Blood pressure throughout systemic circulation

The pressure generated by left ventricular contraction is the driving force for the flow of blood through the entire systemic circulation, from the aorta all of the way back to the right atrium. The mean pressure in the aorta and large arteries is typically very high (90 to 100 mmHg) due to the continual addition of blood to the system by the pumping action of the heart. However, this pressure is pulsatile; in other words, it fluctuates because of the alternating contraction and relaxation phases of the cardiac cycle. In a healthy resting adult, systolic pressure is approximately 120 mmHg and diastolic pressure is approximately 80 mmHg (see Figure 15.2). The pulse pressure is the difference between the systolic and diastolic pressures:

Systolic Pressure

100-J I . , I 1 ; i/\ } Pulse Pressure i i . . i 1 1 i i i i i

100-J I . , I 1 ; i/\ } Pulse Pressure i i . . i 1 1 i i i i i

Arteries Arterioles Capillaries Venules Veins Right Atrium

Figure 15.2 Pressures throughout the systemic circulation. At rest, blood pressure in the aorta and the other large arteries fluctuates between a low pressure of 80 mmHg during diastole and a high pressure of 120 mmHg during systole. The difference between diastolic and systolic pressure is the pulse pressure. Mean arterial pressure in these arteries is approximately 93 mmHg. As the blood continues forward and flows through the arterioles, the pulse pressure is dampened. Because of the high resistance to blood flow in these vessels, the overall pressure drops dramatically. Furthermore, fluctuations between diastolic and systolic pressure are eliminated so that the blood pressure becomes nonpulsatile. Blood pressure continues to decline, although at a slower rate, as blood flows through the capillaries and veins back toward the heart.

Arteries Arterioles Capillaries Venules Veins Right Atrium

Figure 15.2 Pressures throughout the systemic circulation. At rest, blood pressure in the aorta and the other large arteries fluctuates between a low pressure of 80 mmHg during diastole and a high pressure of 120 mmHg during systole. The difference between diastolic and systolic pressure is the pulse pressure. Mean arterial pressure in these arteries is approximately 93 mmHg. As the blood continues forward and flows through the arterioles, the pulse pressure is dampened. Because of the high resistance to blood flow in these vessels, the overall pressure drops dramatically. Furthermore, fluctuations between diastolic and systolic pressure are eliminated so that the blood pressure becomes nonpulsatile. Blood pressure continues to decline, although at a slower rate, as blood flows through the capillaries and veins back toward the heart.

Therefore, using the average values of 120 mmHg (systolic) and 80 mmHg (diastolic), the pulse pressure is 40 mmHg:

Systolic pressure - diastolic pressure = pulse pressure

The mean arterial pressure (MAP) is calculated as follows:

MAP = diastolic pressure + 1/3 (pulse pressure)

Therefore, using these same values, the MAP is 93 mmHg:

At rest, the MAP is closer to the diastolic pressure because the diastolic phase of the cardiac cycle lasts almost twice as long as the systolic phase. During exercise when heart rate increases and the length of diastole decreases, systolic pressure contributes more to the MAP.

As blood flows through the rest of the system, pressure continually falls (see Figure 15.2). Furthermore, the pulsatile nature of blood pressure is lost as blood flows through the arterioles. The pulse pressure is damped out by the considerable resistance offered to blood flow by the arterioles. At the arteriolar end of the capillaries, the blood pressure is 30 to 35 mmHg; at the venular end, the capillary pressure is approximately 10 mmHg. It is important that capillary pressure remain low so as to avoid leakage of fluid out of the capillaries into the tissues. Venous pressure is approximately 6 to 8 mmHg and pressure in the right atrium is close to zero.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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