3.2 - Physiology of Circulation

Blood Flow

The movement of blood through a vessel, tissue, or organ, usually expressed as volume of blood per unit of time; initiated by ventricular contraction

Hydrostatic Pressure

The force exerted by a fluid due to gravitational pull, usually against the wall of the container in which it is located

Blood Pressure

The force exerted by blood upon the walls of the blood vessels or chambers of the heart; a form of hydrostatic pressure

Systemic Arterial Blood Pressure

The pressure of blood flowing in the arteries of the systemic circulation; what "blood pressure" refers to without other descriptors, typically measured via the brachial artery in mmHg

F ∝ ∆P

Blood flow is directly proportional to the pressure gradient; an increase in pressure difference between two points increases blood flow

Vascular Resistance

The opposition to blood flow offered by blood vessels, influenced by vessel diameter, length, and blood viscosity

Total Peripheral Resistance

The resistance to blood flow in the entire cardiovascular system, influenced by blood volume, viscosity, vessel length, diameter, and turbulence

Relationship Between Flow and Resistance

Blood flow is inversely proportional to resistance: increased resistance decreases flow, and decreased resistance increases flow

Radius as the Key Resistance Factor

Of blood volume, viscosity, vessel length, and radius, only radius can be changed rapidly (via vasoconstriction/vasodilation), and small changes in radius greatly affect flow and resistance

Hypovolemia

Low blood volume, which may be caused by bleeding, dehydration, vomiting, severe burns, or certain hypertension medications; may be asymptomatic until 10-20 percent of blood volume is lost

Hypervolemia

Excessive fluid volume, which may be caused by retention of water and sodium, as seen in heart failure, liver cirrhosis, and some kidney diseases

Blood Volume and Flow Relationship

As blood volume decreases, pressure and flow decrease; as blood volume increases, blood pressure and blood flow increase

Viscosity

The thickness of a fluid that affects its ability to flow; directly proportional to resistance and inversely proportional to flow

Determinants of Blood Viscosity

The formed elements (mostly erythrocytes) and plasma proteins (mostly produced by the liver) are the two primary determinants of blood viscosity

Polycythemia and Anemia Effect on Viscosity

Conditions affecting erythropoiesis, such as polycythemia (increases viscosity) or anemia (decreases viscosity), can alter blood viscosity

Liver Dysfunction Effect on Viscosity

Liver abnormalities such as hepatitis, cirrhosis, alcohol damage, and drug toxicities decrease plasma protein levels, which decreases blood viscosity

Vessel Length and Resistance

The length of a vessel is directly proportional to its resistance; the longer the vessel, the greater the resistance and the lower the flow

Vessel Length in Adults

Vessel length increases throughout childhood growth but is unchanging in adults under normal physiological circumstances; overall vessels decrease in length only with loss of mass or amputation

Approximate Vessel Length in an Average Adult

An individual weighing 150 pounds has approximately 60,000 miles of vessels; gaining about 10 pounds adds roughly 2,000 to 4,000 miles of vessels

Vessel Diameter and Resistance

Vessel diameter changes quickly throughout the body; increased diameter means less blood contacts the vessel wall, lowering friction and resistance and increasing flow, while decreased diameter increases resistance and decreases flow

Vascular Tone (in Resistance Context)

The contractile state of vessel smooth muscle; the primary determinant of vessel diameter and thus of resistance and flow

R ∝ 1/r4

Resistance is inversely proportional to the fourth power of the vessel radius, meaning small changes in radius cause large changes in resistance and flow

Effect of Radius Constriction Example

If an artery or arteriole constricts to one-half its original radius, resistance to flow increases 16 times

Effect of Radius Dilation Example

If an artery or arteriole dilates to twice its initial radius, resistance decreases to 1/16 of its original value and flow increases 16 times

Turbulence

The irregular and chaotic flow of blood within vessels, arising when blood velocity surpasses a threshold, disrupting laminar flow and increasing resistance and cardiac workload

Laminar Flow

Smooth blood flow without interruption

Causes of Turbulence

Abrupt changes in vessel geometry (such as branching points or areas of vessel narrowing/stenosis) and conditions like atherosclerosis (plaque buildup) can disrupt normal flow and create turbulence

Compliance

The ability of any compartment to expand to accommodate increased content; greater arterial compliance allows more effective expansion for blood flow surges without increased resistance or pressure

Veins vs Arteries: Compliance

Veins are more compliant than arteries and can expand to hold more blood

Effect of Arterial Stiffening on Compliance

Vascular diseases causing arterial stiffening reduce compliance and increase resistance to blood flow, resulting in more turbulence, higher pressure, reduced blood flow, and increased cardiac workload

Blood Flow (F) - Key Term Definition

The volume of blood flowing per unit of time through a vessel or group of vessels; total blood flow equals cardiac output

Blood Pressure (BP) - Key Term Definition

The hydrostatic pressure in the arterial system that pushes blood through capillary beds

Circulatory Pressure

The pressure difference between the base of the ascending aorta and the entrance to the right atrium

Peripheral Resistance (PR)

Opposition to blood flow

Resistance (R) - Key Term Definition

A force that opposes movement (in this case, blood flow)

Venous Pressure

The hydrostatic pressure in the venous system

Viscosity - Key Term Definition

A resistance to flow due to interactions among molecules within a liquid

F ∝ ΔP/R

Flow is directly proportional to the pressure gradient and inversely proportional to resistance

F ∝ BP/PR

Flow is directly proportional to blood pressure and inversely proportional to peripheral resistance

Velocity of Blood Flow

The speed at which blood moves through the circulatory system, influenced by heart rate, blood vessel diameter, and overall cardiovascular health

Arterioles as Resistance Vessels

Due to their small lumen, arterioles dramatically slow blood flow from arteries and are the site of greatest resistance in the entire vascular network

Capillary Total Cross-Sectional Area

The total cross-sectional area of the body's capillary beds is far greater than any other vessel type because there are vastly more capillaries than other vessel types, despite each individual capillary being much smaller in diameter than an arteriole

Site of Greatest Blood Pressure Drop

Blood pressure drops unevenly as blood travels from arteries to arterioles, capillaries, venules, and veins, with the biggest drop and greatest resistance occurring at the arterioles

Why Arterioles Regulate Blood Pressure Most

Since arterioles are the site of the greatest pressure drop and resistance, their vasodilation and vasoconstriction play a more significant role in regulating blood pressure than do other vessels

Blood Velocity Pattern Through Vessels

Velocity of blood flow decreases dramatically from arteries to arterioles to capillaries (allowing more time for exchange), then increases again as blood flows through veins back to the heart