The vascular component of the cardiovascular system consists of the arteries and their multiple branches, which carry blood away from the heart; the capillaries, which are one cell layer thick so that exchange can occur between the blood and the interstitial space and tissue cells; and the veins and their multiple branches, which carry blood back to the heart. The functions of the different conduits that carry blood are matched by the anatomy of the vessels.
All blood flow is dependent on a pressure gradient. The pressure in the arteries is greater than the pressure in the capillaries which in turn is greater than the pressure in the veins. Thus in the systemic circulation, blood flows from the aorta to the right atrium along a pressure gradient which is generated by the heart pumping a volume of blood into the aorta from where the blood moves to the peripheral arteries. The pressure produced in the aorta during ventricular ejection (systole) usually peaks at 120 mm Hg. During relaxation of the ventricle (diastole), pressure in the aorta falls to approximately 80 mm Hg. Thus blood pressure is generally reported as 120/80. The difference between the systolic pressure and the diastolic pressure is the pulse pressure. In this example, the pulse pressure is equal to 40 mm Hg. The average pressure during a cardiac cycle is the mean arterial blood pressure (MABP). This can be estimated as (1/3 × pulse pressure) + diastolic pressure. In this example, this equals 93 mm Hg. This equation is a good estimate of MABP at resting heart rate when diastole is approximately 2/3 of the cardiac cycle. At higher heart rates, the estimate is not as accurate. The regulation of MABP is discussed in Chapter 19.
The properties of the vascular wall are important in determining arterial blood pressure. The aorta is a large vessel with a thick wall containing smooth muscle and connective tissue—predominantly elastin and collagen. Elastin confers properties to the wall such that when the aorta is distended from the ejection of the stroke volume by the ventricle, the aortic wall returns back to its pre-ejection state during diastole. This recoil property of the aorta maintains pressure in the aorta and prevents arterial pressure from falling to 0 during ventricular relaxation. If the aorta were a rigid tube, pressure in the aorta would drop to 0 when the ventricle relaxed. The more compliant the aorta, the better arterial pressure is maintained during diastole. When the aorta is stiff, pressure tends to fall more during diastole. As people age and the aorta becomes less compliant (stiffer), the change in pressure in the aorta will be greater for the same SV than it was when they were younger.
In progressing from the aorta to the large arteries, the small arteries, and the arterioles, the diameter of the vessels decreases, the number of vessels increases (due to branching ...