Chapter 14: Circulation; Biophysics of Pressure, Flow, and Resistance

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Systemic and Pulmonary Circulation

The circulatory system acts as the primary transport mechanism in the human body, supplying essential nutrients, removing metabolic waste products, and transporting chemical messengers such as hormones. By continuously circulating blood, it maintains a stable physical and chemical environment within tissue fluids to support optimal cellular function. The circulation is functionally split into two main circuits: the systemic circulation, which services the peripheral tissues of the entire body, and the pulmonary circulation, which routes blood through the lungs for gas exchange. The movement of blood through these circuits depends on dynamic relationships between driving pressure differences and vascular resistance.

Each structural segment of the vascular tree possesses unique anatomical characteristics tailored to its functional demands. Arteries act as high-pressure conduits that rapidly transport blood away from the heart, while muscular arterioles regulate local perfusion according to tissue requirements. Thin-walled capillaries allow efficient substance diffusion, whereas veins and venules serve as low-pressure conduits and controllable blood reservoirs. Physical parameters, including total cross-sectional area, blood flow velocity, and local intravascular pressures, vary widely across these vascular divisions.

Primary Functions of the Circulation

• Serves tissue metabolic needs continuously.
• Transports vital nutrients to body tissues.
• Removes metabolic waste products from tissues.
• Delivers hormones between different body regions.
• Maintains optimal tissue fluid cellular environments.
• Controls tissue blood flow according to nutrient needs.
• Provides high renal blood flow for filtration functions.
• Regulates cardiac output and systemic arterial pressure.

Physical Characteristics of Vascular Divisions

• Divided into systemic and pulmonary circulatory circuits.
• Systemic circulation supplies all non-lung body tissues.
• Arteries transport blood under high pressure rapidly.
• Arteries possess thick, strong vascular wall structures.
• Arterioles act as variable control conduits.
• Arteriolar muscle walls dilate or constrict blood vessels.
• Capillaries exchange nutrients, fluids, and electrolytes.
• Capillary walls contain minute water-permeable pores.
• Venules collect blood from local capillary networks.
• Veins transport blood back toward the heart.
• Veins serve as major controllable blood reservoirs.
• Low venous pressure allows thin vascular wall construction.

Blood Volume Distribution Across Circuits

• Systemic circulation contains eighty-four percent total volume.
• Heart and lungs contain sixteen percent total volume.
• Systemic veins hold sixty-four percent total volume.
• Systemic arteries hold thirteen percent total volume.
• Systemic arterioles and capillaries hold seven percent volume.
• Heart cavities contain seven percent total volume.
• Pulmonary vessels contain nine percent total volume.
• Capillaries hold surprisingly low total blood volume.

Vascular Cross-Sectional Areas and Flow Velocities

• Aorta cross-sectional area equals two point five square centimeters.
• Small arteries total twenty square centimeters cross-sectional area.
• Arterioles total forty square centimeters cross-sectional area.
• Capillaries total twenty-five hundred square centimeters area.
• Venules total two hundred fifty square centimeters area.
• Small veins total eighty square centimeters area.
• Venae cavae total eight square centimeters area.
• Vein cross-sectional areas exceed corresponding arterial areas.
• Flow velocity inversely relates to total cross-sectional area.
• Aortic blood velocity averages thirty-three centimeters per second.
• Capillary blood velocity averages zero point three millimeters per second.
• Blood remains inside capillaries for one to three seconds.

Intravascular Pressures Across Vascular Beds

• Mean aortic pressure averages one hundred millimeters mercury.
• Pulsatile arterial pressure alternates between systolic and diastolic.
• Systemic arterial systolic pressure reaches one hundred twenty.
• Systemic arterial diastolic pressure drops to eighty.
• Venae cavae pressure drops to zero millimeters mercury.
• Arteriolar capillary end pressure measures thirty-five millimeters mercury.
• Venous capillary end pressure measures ten millimeters mercury.
• Average functional capillary pressure equals seventeen millimeters mercury.
• Low capillary pressure prevents plasma leakage through pores.
• Pulmonary artery systolic pressure averages twenty-five millimeters mercury.
• Pulmonary artery diastolic pressure averages eight millimeters mercury.
• Mean pulmonary arterial pressure equals sixteen millimeters mercury.
• Mean pulmonary capillary pressure equals seven millimeters mercury.

Fundamental Principles of Circulatory Function

• Local tissue blood flow matches specific tissue needs.
• Active tissues demand up to thirty times baseline flow.
• Microvessels monitor oxygen, nutrients, and waste accumulation.
• Local vessel dilation directly regulates local tissue perfusion.
• Nervous system and hormones assist tissue flow regulation.
• Cardiac output equals sum of local tissue flows.
• Heart responds automatically to returning venous blood volume.
• Nervous signals assist heart in pumping needed output.
• Arterial pressure regulation functions independently of local flow.
• Pressure drops trigger rapid compensatory nervous reflexes.
• Nervous reflexes increase heart pumping force rapidly.
• Venous reservoir contraction increases blood return to heart.
• Generalized arteriolar constriction increases systemic pressure.
• Kidneys provide long-term pressure control via hormones.

Interrelationships of Pressure, Flow, and Resistance

• Blood flow depends on pressure gradient and resistance.
• Pressure difference pushes blood through vascular segments.
• Friction against vessel endothelium creates vascular resistance.
• Ohm's law defines blood flow relationships mathematically.
• Blood flow directly correlates with pressure differences.
• Blood flow inversely correlates with vascular resistance.
• Pressure differences determine flow rather than absolute pressure.
• Equal pressure at vessel ends results in zero flow.

Principles of Measurement and Blood Flow Dynamics

• Blood flow measures volume passing points per time.
• Resting cardiac output equals five thousand milliliters per minute.
• Flowmeters measure intravascular blood flow rates electronically.
• Electromagnetic flowmeters generate voltage across magnetic fields.
• Moving blood generates electrical voltage inside magnetic fields.
• Generated voltage directly correlates with blood flow speed.
• Electromagnetic probes record rapid pulsatile flow changes.
• Ultrasonic Doppler flowmeters transmit high-frequency sound waves.
• Moving red blood cells reflect ultrasonic waves backward.
• Doppler effect causes frequency shifts in reflected waves.
• Frequency differences determine precise blood flow velocity.
• Constant vessel diameters allow accurate volume flow calculations.