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Blood Vessel Diameter and Resistance

• Smaller tube diameter = greater resistance
• Fluid is slower at walls, faster in center
• Small-diameter arterioles are a major factor in resistance

Relationship Between Blood Flow, Blood Pressure, and Resistance

• Blood flow is directly proportional to the pressure gradient
• If blood flow increases, pressure increases
• Blood flow is inversely proportional to peripheral resistance
• If resistance increases, blood flow decreases

Systemic Blood Pressure

• The pumping action of the heart generates blood flow
• Pressure results when flow is opposed by resistance
• Blood pressure near the heart is pulsatile (BP rises and falls)
• Systemic pressure is highest in the aorta, declines throughout the pathway, and is 0 mm Hg when returning to the heart at the right atrium
• The steepest drop occurs in arterioles

Blood Pressure throughout the Circulatory System

• Arterial Blood Pressure:
  • Systolic pressure: pressure exerted during ventricular contraction
  • Diastolic pressure: lowest level of arterial pressure during ventricular relaxation
  • Pulse pressure = difference between systolic and diastolic pressure
  • Mean arterial pressure (MAP): pressure that propels the blood to the tissues
• Pulse Points:
  • Capillary Blood Pressure: low capillary pressure is desirable, high BP would rupture fragile, thin-walled capillaries
  • Venous Blood Pressure: changes little during the cardiac cycle, small pressure gradient, about 15 mm Hg, low pressure due to cumulative effects of peripheral resistance

Factors Aiding Venous Return

• Valves prevent backflow of blood
• Most abundant in veins of the limbs
• Veins

Physiology of Circulation: Definition of Terms

• Blood flow: volume of blood flowing through a vessel, an organ, or the entire circulation in a given period
• Blood pressure (BP): the pressure gradient provides the driving force that keeps blood moving from higher to lower pressure areas
• Resistance (peripheral resistance): measure of the amount of friction blood encounters, generally encountered in the peripheral systemic circulation

Sources of Resistance

• Blood viscosity: the "stickiness" of the blood due to formed elements and plasma proteins, greater viscosity = greater resistance
• Total blood vessel length: longer vessel = greater resistance
• Blood vessel diameter: smaller diameter = greater resistance

Maintaining Blood Pressure

• Requires cooperation of the heart, blood vessels, kidneys, and brain
• Blood pressure = CO x PR (and CO depends on blood volume)
• Blood pressure varies directly with CO, PR, and blood volume
• Changes in one variable are quickly compensated for by changes in the other variables

Control of Blood Pressure

• Short-term controls: neural and hormonal controls
• Long-term controls: renal regulation
• Counteract fluctuations in blood pressure by altering peripheral resistance or blood volume

Short-Term Mechanisms: Neural Controls of Blood Pressure

• Neural controls operate via reflex arcs involving baroreceptors, vasomotor centers, and vasomotor fibers
• Baroreceptors are located in carotid sinuses, aortic arch, and walls of large arteries of the neck and thorax
• Vasomotor center: a cluster of sympathetic neurons in the medulla that oversees changes in blood vessel diameter, maintains vasomotor tone (moderate constriction of arterioles)
• Cardiovascular center: vasomotor center plus the cardiac centers that integrate blood pressure control by altering cardiac output and blood vessel diameter

Short-Term Mechanisms: Hormone Controls of Blood Pressure

• Adrenal medulla hormones: Norepinephrine and epinephrine cause generalized vasoconstriction and increased cardiac output
• Antidiuretic hormone (ADH): causes intense vasoconstriction in cases of extremely low BP
• Atrial natriuretic peptide (ANP): causes blood volume and blood pressure to decline, causes generalized vasodilation

Long-Term Mechanisms: Renal Regulation of Blood Pressure

• Kidneys act directly and indirectly to regulate arterial blood pressure by altering blood volume