Blood Pressure Regulation: Short-Term & Long-Term Mechanisms

Blood Pressure (BP) is the force that blood exerts against the walls of blood vessels, measured in $\mathrm{mmHg}$ .
Systolic Blood Pressure (SBP): the peak pressure during heart contraction.
Diastolic Blood Pressure (DBP): the minimum pressure during heart relaxation.
BP values for a typical young adult at rest: SBP ≈ $120\ \mathrm{mmHg}$ , DBP ≈ $80\ \mathrm{mmHg}$ , often recorded as $SBP/DBP = 120/80.$
Pulse Pressure (PP): the difference between systolic and diastolic pressures.
- $PP = SBP - DBP$
Mean Arterial Pressure (MAP): a weighted average pressure driving blood through the circulatory system.
- MAP is slightly less than the arithmetic mean of SBP and DBP.
- $MAP = DBP + \frac{1}{3}\,PP = DBP + \frac{1}{3}(SBP - DBP)$
Blood Pressure (BP) relation to cardiac output and peripheral resistance:
- $BP = CO \times TPR$
- Cardiac Output: $CO = HR \times SV$
- Alternative expression: $MAP \approx HR \times SV \times PR$ where PR stands for peripheral resistance.
Typical resting values used in calculations:
- SBP ≈ $120\ \mathrm{mmHg}$ , DBP ≈ $80\ \mathrm{mmHg}$
- BP is read as approximately 120/80.
Key concepts:
- BP is determined by heart rate, stroke volume, and peripheral resistance.
- Regulation occurs on two timescales:
- Short-term (seconds to minutes): neural reflexes and hormones.
- Long-term (hours to days): kidney-mediated mechanisms that regulate blood volume.

Baroreceptors are stretch-sensitive receptors that monitor BP continuously (minute-to-minute control).
Primary locations:
- Carotid sinus (carotid arteries)
- Aortic arch
Their signals are transmitted to the brain via:
- Glossopharyngeal nerve (IX) from carotid sinus baroreceptors
- Vagus nerve (X) from aortic arch baroreceptors
Central processing occurs in the medulla oblongata at two major centers:
- Cardioregulatory Center
- Vasomotor Center
Efferent responses:
- Increased parasympathetic stimulation of the heart → decreases heart rate (via SA node).
- Increased sympathetic stimulation of the heart → increases heart rate and stroke volume.
- Increased sympathetic stimulation of blood vessels → vasoconstriction.
Net short-term effect:
- If blood pressure increases: parasympathetic activity rises and sympathetic activity to the heart and vessels decreases → BP falls.
- If blood pressure decreases: parasympathetic activity diminishes and sympathetic activity increases → BP rises.
Key concept: this is a negative feedback mechanism that rapidly stabilizes MAP within seconds to minutes.

Triggered by a rise in sympathetic activity or a sudden change in activity or a large drop in BP.
Mechanism:
- Sympathetic nerve fibers stimulate the adrenal medulla to release catecholamines (epinephrine and norepinephrine).
Net effect on BP:
- Increases heart rate and contractility (CO↑)
- Increases peripheral vasoconstriction (TPR↑)
- Result: elevation of BP to restore perfusion during stress or sudden BP drops.

Chemoreceptors respond to chemical changes in the blood.
Primary triggers:
- Decreased oxygen (O₂)
- Increased carbon dioxide (CO₂)
- Resulting changes in blood pH (acid-base balance)
Effect on BP:
- Changes in chemoreceptor activity influence autonomic outflow to adjust BP as part of maintaining adequate gas exchange and tissue perfusion.

Emergency response when blood flow to the medulla oblongata is severely restricted.
Mechanism:
- Ischemia to the brain stimulates the vasomotor center to induce widespread vasoconstriction.
Purpose:
- Rapidly raise arterial pressure to restore cerebral perfusion in critical situations.

Primary role: regulate blood volume and vascular resistance over hours to days via kidney function.
Trigger for renin release:
- Low blood volume, low MAP, or reduced renal blood flow (RBF) → juxtaglomerular cells release renin.
Biochemical cascade:
- Renin catalyses the formation of Angiotensin I from angiotensinogen.
- Angiotensin I is converted to Angiotensin II in the lungs via angiotensin-converting enzyme (ACE): $ext{Angiotensin I} \xrightarrow{ACE} \text{Angiotensin II}$
Effects of Angiotensin II:
- Stimulates secretion of aldosterone and antidiuretic hormone (ADH).
Consequences for blood pressure and volume:
- Aldosterone and ADH increase Na⁺/Cl⁻ and water reabsorption in renal tubules (increase blood volume).
- Increased blood volume leads to higher MAP.

Trigger:
- Decreased blood pressure detected by baroreceptors.
Actions:
- ADH causes vasoconstriction of blood vessels (increases peripheral resistance).
- ADH decreases the rate of urine production by the kidneys, helping to retain water and maintain blood volume.
Net effect: supports maintenance of blood pressure over longer periods by modulating volume and vascular tone.

Nature:
- ANP is a polypeptide released from atrial myocytes.
Primary targets and effects:
- Acts on the kidneys to increase urine production and Na⁺ loss in urine (natriuresis).
- Dilates arteries and veins, reducing peripheral resistance.
Hemodynamic consequences:
- Decrease in blood volume, venous return, and peripheral resistance → decreased BP.

Blood pressure is regulated by two complementary systems:
- Short-term regulatory mechanisms (seconds to minutes): baroreceptor reflex, adrenal medullary mechanism, chemoreceptor reflexes, CNS ischemic response.
- Long-term regulatory mechanisms (hours to days): RAAS, ADH, ANP.
Key relationships and equations to remember:
- $PP = SBP - DBP$
- $MAP = DBP + \frac{1}{3}PP = DBP + \frac{1}{3}(SBP - DBP)$
- $BP = CO \times TPR$
- $CO = HR \times SV$
- $MAP \approx HR \times SV \times PR$
Clinical relevance:
- The RAAS, ADH, and ANP systems are common targets for treating hypertension and volume disorders.
- Understanding baroreceptor and chemoreceptor reflexes is essential for interpreting orthostatic changes and autonomic dysfunction.
Foundational note:
- The material aligns with Seeley’s Anatomy & Physiology (12th ed.) and is used in medical education to illustrate cardiovascular homeostasis.