MEDICAL BIOLOGY, HUMAN ANATOMY, PHYSIOLOGY AND PATHOLOGY - Ya.I. Fedoniuk 2010

ANATOMY, PHYSIOLOGY, PATHOLOGY

SECTION 5. BLOOD AND LYMPH CIRCULATION AND ITS PATHOLOGY

PHYSIOLOGY OF BLOOD VESSELS

1. BASIC PRINCIPLES OF HEMODYNAMICS

The science that studies the movement of Blood through vessels is called hemodynamics. According to its laws, blood flow is determined by the pressure gradient, which is the difference between the pressure in the left ventricle during its systole and the pressure in the right atrium during its diastole. For example, at the end of systole, the pressure in the left ventricle reaches 120-130 mmHg, while at this time in the right atrium, which is in diastole, the pressure drops to 0 mmHg. Thus, a pressure gradient of 120-130 mmHg is created.

The Heart ejects 70-80 ml of blood into the vessels only during systole, yet blood flows continuously through The Vascular System. This continuous blood flow is caused by the elasticity of the arterial vessels. Following ventricular systole, arterial pressure rises sharply, and the arterial walls stretch. During diastole, blood pressure in the Arteries decreases, and the vascular walls return to their initial state due to their elasticity. They exert pressure on the blood, propelling it forward and ensuring a steady flow through the vessels. The buffering vessels—the aorta and large arteries—facilitate this continuous blood flow.

Blood flow through the vessels depends on total peripheral vascular resistance. Vascular resistance is directly proportional to blood viscosity and inversely proportional to the radius of the vessel. The primary resistance to blood flow occurs in resistive vessels, namely arterioles.

Another key hemodynamic parameter is linear blood flow velocity, which reflects the speed at which blood particles move along a vessel. This hemodynamic index depends on the cross-sectional area of the vessel: the larger the cross-sectional area, the lower the velocity of the blood flowing through it. That is why in the aorta, which has a cross-sectional area of 2.5 cm², the linear velocity is highest at 20 cm/s, whereas in the capillaries, whose total cross-sectional area is 2500 cm², the linear velocity is very low, ranging from 0.03 to 1.05 cm/s.

Volumetric blood flow rate is the volume of blood passing through a vessel's cross-section per unit of time. The volume of blood flowing through each section of the vascular bed per unit of time remains constant; that is, in 1 minute, the exact same amount of blood flows through the aorta, the pulmonary arteries, or the combined cross-section at any level of arteries, Veins, or capillaries. This volume of blood represents the Cardiac Output per minute.