Blood Circulation

Blood Circulation

The blood must continue to circulate to sustain life. The heart acts as the circulation pump, and the arteries are pressurized reservoirs and channels. The arterioles control distribution of blood via resistance, the capillaries provide sites for exchange, and the venules and veins collect blood, acting as reservoirs and conducts. It is essential to define three related terms: blood flow, blood pressure, and resistance:

■■ Blood flow: The amount orvolumeof bloodthat flows through blood vessels, organs, or thesystemic­ circulation, in milliliters per minute (mL/min). Throughout the body, blood flow is equivalent to cardiac output. When resting, this is relatively constant, yet in certain body organs, blood flow may be different from others because of individual requirements. For example, blood flow to the skin increases when environmental temperature rises. Brain blood flow autoregulation is abolished when abnormally high carbon dioxide levels persist.

■■ Blood pressure: It is defined as the force thatblood exerts against the inner walls of blood vessels. It most commonly refers to pressure in arteries supplied by the aortic branches, even though it actually occurs throughout the vascu-lar system. Blood flow is generated by the heart’s pumping action, and blood pressure results from resistance opposing blood flow. Blood pressure is expressed in millimeters of mercury (mm Hg). A blood pressure of 120 mm Hg is the same as a column of mercury that is 120 mm in height.

■■ Resistance: The friction between blood and bloodvessel walls. Blood pressure must overcome this force for the blood to continue flowing. Factors that alter peripheral resistance therefore change blood pressure. Viscosity is defined as the ease with which a fluid’s molecules flow past one another. The higher the viscosity, the greater the resistance to flow. Blood viscosity is increased by blood cells and plasma proteins. The greater the resistance, the more force needed to move the blood. Blood pressure rises as blood viscosity­ increases, and vice versa. Conditions such as exces-sive numbers of red blood cells, which is known as polycythemia­, can cause both blood viscosity and resistance to increase. Some anemias, which cause low red blood cell counts, reduce viscosity and peripheral resistance.

Total blood vessel length and resistance are inter-related. There is more resistance over a longer vessel length in comparison with a shorter vessel length. As an infant grows as a child and then an adult, blood vessels lengthen. Therefore, the individual’s blood pressure and peripheral resistance increase with growth. In healthy people blood viscosity and vessel length are basically constant because they are rela-tively unchanging once adulthood is reached. Blood vessel diameter changes often, however, and this does change peripheral resistance. Fluid near a channel or tube wall slows down because of friction. How-ever, in the center of the channel or tube, fluid flows quicker since it experiences less friction. The smaller is a channel or tube, the greater is the friction. This is because more of the fluid contacts the walls, slowing its movement.

When the blood experiences a quick change in ves-sel diameter, or the tube wall has protruding or rough areas, often due to atherosclerotic plaques, the smooth laminar blood flow becomes turbulent flow. This is an irregular fluid motion. Blood from different lamina, or layers of the tube’s cross section, mix. The turbu-lence that is produced greatly increases resistance. Other factors may increase peripheral resistance, such as when sympathetic stimulation or epinephrine levels in the blood are increased.

Relationship Between Flow, Pressure, and Resistance

Blood pressure is calculated by multiplying cardiac output by peripheral resistance. Normal arterial pres-sure is maintained by regulating these two factors. Ideally, the volume of blood discharged from the heart should be equal to the volume entering the atria and ventricles. Fiber length and force of contraction are interrelated, because of the stretching of the cardiac muscle cell just before contraction. Known as the Frank-Starling law of the heart, it is important duringexercise when greater amounts of blood return to the heart from the veins.

Peripheral resistance also controls blood pres-sure. Changes in the diameters of arterioles regulate peripheral resistance. The vasomotor center of the medulla oblongata controls peripheral resistance.When arterial­ blood pressure increases suddenly, baroreceptors in the aorta and carotid arteries alert the ­vasomotor center, which vasodilates the vessels to decrease peripheral resistance. Carbon dioxide, oxygen, and hydrogen ions also influence peripheral resistance by affecting precapillary sphincters and smooth arteriole wall muscle.