Physiology of Circulation: Flow, Pressure, Resistance, and Blood Pressure Regulation

Physiology of Circulation

19.6 Flow, Pressure, and Resistance

The physiology of circulation involves understanding how blood moves through vessels, the forces involved, and the factors that oppose flow. These principles are essential for maintaining tissue perfusion and overall cardiovascular health.

• Blood flow: The volume of blood moving through a vessel, organ, or the entire circulation in a given period.

  • Measured in mL/min; for the entire vascular system, it equals cardiac output (CO).

  • Relatively constant at rest but varies by organ based on metabolic needs.

• Blood pressure (BP): The force per unit area exerted on the wall of a blood vessel by the blood.

  • Expressed in millimeters of mercury (mm Hg).

  • Measured as systemic arterial BP in large arteries near the heart.

  • Pressure gradient drives blood from higher to lower pressure areas.

• Resistance (peripheral resistance): Opposition to blood flow, mainly due to friction between blood and vessel walls.

  • Measured in the peripheral (systemic) circulation.

  • Three main sources: blood viscosity, total blood vessel length, blood vessel diameter.

Blood Viscosity

• Refers to the thickness or "stickiness" of blood due to formed elements and plasma proteins.

• Higher viscosity means increased resistance to flow.

• Example: Polycythemia increases viscosity, raising resistance.

Total Blood Vessel Length

• Longer vessels encounter more resistance.

• Example: Obesity increases vessel length, raising resistance.

Blood Vessel Diameter

• Most influential factor in resistance.

• Frequent changes in diameter alter peripheral resistance.

• Fluid near vessel walls moves slower than in the center (laminar flow).

• Resistance varies inversely with the fourth power of vessel radius: $R \propto \frac{1}{r^4}$ Doubling radius decreases resistance to 1/16th.

• Abrupt changes or obstructions (e.g., plaques) disrupt laminar flow, causing turbulent flow and increased resistance.

Example: Milk Shake and Straws

• Drinking a thick milkshake through a narrow straw requires more effort (higher resistance) than through a wide straw.

Relationship Between Flow, Pressure, and Resistance

Blood flow is determined by the pressure gradient and resistance within vessels.

• Blood flow ($F$) is directly proportional to the pressure gradient ($\Delta P$): $F \propto \Delta P$

• Blood flow is inversely proportional to resistance ($R$): $F \propto \frac{1}{R}$

• Combined equation: $F = \frac{\Delta P}{R}$

• Resistance is the most important factor for local blood flow regulation, as vessel diameter can change rapidly.

19.7 Systemic Blood Pressure

Systemic blood pressure is generated by the heart and opposed by vascular resistance. It is highest in the aorta and decreases throughout the systemic circuit, with the steepest drop in arterioles.

• Pressure profile:

Arterial Blood Pressure

• Determined by:

  1. Elasticity (compliance/distensibility) of arteries near the heart

  2. Volume of blood forced into arteries at any time

• Blood pressure near the heart is pulsatile (rises and falls with each heartbeat).

• Systolic pressure: Pressure during ventricular contraction (average 120 mm Hg).

• Diastolic pressure: Pressure during ventricular relaxation (lowest level).

• Pulse pressure: Difference between systolic and diastolic pressures.

• Pulse: Throbbing of arteries due to pulse pressure, palpable under the skin.

Mean Arterial Pressure (MAP)

• MAP is the pressure that propels blood to tissues.

• Calculated as: $\text{MAP} = \text{Diastolic Pressure} + \frac{1}{3} \text{Pulse Pressure}$

• Example: BP = 120/80 mm Hg Pulse Pressure = 120 - 80 = 40 mm Hg MAP = 80 + (1/3) × 40 = 80 + 13 ≈ 93 mm Hg

• Pulse pressure and MAP decline with increasing distance from the heart.

Clinical Signs

• Vital signs: Pulse, blood pressure, respiratory rate, body temperature.

• Radial pulse: Most commonly measured at the wrist.

• Pressure points: Arteries close to the body surface can be compressed to stop blood flow during hemorrhage.

Body Sites Where Pulse is Easily Palpated

Artery	Location
Superficial temporal	Temple
Facial	Jaw
Common carotid	Neck
Brachial	Arm
Radial	Wrist
Femoral	Groin
Popliteal	Knee
Posterior tibial	Ankle
Dorsalis pedis	Foot

Measuring Blood Pressure

• Measured indirectly by auscultatory methods using a sphygmomanometer.

• Steps:

  1. Wrap cuff around upper arm.

  2. Increase pressure until it exceeds systolic pressure.

  3. Release pressure slowly, listen for Korotkoff sounds with a stethoscope.

• Systolic pressure: First sound heard, normally <120 mm Hg.

• Diastolic pressure: When sounds disappear, normally <80 mm Hg.

The Muscular Pump

• Venous return is aided by skeletal muscle contraction, which compresses veins and pushes blood toward the heart.

• Venous valves prevent backflow.

19.8 Regulation of Blood Pressure

Blood pressure regulation involves the heart, blood vessels, kidneys, and is supervised by the brain. Three main factors regulate BP: cardiac output (CO), peripheral resistance (PR), and blood volume.

• BP varies directly with CO, PR, and blood volume.

Major Factors That Increase MAP

Factor	Effect
Stroke volume	↑ Cardiac output
Heart rate	↑ Cardiac output
Diameter of blood vessels	↓ Peripheral resistance if increased
Blood viscosity	↑ Peripheral resistance
Blood vessel length	↑ Peripheral resistance

Regulation Mechanisms

• Short-term regulation:

  • Neural controls (reflex arcs involving cardiovascular center, baroreceptors, chemoreceptors, higher brain centers)

  • Hormonal controls (adrenal medulla hormones, angiotensin II, ADH, atrial natriuretic peptide)

• Long-term regulation:

  • Renal controls (direct and indirect mechanisms via kidneys)

Short-Term Regulation: Neural Controls

• MAP is maintained by altering vessel diameter and blood distribution.

• Neural controls operate via reflex arcs:

  • Cardiovascular center of medulla: Sympathetic neurons regulate heart and vessel tone.

  • Baroreceptors: Detect changes in pressure, located in carotid sinuses, aortic arch, and large arteries.

  • Chemoreceptors: Detect changes in CO2, pH, and O2.

  • Higher brain centers: Hypothalamus and cerebral cortex can modify BP during stress or exercise.

Baroreceptor Reflexes

• High MAP:

  • Stimulates baroreceptors, increases input to vasomotor center.

  • Inhibits vasomotor and cardioacceleratory centers, stimulates cardioinhibitory center.

  • Results in vasodilation and decreased BP.

• Low MAP:

  • Initiates reflex vasoconstriction, increases CO and BP.

  • Carotid sinus reflex ensures blood flow to brain.

  • Aortic reflex maintains BP in systemic circuit.

• Baroreceptors adapt to chronic high BP (hypertension), becoming less effective.

Chemoreceptor Reflexes

• Detect increased CO2, decreased pH or O2.

• Increase BP by signaling cardioacceleratory and vasomotor centers.

Influence of Higher Brain Centers

• Hypothalamus increases BP during stress.

• Redistributes blood flow during exercise and temperature changes.

Short-Term Mechanisms: Hormonal Controls

• Adrenal medulla hormones:

  • Epinephrine and norepinephrine increase CO and vasoconstriction.

• Angiotensin II: Potent vasoconstrictor.

• ADH: High levels cause vasoconstriction.

• Atrial natriuretic peptide (ANP): Decreases BP by antagonizing aldosterone, reducing blood volume.

Table: Effects of Selected Hormones on Blood Pressure

Hormone	Effect on BP	Variable Affected	Site of Action
Epinephrine/Norepinephrine	Increase	CO and vasoconstriction	Heart, arterioles
Angiotensin II	Increase	Total peripheral resistance	Arterioles
ADH	Increase	Total peripheral resistance, blood volume	Arterioles, kidney tubule cells
Aldosterone	Increase	Blood volume	Kidney tubule cells
ANP	Decrease	Total peripheral resistance	Arterioles

Long-Term Mechanisms: Renal Regulation

• Baroreceptors adapt to chronic BP changes; kidneys regulate BP long-term by altering blood volume.

• Direct renal mechanism:

  • Increased BP/blood volume → more urine produced → BP decreases.

  • Decreased BP/blood volume → kidneys conserve water → BP increases.

• Indirect renal mechanism (renin-angiotensin-aldosterone):

  • Decreased BP → renin release from kidneys → angiotensinogen converted to angiotensin I → angiotensin II (via ACE).

  • Angiotensin II:

    • Stimulates aldosterone secretion.

    • Causes ADH release.

    • Triggers thirst center.

    • Acts as a vasoconstrictor.

Summary of Blood Pressure Regulation

• Goal: Maintain BP high enough for tissue perfusion, but not so high as to damage vessels.

• Low BP to brain → inadequate perfusion → loss of consciousness.

• High BP → risk of stroke and vessel damage.

Homeostatic Imbalances in Blood Pressure

Hypertension

• Sustained arterial pressure ≥140/90 mm Hg.

• Prehypertension: Values elevated but not in hypertension range; may be transient or persistent (e.g., obesity).

• Prolonged hypertension: Major cause of heart failure, vascular disease, renal failure, stroke.

• Accelerates atherosclerosis.

Primary Hypertension

• 90% of cases; no identifiable cause.

• Risk factors: Heredity, diet, obesity, age, diabetes, stress, smoking.

• Managed by lifestyle changes and antihypertensive drugs.

Secondary Hypertension

• Less common; due to identifiable disorders (renal, endocrine).

• Treatment targets underlying cause.

Hypotension

• Low BP (<90/60 mm Hg); usually not a concern unless tissue perfusion is inadequate.

• Associated with long life and lack of cardiovascular illness.

• Orthostatic hypotension: Temporary drop in BP when rising quickly.

• Chronic hypotension: Poor nutrition, Addison's disease, hypothyroidism.

• Acute hypotension: Sign of circulatory shock.

Circulatory Shock

• Blood vessels inadequately filled; cannot circulate blood normally.

• Types:

  • Hypovolemic shock: Large-scale blood loss.

  • Vascular shock: Extreme vasodilation, decreased resistance.

  • Cardiogenic shock: Heart cannot sustain adequate circulation.

Additional info: All equations and tables have been expanded and clarified for academic completeness. Diagrams referenced are described in text for study purposes.