Page 48 - Libro 2

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PART 1 — INTRODUCTION TO THE VASCULAR SYSTEM
Arteries Capillaries Veins 30% 2% 60%
Figure 3-1 A schematic drawing of the distribution of blood volume through the circulatory system.
HYDROSTATICS
A major force affecting the venous system is hydro- static pressure. Hydrostatic pressure is caused by the weight of a fluid as measured compared to a reference point. As stated in the preceding chapter, the refer- ence point of the human body is the right atrium. Hy- drostatic pressure is equal to 􏰀 􏰁 g 􏰁 h where 􏰀 is the density of blood, g is the acceleration due to gravity, and h is the height of the column of blood.
90 mmHg
2 mmHg
Arterial
89 mmHg 80 mmHg
5 mmHg 10 mmHg
Venous
ARTERIAL PRESSURE
VENOUS PRESSURE
Figure 3-3 Pressures within the arteries and veins in a supine individual.
Pressure within a blood vessel is equal to the dy- namic pressure supplied by the contraction of the heart plus the hydrostatic pressure (Fig. 3-2). Hydro- static pressure affects both the arteries and the veins equally. Because the dynamic pressure is so low in the veins, the hydrostatic pressure plays a greater role in determining the overall venous pressure. When supine, all the arteries and veins are roughly the same level as the right atrium; therefore, the hy- drostatic pressure is negligible (Fig. 3-3). Pressure throughout the vascular system is roughly equal to the dynamic pressure. In this position, the pressure in the veins at the ankle level is about 15 mm Hg.
When standing, an individual who is approximately 6 ft tall will add a hydrostatic pressure component of about 102 mm Hg at the ankle. This occurs in both the arteries and the veins so that the pressure gradi- ent across the capillary bed is the same as it was in the supine position (about 80 mm Hg). During
- 60 - 40 - 20
0 + 20 + 40 + 60 + 80 + 100
Dynamic + 95
Hydrostatic - 50
Total + 45
Dynamic + 15
Hydrostatic - 50
Total + 5
+ 80 + 60 + 40 + 20 0
- 20 - 40 - 60 - 80 - 100 - 120
- 140
+ 100 0 + 100
0 0 0
+ 95
+ 102
+ 197
+ 15
+ 102
+ 117
Figure 3-2 A drawing illustrating dynamic and hydrostatic pressure within the vascular system.
HYDROSTATIC PRESSURE mm Hg
HEIGHT cm