Physiology of Circulation and Blood Pressure Regulation

The Cardiovascular System: Blood Vessels

Physiology of Circulation

This section explores the fundamental principles governing blood flow, pressure, and resistance within the circulatory system, as well as the mechanisms regulating blood pressure and associated homeostatic imbalances.

Definition of Terms

• 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 whole vascular system, it is equivalent to cardiac output (CO).

• Blood Pressure (BP): The force per unit area exerted on the wall of a blood vessel by the blood, expressed in mmHg. Systemic arterial BP is measured in large arteries near the heart.

• Pressure Gradient: The difference in pressure that drives blood from areas of higher to lower pressure.

• Resistance (Peripheral Resistance): The opposition to blood flow, primarily due to friction between blood and vessel walls. Major sources include blood viscosity, vessel length, and vessel diameter.

Sources of Resistance

• Blood Viscosity: Thickness of blood due to formed elements and plasma proteins. Increased viscosity raises resistance.

• Total Blood Vessel Length: Longer vessels increase resistance.

• Blood Vessel Diameter: Has the greatest influence on resistance. Resistance varies inversely with the fourth power of vessel radius ($R \propto \frac{1}{r^4}$). Small-diameter arterioles are major determinants of peripheral resistance.

• Example: Doubling vessel radius decreases resistance to 1/16 of its original value.

Relationship Between Flow, Pressure, and Resistance

• Blood Flow (F): Directly proportional to the pressure gradient ($\Delta P$) and inversely proportional to resistance (R).

• Formula:

$F = \frac{\Delta P}{R}$

• Resistance is the most important factor influencing local blood flow, as vessel diameter can be easily altered.

Systemic Blood Pressure

Blood pressure is generated by the heart's pumping action and is opposed by resistance. It is highest in the aorta and declines throughout the circulatory pathway, with the steepest drop in arterioles.

Arterial Blood Pressure

• Determined by arterial elasticity and the volume of blood forced into arteries.

• Systolic Pressure: Pressure during ventricular contraction (average 120 mmHg).

• Diastolic Pressure: Lowest aortic pressure during heart relaxation.

• Pulse Pressure: Difference between systolic and diastolic pressure.

• Mean Arterial Pressure (MAP): The pressure that propels blood to tissues. Calculated as:

$\text{MAP} = \text{Diastolic Pressure} + \frac{1}{3} \text{Pulse Pressure}$

• Example: If BP = 120/80 mmHg, Pulse Pressure = 40 mmHg, MAP = 80 + (1/3) * 40 = 93 mmHg.

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

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

• Pressure Points: Sites where arteries are close to the body surface and can be compressed to stop blood flow during hemorrhage.

Capillary Blood Pressure

• Ranges from 35 mmHg at the start to ~17 mmHg at the end of capillary beds.

• Low pressure prevents rupture of fragile capillaries and allows for filtration into interstitial spaces.

Venous Blood Pressure

• Low and changes little during the cardiac cycle (gradient ~15 mmHg).

• Low pressure requires adaptations for venous return:

  • Muscular Pump: Skeletal muscle contractions push blood toward the heart; valves prevent backflow.

  • Respiratory Pump: Breathing changes pressure, moving blood toward the heart.

  • Sympathetic Venoconstriction: Smooth muscle constriction under sympathetic control pushes blood toward the heart.

Regulation of Blood Pressure

Blood pressure is regulated by the heart, blood vessels, and kidneys, under supervision of the brain. Three main factors are cardiac output (CO), peripheral resistance (PR), and blood volume.

• Blood pressure varies directly with CO, PR, and blood volume.

• Key Formulas:

$F = \frac{\Delta P}{R}$ $CO = \frac{\Delta P}{R}$ $\Delta P = CO \times R$ $MAP = SV \times HR \times R$

• Anything increasing stroke volume (SV), heart rate (HR), or resistance (R) increases MAP.

• Regulation occurs via short-term (neural and hormonal) and long-term (renal) mechanisms.

Short-Term Regulation: Neural Controls

• Neural mechanisms alter vessel diameter and blood distribution.

• Operate via reflex arcs involving:

  • Cardiovascular center (medulla oblongata)

  • Baroreceptors

  • Chemoreceptors

  • Higher brain centers

• Cardiovascular Center: Clusters of sympathetic neurons in the medulla oblongata, including cardiac and vasomotor centers.

• Baroreceptor Reflexes:

  • High MAP stimulates baroreceptors, inhibiting vasomotor and cardioacceleratory centers, stimulating cardioinhibitory center, resulting in decreased BP via vasodilation and reduced CO.

  • Low MAP triggers reflex vasoconstriction, increasing CO and BP.

  • Baroreceptors adapt to sustained BP changes and become less effective.

• Chemoreceptor Reflexes: Detect increased CO2, decreased pH or O2, and increase BP by stimulating CO and vasoconstriction.

• Higher Brain Centers: Hypothalamus and cerebral cortex can modify BP during stress, exercise, and temperature changes.

Short-Term Regulation: Hormonal Controls

• Hormones regulate BP by altering peripheral resistance or blood volume.

• Adrenal Medulla Hormones: Epinephrine and norepinephrine increase CO and vasoconstriction.

• Angiotensin II: Stimulates vasoconstriction.

• ADH: High levels cause vasoconstriction.

• Atrial Natriuretic Peptide: Decreases BP by reducing blood volume.

Long-Term Regulation: Renal Controls

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

• Direct Renal Mechanism: Increased BP or volume leads to more urine output, reducing BP; decreased BP causes water conservation, raising BP.

• Indirect Renal Mechanism (Renin-Angiotensin-Aldosterone):

  • Low BP triggers renin release from kidneys.

  • Renin converts angiotensinogen (from liver) to angiotensin I.

  • Angiotensin-converting enzyme (mainly from lungs) converts angiotensin I to angiotensin II.

  • Angiotensin II stabilizes BP and ECF by stimulating aldosterone, causing ADH release, triggering thirst, and acting as a potent vasoconstrictor.

Summary of Blood Pressure Regulation

• The goal is to maintain BP high enough for adequate tissue perfusion, but not so high as to damage vessels.

• Too low BP to the brain causes loss of consciousness; too high can cause stroke.

Homeostatic Imbalances in Blood Pressure

Blood pressure can be transiently elevated or lowered due to various factors. Chronic imbalances can lead to significant health issues.

• Hypertension: Sustained BP of 140/90 mmHg or higher. Prehypertension is elevated but not yet hypertensive. Prolonged hypertension is a major cause of heart failure, vascular disease, renal failure, and stroke. Risk factors include heredity, diet, obesity, age, diabetes, stress, and smoking. Managed by lifestyle changes and antihypertensive drugs.

• Primary Hypertension: No identifiable cause; accounts for 90% of cases.

• Hypotension: BP below 90/60 mmHg. Usually not a concern unless it causes inadequate tissue perfusion. Often associated with longevity and low cardiovascular risk.

• Circulatory Shock: Blood vessels inadequately fill and cannot circulate blood normally, leading to insufficient tissue perfusion.