Review of hemodynamics

CHAPTER 43


Review of hemodynamics


Hemodynamics is the study of the movement of blood throughout the circulatory system, along with the regulatory mechanisms and driving forces involved. Concepts introduced here reappear throughout the chapters on cardiovascular drugs. Accordingly, I urge you to review these now. Because this is a pharmacology text, and not a physiology text, discussion is limited to hemodynamic factors that have particular relevance to the actions of drugs.




Overview of the circulatory system


The circulatory system has two primary functions: (1) delivery of oxygen, nutrients, hormones, electrolytes, and other essentials to cells; and (2) removal of carbon dioxide, metabolic wastes, and other detritus from cells. In addition, the system helps fight infection.


The circulatory system has two major divisions: the pulmonary circulation and the systemic circulation. The pulmonary circulation delivers blood to the lungs. The systemic circulation delivers blood to all other organs and tissues. The systemic circulation is also known as the greater circulation or peripheral circulation.



Components of the circulatory system


The circulatory system is composed of the heart and blood vessels. The heart is the pump that moves blood through the arterial tree. The blood vessels have several functions:



Arteries and veins differ with respect to distensibility (elasticity). Arteries are very muscular, and hence do not readily stretch. As a result, large increases in arterial pressure (AP) cause only small increases in arterial diameter. Veins are much less muscular, and hence are 6 to 10 times more distensible. As a result, small increases in venous pressure cause large increases in vessel diameter, which produces a large increase in venous volume.



Distribution of blood


The adult circulatory system contains about 5 L of blood, which is distributed throughout the system. As indicated in Figure 43–1, 9% is in the pulmonary circulation, 7% is in the heart, and 84% is in the systemic circulation. Within the systemic circulation, however, distribution is uneven: most (64%) of the blood is in veins, venules, and venous sinuses; the remaining 20% is in arteries (13%) and arterioles or capillaries (7%). The large volume of blood in the venous system serves as a reservoir.


image
Figure 43–1  Distribution of blood in the circulatory system.
Note that a large percentage of the blood resides in the venous system.


What makes blood flow?


Blood moves within vessels because the force that drives flow is greater than the resistance to flow. As indicated in Figure 43–2, the force that drives blood flow is the pressure gradient between two points in a vessel. Obviously, blood will flow from the point where pressure is higher toward the point where pressure is lower. Resistance to flow is determined by the diameter and length of the vessel, and by blood viscosity. From a pharmacologic viewpoint, the most important determinant of resistance is vessel diameter: The larger the vessel, the smaller the resistance. Accordingly, when vessels dilate, resistance declines, causing blood flow to increase—and when vessels constrict, resistance rises, causing blood flow to decline. In order to maintain adequate flow when resistance rises, blood pressure must rise as well.


image
Figure 43–2  Forces that promote and impede flow of blood.
Blood flows from the point of higher pressure toward the point of lower pressure. Resistance to flow is determined by vessel diameter, vessel length, and blood viscosity.


How does blood get back to the heart?


As indicated in Figure 43–3, pressure falls progressively as blood moves through the systemic circulation. Pressure is 120 mm Hg when blood enters the aorta, 30 mm Hg when blood enters capillaries, and only 18 mm Hg when blood leaves capillaries, and then drops to negative values (0 to −5 mm Hg) in the right atrium. (Negative atrial pressure is generated by expansion of the chest during inspiration.)


image
Figure 43–3  Distribution of pressure within the systemic circulation.
Note that pressure is highest when blood leaves the left ventricle, falls to only 18 mm Hg as blood exits capillaries, and reaches negative values within the right atrium.

Given that pressure is only 18 mm Hg when blood leaves capillaries, we must ask, “How does blood get back to the heart? After all, a pressure of 18 mm Hg does not seem adequate to move blood from the feet all the way up to the thorax.” The answer is that, in addition to the small pressure head in venules, three mechanisms help ensure venous return. First, negative pressure in the right atrium helps “suck” blood toward the heart. Second, constriction of smooth muscle in the venous wall increases venous pressure, which helps drive blood toward the heart. Third, and most important, the combination of venous valves and skeletal muscle contraction constitutes an auxiliary “venous pump.” As indicated in Figure 43–4A, the veins are equipped with a system of one-way valves. When skeletal muscles contract (Fig. 43–4B), venous blood is squeezed toward the heart—the only direction the valves will permit.


image
Figure 43–4  Venous valves and the auxiliary venous “pump.”
A, Veins and their one-way valves in the leg. Note that the configuration of these valves ensures that blood will move toward the heart. B, Contraction of skeletal muscle pumps venous blood toward the heart.


Regulation of cardiac output


In the average adult, cardiac output is about 5 L/min. Hence, every minute the heart pumps the equivalent of all the blood in the body. In this section, we consider the major factors that determine how much blood the heart pumps.



Determinants of cardiac output


The basic equation for cardiac output is


Approx imate  child“s  dose=                                                                                    Body  surface  area  of  the  child×Adult  dose1.73  m2


CO = HR × SV

where CO is cardiac output, HR is heart rate, and SV is stroke volume. According to the equation, an increase in HR or SV will increase CO, whereas a decrease in HR or SV will decrease CO. For the average person, heart rate is about 70 beats/min and stroke volume is about 70 mL. Multiplying these, we get 4.9 L/min—the average value for CO.

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Jul 24, 2016 | Posted by in NURSING | Comments Off on Review of hemodynamics

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