The arterial wall has 3 linings:
Tunica Intima: The smooth innermost layer; consists of epithelial lining of endothelium, base membrane and connective tissue.
Tunica Media: The muscular middle layer provides stiffness, elastic and strength to the artery. This layer contains mostly smooth muscle and elastic fibers in larger arteries
Tunica Adventitia: The outer layer; is composed of loose connective tissue, contains the vasa vasorum “vessels of vessels” and supply blood to the larger arteries.
Understanding the arterial system
The pumping action of the heart is broken down into diastole and systole. The heart relaxes and fills with blood during diastole, at the end of diastole, the heart contracts and pumps blood into the ventricles. The ventricles then contract, a process known as ventricular systole, and pump the blood out of the heart.
The pressure in the heart is greater and decreases as the blood moves to the rest of the body. The higher the pressure from the heart, the faster the flow of blood. The difference in pressure helps to maintain the flow of blood to the body. The greater the pressure difference (pressure gradient) the greater the flow rate.
Blood pressure is affected by:
• peripheral resistance
• cardiac output
• blood volume
• vessel elasticity
Note: Increase in peripheral resistance, blood volume, and cardiac output results in higher blood pressure and decrease in these three factors results to lower blood pressure. Decrease in vessel elasticity results in higher blood pressure
Three main sources of peripheral resistance include:
• Blood vessel length
– The longer the total vessel length, the greater the resistance and the blood pressure
• The blood vessel diameter. (the most significant effect)
– larger diameter, less pressure
– smaller diameter, more pressure
• Blood viscosity
– Hematocrit is affected by increase in red blood cells or less plasma in the blood leading to increase in blood viscosity.
– Decreased hematocrit is caused by fewer red blood cells or increased plasma leading to decrease blood viscosity.
General rule: Increased blood viscosity (thicker blood) equals greater resistance and higher blood pressure (slower flow).
Summary: the flow of blood is dependent on pressure and resistance.
Types of Energy
The overall energy contained in a moving fluid is the sum of potential (pressure), kinetic, and gravitational energies.
Potential energy (pressure) is stored energy. It is the main form of energy in flowing blood formed by the pumping action of the heart.
Kinetic energy (motion) is the ability of flowing blood to do work due to speed.
Gravitational energy is (potential)stored energy in an elevated object. Hydrostatic pressure is related to pressure or mass of blood pressing on a vessel. The BP measurement is dependent on patient position and location of measurement. Thus, Hydrostatic pressure is the mass within the blood column from the heart to the level of area being measured.
For fluid to flow from one segment to another, pathway and pressure gradient are needed. The highest pressure is in the arteries arising from the ventricles, decreases as blood travels through the vascular network and reaches its lowest point in the veins, which returns blood to the atria.
Note: Pressure difference is needed to ensure the flow of blood from the ventricles through the arteries, capillaries, veins back to the atria. Liquids and gases flow from areas of high pressure to areas of low pressure.
Energy is lost in the circulation due to viscosity, friction and inertia.
• Viscosity- thickness of the blood, due to elevated hematocrit.
• Friction- flow of energy is converted to heat due to rubbing of red blood cells against another.
• Inertia- is the resistance of moving fluid to changes in speed and direction. This usually occurs during phasic, pulsatile flow and velocity changes at a stenotic site. When there is a decrease in vessel size there is an increase in velocity. There is a velocity decrease as the blood flows out of the stenosis into a normal vessel diameter.