Blood pressure is the force the blood exerts against the walls of the blood vessels. Filtration in capillaries depends upon blood pressure; filtration brings nutrients to tissues. Blood pressure is one of the “vital signs” often measured, and indeed a normal blood pressure is essential to life pumping of the ventricles creates blood pressure, which is measured in mmHg (millimeters of mercury). When a systemic blood pressure reading is taken, two numbers are obtained: systolic and diastolic, as in 110/70 mmHg. Systolic pressure is always the higher of the two and represents the blood pressure when the left ventricle is contracting. The lower number is the diastolic pressure, when the left ventricle is relaxed and does not exert force. Diastolic pressure is maintained by the arteries and arterioles and is discussed in a later section. Systemic blood pressure is highest in aorta,
which receives all the blood pumped by the left ventricle. As blood travels farther away from the heart,
blood pressure decreases. The brachial artery is most often used to take a blood pressure reading; here a normal systolic range is 90 to 120 mmHg and a normal diastolic range is 60 to 80 mmHg. In the arterioles, blood pressure decreases further, and systolic and diastolic pressures merge into one pressure. At the arterial end of capillary networks, blood pressure is about 30 to 35 mmHg, decreasing to 12 to 15 mmHg at the venous end of capillaries. This is high enough to filter, low enough to prevent rupture of the capillaries. As blood flows through veins, the pressure decreases, blood pressure approaches zero as blood enters the right atrium. The upper limit of the normal blood pressure range is now 120/80 mmHg. The levels of 125 to 139/85 to 89 mmHg, once considered high-normal, are now a called “prehypertension,” that is, with the potential to become even higher. A systemic blood pressure consistently higher than the normal range is called hypertension. A lower than normal blood pressure is called hypotension. The regulation of systemic blood pressure is discussed in a later section. Pulmonary blood pressure is created by the right ventricle, which has relatively thin walls and thus exerts about one-sixth the force of the left ventricle. The result is that pulmonary arterial pressure is always low:20 to 25/8 to 10 mmHg, and in pulmonary capillaries is lower still. This is important to prevent filtration in pulmonary capillaries, which in turn prevents tissue fluid from accumulating in the alveoli of the lungs.
Heart rate and force—in general, if heart rate and force increase, blood pressure increases; this is what happens during exercise. However, if the heart is beating extremely rapidly, the ventricles may not fill completely between beats, and cardiac output and blood pressure will decrease.
Peripheral resistance—this term refers to the resistance the vessels offer to the flow of blood.
The arteries and veins are usually slightly constricted, which maintains normal diastolic blood pressure. It may be helpful to think of the vessels as the “container” for the blood. If a person’s body has 5 liters of blood, the “container” must be smaller in order for the blood to exert a pressure against its walls. This is what normal vasoconstriction does: It makes the container (the vessels) smaller than the volume of blood so that the blood will exert pressure even when the left ventricle is relaxed.
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