Blood Vessels: Blood Pressure

Blood Flow Rate

The rate at which blood moves through blood vessels. Calculated as (High pressure - Low pressure) / Resistance

Resistance (Resistance to Flow)

The sum of all forces that oppose the movement of fluid through blood vessels. Increased resistance leads to decreased blood flow rate. Decreased resistance leads to increased blood flow rate. Resistance also impacts blood pressure

Factors that alter resistance to flow

Blood viscosity Thicker blood = more resistance = less flow. Depends on plasma vs. formed elements.

Blood vessel length. Longer vessels = more resistance = less flow. Changes with conditions like obesity.

Blood vessel radius/diameter. Smaller radius (vasoconstriction) = ↑ resistance, ↓ flow. Larger radius (vasodilation) = ↓ resistance, ↑ flow.

Blood Viscosity

The thickness of blood

Blood pressure

The force of blood pushing against vessel walls (a hydrostatic pressure). Usually refers to systemic arterial pressure, which decreases as blood moves away from the heart.

Types of Blood Pressure discussed

Arterial blood pressure: Higher pressure, measured in arteries

Venous blood pressure: Lower pressure, measured in veins

Systemic blood pressure: Measured in the systemic circulation

Pulmonary blood pressure: Measured in the pulmonary circulation

Systolic pressure: Higher pressure, measured during systole (heart contraction). Normal value is 120 mmHg or less

Diastolic pressure: Lower pressure, measured during diastole (heart relaxation). Normal value is 80 mmHg or less

How Systemic Arterial Blood Pressure is reported

Always reported as Systolic Pressure / Diastolic Pressure

Mean Arterial Pressure (MAP)

Average arterial pressure.
Formula: Diastolic + (Systolic − Diastolic)/3
Normal = ≤100 mmHg, usually closer to diastolic.
"Blood pressure" often means MAP.

Major factors determining Blood Pressure

BP depends on HR, SV, and PR.
↑ in any = ↑ BP; ↓ = ↓ BP.
The body adjusts these to keep BP normal.

Regulation of Blood Pressure

Blood pressure must be maintained within a normal range to propel blood through arteries to capillaries. Regulation involves fast, neural control mechanisms and slow, hormonal control mechanisms

Neural Control Mechanisms of Blood Pressure

Provide second-to-second / short-term regulation using a neuronal reflex arc called the peripheral baroreceptor reflex

Components of the Peripheral Baroreceptor Reflex

Peripheral baroreceptors: Stretch receptors in the aorta and carotid arteries. They turn stretch from BP into action potentials (APs). More BP = more APs, less BP = fewer APs.

Medulla oblongata: CIC lowers HR. CAC increases HR. Vasomotor center controls vessel diameter (PR)

Effectors: Heart adjusts HR & SV. Arterioles adjust PR through vasoconstriction/dilation

Peripheral Baroreceptor Reflex Response to a Rise in Blood Pressure

Baroreceptors detect ↑BP → more APs

More APs → CIC activated

CIC → ↓ HR via more parasympathetic input → ↓ BP

APs inhibit vasomotor center → ↓ sympathetic signals

Less sympathetic input → vasodilation

Vasodilation → ↓ PR → ↓ BP

Peripheral Baroreceptor Reflex Response to a Drop in Blood Pressure

Baroreceptors detect ↓BP → fewer APs

Fewer APs → CIC less active, CAC more active

CAC → ↑ sympathetic, ↓ parasympathetic → ↑ HR & SV → ↑ BP

Vasomotor center more active → ↑ sympathetic signals

More sympathetic input → vasoconstriction

Vasoconstriction → ↑ PR → ↑ BP

Hormonal Control Mechanisms of Blood Pressure

Perform long-term regulation of blood pressure using hormones that alter blood volume. Recall that increased blood volume increases SV, which increases BP

Hormonal Response to Decreases in Blood Volume and Blood Pressure

Kidneys detect ↓ BV/BP → release Renin

Renin starts RAAS

Renin → Angiotensinogen → Angiotensin I

ACE converts it to Angiotensin II

• Angiotensin II effects:

  • ADH release → ↑ water retention → ↑ BV → ↑ BP

  • Aldosterone release → ↑ water retention → ↑ BV → ↑ BP

  • Hypothalamus → ↑ thirst → ↑ intake → ↑ BV → ↑ BP

  • Vasoconstriction → ↑ PR → ↑ BP

Hormonal Response to Increases in Blood Volume and Blood Pressure

• Heart stretch → releases ANP & BNP

• These hormones:

  • ↑ urine output → ↓ BV, SV, BP

  • ↓ thirst → ↓ intake → ↓ BV, SV, BP

    • Cause vasodilation → ↓ PR → ↓ BP

Hypertension

Chronic high blood pressure

Classes of drugs to treat Hypertension

Diuretics: Increase urine → ↓ BV

Beta Blockers: ↓ HR & SV

ACE Inhibitors: Block Angiotensin II → ↓ BV