Week 3: Blood Pressure & Homeostasis

Key Terms

• Flow – volume of blood moving through a vessel per unit time.
• Perfusion – flow per mass/volume of tissue (delivery of O₂ & nutrients).
• Blood Pressure (BP) – force of blood on arterial wall.

Normal Values & Calculations

• Normal adult BP ≈ 120/80\,\text{mmHg}.
• Systolic (contraction) / Diastolic (relaxation).
• Pulse Pressure: PP = \text{systolic} - \text{diastolic}.
• Mean Arterial Pressure: MAP = \dfrac{\text{systolic}-\text{diastolic}}{3} + \text{diastolic}.
• Hypertension ≥ 140/90\,\text{mmHg}; Hypotension < normal.

Determinants of BP

• Peripheral Resistance
  • Viscosity ↑ (dehydration, polycythemia) → BP ↑; viscosity ↓ (anemia, hypoproteinemia) → BP ↓.
  • Vessel Length ↑ → resistance ↑.
  • Radius ↓ (vasoconstriction) → resistance ↑ & BP ↑; radius ↑ (vasodilation) → BP ↓.
• Cardiac Output (CO)
  • CO = HR \times SV; HR or SV ↑ → BP ↑; HR or SV ↓ → BP ↓.
• Blood Volume
  • Volume ↑ (fluid retention) → BP ↑; Volume ↓ (bleeding, dehydration) → BP ↓.

Vascular Aging & Disease

• Arteriosclerosis – loss of arterial elasticity.
• Atherosclerosis – lipid/calcific plaques; both elevate BP.

Regulation Mechanisms

• Local (Autoregulation): histamine, prostaglandins, angiogenesis.
• Neuronal: sympathetic vasomotor center; baro-, chemo-, medullary ischemic reflexes.
• Hormonal:
  • Angiotensin II – potent vasoconstrictor, ↑ volume.
  • Aldosterone – Na⁺/water retention → volume ↑.
  • ADH – water reabsorption → volume ↑.
  • Catecholamines – vasoconstriction, CO ↑.
  • ANP – Na⁺/water excretion → volume ↓.

Capillary Exchange

• Diffusion (small solutes & gases).
• Transcytosis (large molecules in vesicles).
• Filtration (arterial end, hydrostatic pressure) & Reabsorption (venous end, colloid osmotic pressure).

Venous Return Aids

• Venous valves.
• Skeletal muscle pump.
• Respiratory pump.
• Gravity (head/neck).

Homeostatic Imbalances

• Thrombus – stationary clot; Embolus – traveling fragment.
• Transient Ischemic Attack (TIA) – brief cerebral ischemia, no permanent damage.
• Cerebrovascular Accident (Stroke)
  • Ischemic (≈80\%) – vessel occlusion.
  • Hemorrhagic – vessel rupture.
• Circulatory Shock
  • Cardiogenic – pump failure (MI).
  • Low Venous Return:
  – Hypovolemic (blood/fluid loss).
  – Obstructed venous return (compression).
  – Venous pooling (gravity, neurogenic, vasodilation).
  • Mixed: Septic, Anaphylactic.
• Edema – fluid accumulation from ↑filtration, ↓reabsorption, or blocked lymphatics.

Quick Rules to Remember

• Smaller radius ⇒ resistance ↑ ⇒ flow ↓.
• As resistance ↓, flow ↑.
• BP decreases with distance from heart.
• PP, MAP, and vessel radius changes are rapid indicators of arterial health and perfusion status.