole) blood pressure tends to fall to a minimum. This is diastolic blood pressure. The difference between these two pressures gives an index of the capacity of the circulatory system to sustain pressure.
In the adult human, the actual pressure observed for the large arteries of the arm normally varies between 115 to 150 mm Hg. The pressure is less in smaller arteries and larger in the aorta. The pressure varies with sex, higher in males than females. It also varies during mental and physical work. It shows a tendency to fall in fatigue, also it shows more or less fluctuations during physical and psychological conditions.
Venous Blood Pressure:
The pressure observed in veins of the limb of mammals is usually less than 10 mm Hg and shows a fairly regular tendency to decrease along the venous system towards the heart. In jugular vein pressure less than 1 mm Hg.
The capillary pressure in different animals is roughly measured and the figure has been obtained between 18 to 40 mm Hg in mammals; which means it is much lowered than the arteries.
Measurement of Arterial Blood Pressure:
The arterial blood pressure is measured in man by means of a sphygmomanometer. This consists of a rubber bag (covered with a cloth envelope) that is wrapped around the upper arm over the brachial artery.
One tube connects the inside of the bag with a manometer containing mercury. Another tube connects the inside of the bag to a hand-operated pump with a release valve.
Method- Ask your partner to sit at ease on a table with the arm resting upon the table and at about the level of the heart. After removing the shirt sleeve from his left arm, place the pressure sleeve snugly on the upper left arm without much pressure and wrap the remaining strip of cloth around the arm and tuck it under the pressure sleeve. There are two methods by which blood pressure can be measured; the palpatory or auscultatory method.
The Palpatory Method:
Only systolic blood pressure can be measured by this method. Place the armlet in the proper position and by using the rubber bulb and screw valve closed, inflate the pressure sleeve; at the same time by using the fingertips of your left hand, feel the radial pulse. Inflate the pressure sleeve (up to 155 mm Hg) until the pulse disappears. Now slowly deflate the pressure sleeve by using the pressure release valve and when the pulse appears, note the reading on the mercury column. This is the systolic pressure.
The Auscultatory Method:
Both the systolic and the diastolic pressures can be measured by this method. Follow the method described above and place a stethoscope bulb over the brachial artery close to the pressure sleeve. Now inflate the pressure sleeve up to 150 mm Hg and by using the pressure release valve, deflate very slowly. At a certain point, a sound is heard with each heartbeat, this shows that the pressure in the brachial artery is capable of overcoming the outside pressure and the blood has started flowing through the artery. Now increase the pressure very slowly and the highest pressure point at which the heartbeat can be heard is the systolic pressure. For measuring diastolic pressure gradually deflate the pressure sleeve and notice the change from sharp and clear clicking to murmuring sound of the blood flow. This sound is similar to the continuous flowing of river water, or it can be limited by placing the index finger on your ear and stroking it with the large next finger. Reduce the pressure still further and the lowest pressure at which the sound can still be heard is the diastolic pressure. Repeat twice and not any change in reading.
Normal systolic blood pressure is considered about 120 mm Hg. and diastolic about 80 mm Hg. These values differ with sex, age, exercise, sleep, etc.