I. Flow– the volume of blood moving from an area of high pressure to one of low pressure over time. F = P/R
II. Pressure– force exerted by the blood, the driving pressure is what results in blood flow; greatest closest to the pump. The left ventricle certainly has the highest pressure due to the hardest job of all heart chambers.
III. Resistance – difficulty of the blood to flow as a result of the friction between the blood and the vessel wall and within the blood itself
A. Resistance is directly proportional to blood viscosity (measured with the hematocrit test)
B. Resistance is directly proportional to blood vessel length –but this remains constant
C. Resistance is indirectly proportional to the fourth power of the vessel radius –thus a small change in radius has a huge effect on resistance
IV. Laminar vs. Turbulent Flow
A. Laminar flow occurs in most circulation. Velocity of blood is greatest at the center of the blood vessel and the least next to the wall
B. Turbulent flow does not flow straight and requires a greater pressure for blood flow. It produces noise. (Heart murmurs are due to turbulent flow as well)
Measuring blood pressure on the brachial artery demonstrates laminar vs. turbulent flow as the point at which there is no blood flow due to a very tight cuff on the brachial artery is the systolic pressure (normal 120 mm Hg) and the point at which enough pressure has been released from the cuff and blood flow is again laminar is the diastolic pressure (normal 80 mm Hg). Between 120 and 80 as the cuff pressure is gradually released demonstrates turbulent flow in which a thumping sound can be heard through a stethoscope distal to the cuff.
V. Velocity of blood flow –Inversely related to total cross sectional area of a vessel, thus velocity is lowest in capillaries.
VI. Morphology of the Vessels
A. Arteries– thick walled vessels that carry blood away from the heart, starting with the largest artery, the aorta, and traveling to smaller and smaller arteries until you arrive at the arterioles – which are tiny arteries- and they end at the terminal arterioles. Arterioles are primary resistance vessel controlling distribution of blood. Arteries have elastic recoil and a pulse.
-Thick Tunica externa (CT, elastic fibers)
-Thick Tunica media (smooth muscle, elastic fibers)
-Tunica interna (endothelium)
B. Capillaries– tiny thin-walled blood vessels whose wall is only one cell layer thick. Diffusion occurs through capillaries. Typically link arterioles and venules.
-Only endothelium (tunica interna)
C. Veins– blood leaves the capillary beds by traveling into tiny veins called venules, then into larger and large veins until the largest veins in the body (superior & inferior vena cava). Walls are thin and not very muscular, pressures are low; veins have valves to help with one-way flow and course through skeletal muscles. Serve as blood reservoirs with large lumens.
-Same three layers as arteries, but very, very thin in comparison.