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MCAT Content / Circulatory System / Arterial And Venous Systems Pressure And Flow Characteristics

Arterial and venous systems; pressure and flow characteristics

Topic: Circulatory System

Blood vessels include arteries, capillaries, and veins which are responsible for transporting blood throughout the body. Three key factors that influence blood circulation are resistance, flow, and pressure.

The blood from the heart is carried through the body by a complex network of blood vessels. Arteries take blood away from the heart. The main artery is the aorta that branches into other major arteries, which take blood to different limbs and organs. Arterioles diverge into capillary beds. Capillary beds contain a large number (10 to 100) of capillaries that branch among the cells and tissues of the body. The capillaries converge again into venules that connect to minor veins, which connect to major veins that take blood high in carbon dioxide back to the heart.

Resistance to flow must be overcome to push blood through the circulatory system. If resistance increases, either pressure must increase to maintain flow, or flow rate must reduce to maintain pressure. Numerous factors can alter resistance, but the three most important are vessel length, vessel radius, and blood viscosity. With increasing length, increasing viscosity, and decreasing radius, resistance is increased. The arterioles and capillary networks are the main regions of the circulatory system that generate resistance, due to the small diameter of their lumen. Arterioles, in particular, are able to rapidly alter resistance by altering their radius through vasodilation or vasoconstriction. On the other hand, veins have lower resistance and rely on valves to prevent the backflow of blood due to their wider lumen and lower pressure.

Blood pressure is the pressure that blood exerts on the wall of the blood vessels. The pressure originates in the contraction of the heart, which forces blood out of the heart and into the blood vessels. If the flow is impaired through increased resistance, then blood pressure must increase, so blood pressure is often used as a test for circulatory health. Blood pressure can be modulated through altering cardiac activity, vasoconstriction, or vasodilation. The pressure is higher in arteries due to their proximity to the heart and increased muscle tissue which is able to cause contractions. Whereas veins have a lower pressure as blood moving through veins often comes from low-pressure capillary beds.

Flow is the movement of the blood around the circulatory system. Relatively constant flow is required by the body’s tissues, so pressure and resistance are altered to maintain this consistency. Specifically, the volume flow rate in blood vessels is the speed of the blood times the cross-sectional area of the vessel.  This flow rate is relatively constant. A too-high flow can damage blood vessels and tissue, while flow that’s too low means tissues served by the blood vessel may not receive sufficient oxygen to function. Flow is high in arteries due to their narrow lumens and high pressure but lowers in veins due to the lack of pressure.


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Key Points

• Arteries carry blood away from the heart; the main artery is the aorta.

• Smaller arteries called arterioles diverge into capillary beds, which contain 10-100 capillaries that branch among the cells and tissues of the body.

• Capillaries carry blood away from the body and exchange nutrients, waste, and oxygen with tissues at the cellular level.

• Veins are blood vessels that bring blood back to the heart and drain blood from organs and limbs.

• The flow of blood along arteries, arterioles and capillaries is not constant but can be controlled depending upon the body’s requirements.

• Vascular resistance generated by the blood vessels must be overcome by blood pressure generated in the heart to allow blood to flow through the circulatory system.

Key Terms

Vasodilation: The opening of a blood vessel.

Flow: The movement of blood around the body, closely controlled by alterations in resistance and pressure.

Vasoconstriction: The closing or tightening of a blood vessel.

Resistance: The resistance which must be overcome by pressure to maintain blood flow throughout the body.

Pressure: The force which overcomes resistance to maintain blood flow throughout the body

Veins: blood vessels carrying deoxygenated blood back to the heart

Arteries: muscular blood vessels that carry oxygenated blood from the heart to the rest of the body

Aorta: the main artery in the body

Capillary beds: collections of narrow blood vessels where gaseous exchange is possible

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