Blood Pressure and Blood Flow Help

Bllod Pressure Gradient

Blood pressure is critically important because it is the main force causing blood flow. There is a blood pressure gradient ( GRAY -dee-unt), a series of downward “steps” from a region of higher to a region of lower blood pressure. Within the systemic circulation, for example, the BP is highest within the aortic arch. As the aortic arch progressively branches into smaller and smaller arteries farther away from the heart, the BP also declines. This creates a blood pressure gradient or series of downward steps in blood pressure in vessels farther from the heart. Hence, the blood is pushed down a BP gradient, so that it flows into lower-pressure arteries in a series of steps.

Taking Blood Pressure

One of these farther arteries is called the brachial ( BRAY -kee-al) artery, which is named for its location within the upper “arm” (brachi). The brachial artery is the most frequent site used for taking someone’s blood pressure, utilizing a stethoscope and sphygmomanometer ( sfig -moh- muh -NAHM-uh-ter). A sphygmomanometer is literally “an instrument used to measure” (-meter) the “throbbing pulse” (sphygmo) at certain “intervals” (mano).

To be sure, one does hear a dull throbbing sensation when the bell of a stethoscope is placed over the brachial artery and the arm cuff of a sphygmomanometer device is inflated around it. The sphygmomanometer is usually marked off in units of millimeters of mercury, abbreviated as mmHg (Hg is the chemical symbol for the element, mercury).

Systolic Bllod Pressure

As the air is slowly let out of the inflated arm cuff, the first dull throbbing noise one hears through the stethoscope is called the systolic ( sis-TAHL -ik) blood pressure, or SBP. The systolic blood pressure (SBP) is the pressure created by the systole (contracting and emptying phase) of the left ventricle of the heart. A slug of blood is pushed out of the left ventricle with its contraction. Flowing progressively down a BP gradient, this slug of blood finally enters the brachial artery. The blood bulges out the artery somewhat as it passes through it, thereby creating a thumping sound. For a resting adult, the systolic blood pressure (SBP) is usually recorded as about 120 mmHg.

Diastolic Blood Pressure

The diastolic ( DIE -ah-stahl-ik) blood pressure, or DBP, is the blood pressure associated with the diastole (relaxing and filling) phase of the left ventricle. Upon thoughtful reflection, you might well ask, “Why isn’t the diastolic BP just 0 mmHg? Isn’t diastole the resting and filling phase, when the ventricle isn’t even contracting or creating any blood pressure? So, why should there even be any diastolic BP, at all?”  Good question! Glance again at Figure 16.4.

Fig. 16.4 Blood pressure (BP), blood flow, and the brachial artery.

Note that the brachial artery bulges out with the force of the systolic BP against its walls. Now, when the ventricle stops contracting, and diastole begins, there is a powerful elastic recoil, or snapping back force, created by the stretched brachial artery wall coming back to its non-stretched shape. This force of elastic recoil or snapping back of the stretched brachial artery wall is what creates the diastolic BP. The diastolic blood pressure is, then, in a sense, a residual or left-over blood pressure, the force of blood pressure temporarily “stored” in the bulged-out walls of the brachial artery during systole. Because it is a residual BP, the diastolic BP is normally significantly lower than the systolic BP. It averages about 80 mmHg in a resting adult.

The stethoscope and sphygmomanometer measure a resting adult’s blood pressure as about 120/80 mmHg. The SBP is 120, the DBP is 80. The diastolic BP is usually recorded as the level of blood pressure where the dull thumping sound heard through the stethoscope just disappears.

Study suggestion: Ask yourself, “Why does the thumping sound disappear just below the recorded level of the diastolic BP?” Hint: Think about what is happening to the wall of the brachial artery.

Practice problems for these concepts can be found at: Blood And The Circulatory System Test