Blood Pressure And Flow In The Systemic Circulation

Any time someone says they have a blood pressure problem, what the; ire talking about is blood pressure in the arteries of their systemic circulai >n. Like atmospheric pressure, alveolar gas pressures, and blood g ses (chapter 2), blood pressure is measured in millimeters of men iry (mm Ilg). When we say, "Normal blood pressure is 120 over 80 mm Hg or more simply 120/80, we are referring to the pressure of the blood ag nst the inner walls of medium-sized arteries of the systemic circulation, u- illy measured in the arm when we are either sitting quietly in a chair or ing down (fig. 8.1). As pressures go these are low—the equivalent to out 2 pounds per square inch of air pressure in the tires of your car.

The two ligures aie significant. The blood pressure in an artery < the arm rises to 120 mm Hg as the heart pumps blood from its contract! 1 let ventricle; this is the systolic pressure, which is named after the Greek ord meaning "contraction." Between contractions the pressure drops to 81 mm Hg as the left ventricle fills; this is the diastolic pressure, or the pre -ure between contractions. Blood pressure in an artery of the arm is 01 ly H small part of a bigger picture, however From the left ventricle, ble d is

#. hie he4dstam) 4.1y propelled successively through the aorta to large arteries, medium-sized arteries, arterioles, capillaries, venules, and veins, and blood pressure decreases from segment to segment. Within the heart itself—in the left ventricle—systolic blood pressure is 120 mm Hg and diastolic pressure is a mere 10 mm Hg, because the latter drops almost to nothing while the ventricle is filling with blood. The textbook standard of blood pressure is 120/80 mm Hg between the aorta and small arteries, and beyond the arterioles in the capillary bed it drops to about 15 mm Hg. On the venous side of the systemic circulation, blood pressure continues to drop in the venules and veins, and it is essentially o in the vena cava where that vessel opens into the right atrium of the heart (fig. 8.1).

Blood pressure in medium-sized arteries depends both on the heart acting as a pump and on peripheral resistance. The importance of the pump is obvious: a harder-working heart creates more pressure in the system. But the resistance to flow in the arterioles is just as important: as peripheral resistance increases, blood pressure in the arteries also increases. There are many neurological, hormonal, and other physiological factors that influence the heartbeat and peripheral resistance, but they are beyond the scope of this book; here we'll note only that any time you become especially active or anxious, the sympathetic nervous system and hormones from the adrenal gland increase blood pressure by increasing both peripheral resistance and the strength and rate of the heartbeat.

Figure 8.1. This graph shows blood pressure in different parts of the systemic circulation at heart level. The continuous curves in the portions of the graph for large arteries, small arteries, and arterioles represent variations in systolic <top) and diastolic (bottom) blood pressure, and the dashed 1 curve in the same regions I

represents averages (for c example, about 100 mm Hg in <u •arge arteries). Systolic and diastolic pressures are no longer detected separately in capillaries and veins, and blood Pressure drops essentially to 0 Hm Fig where the vena cava empties into the right atrium

Blood pressure also varies in different parts of the body. It increase below the heart and decreases above the heart because the weight of t) column of blood in an artery adds to (or subtracts from) the pressu generated by the heart and by peripheral resistance. In a standing positii with blood pressure in medium-sized arteries at 120/80 mm Hg at he; 1 level, blood pressure will be about 210/170 mm Hg in the arteries of the fc t and about 100/60 mm Hg in the brain (fig. 8.2a). The only circumstani s under which we'll see hlood pressure equalized throughout the body ¡t 120/80 mm Hg is if we neutralize the effect of gravity by lying prone .r supine, by submerging ourselves in water, or by taking up residence it a space capsule that is orbiting earth.

Turning upside down in the headstand reverses the figures seen stand g in a straightforward fashion. Blood pressure will remain at 120/80 at he 1 level, at least if you are not under too much stress, but the pressure in e arm will rise to about 140/100 mm Hg because the arm is alongside the h< d and below the heart instead of level with it. We can calculate that bl> >d

100/60 mm Hg

40/0 mm Hg

210/170 mm Hg

(average of 100 mm Hg)

210/170 mm Hg

(average of 100 mm Hg)

150/110 mm Hg (average of 130 mm

140/100 mm He

150/110 mm Hg (average of 130 mm

140/100 mm He

Figure 8.2b. Calculated arterial bloc» pressure in the headstand in muscul. r arteries in different parts of the bod\

Figure 8.2a. Arterial blood pressure in a standing posture in muscular arteries in different parts of the body.

Figure 8.2b. Calculated arterial bloc» pressure in the headstand in muscul. r arteries in different parts of the bod\

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