Blood Vessel Networks and Pressure Regulation

• How do series and parallel arrangements of blood vessels affect total resistance, and why is this physiologically important?

  • In series (e.g., artery → arteriole → capillary → venule → veins), total resistance Rtotal​ is the sum of all individual resistances:
  • Rtotal = R1 + R2 + R3 + ...This approximates the resistance of the smallest vessel, which gives the highest resistance in the circuit.
  • In parallel (e.g., systemic capillary beds),
  • 1/Rtotal=∑1/Ri
  • Total resistance to flow is 1 over the sum of the inverse resistances. It is less than the resistance of the vessel with the lowest resistance.
  • Allows blood to be evenly distributed while keeping pressure drops small.
  • Physiological relevance:
  • Increases the efficiency of nutrient delivery.
  • Prevents excessive pressure drops.
  • Explains why widespread vasodilation (more vessels in parallel) lowers total peripheral resistance.

  A) parallel

  B) little

  C) large

  D) lowers

  
• How does pressure change throughout the circulatory system, and what determines these changes?

  • Pressure is highest in the large arteries (e.g., aorta) and lowest in the large veins (e.g., vena cava).
  • Largest pressure drop occurs across the smallest vessels. But by having many vessels in parallel, the pressure drop is less than if the vessels were in series.
  • Flow (F) is defined by:
  • F = P ÷ R
  • In the whole body:MAP = CO × TPR
  • To maintain constant flow (CO) despite lower resistance in capillaries, pressure must fall across each segment.
  • Oscillations in pressure are due to the pulsatile nature of systole and diastole but are dampened by arterial compliance.
  • Blood reaches capillaries at a relatively constant pressure to prevent damage and ensure steady flow.

  A) same

  B) drops

  C) big

  D) less

  
• • As the vessels get smaller, the pressure becomes more constant.
  • That is because the elasticity in the arteries can damp down the pressure changes, so that by the time blood reaches the capillaries, it flows smoothly under constant resistance
• Largest drop in pressure occurs across the arterioles.

  Pressure is higher during systole as blood is actively pumped and lower during diastole.

  
• What is total peripheral resistance and how does it differ between systemic and pulmonary circulation?

  • TPR is the total resistance to blood flow in the systemic vasculature, calculated by:
  • TPR = (MAP−RAP)/CO, where MAP is mean arterial pressure and RAP is right arterial pressure.
  • Units: Peripheral Resistance Units (PRU) - 1 PRU = 100 mm.Hg/100ml.s-1
  • Systemic circulation:
  • High pressure, high resistance
  • TPR ≈ 1.08 PRU at rest
  • Pulmonary circulation:
  • A fraction of that in the systemic circulation
  • Low pressure, low resistance
  • Pulmonary resistance ≈ 0.14 PRU
  • Essential to protect delicate alveoli from high pressure damage
• How is blood pressure regulated by the body, and what are the roles of CO and TPR?

  • MAP = CO × TPR
  • MAP (mean arterial pressure) must be kept in a tight range (typically ~93 mmHg):
  • Too high = hypertension
  • Too low = hypotension
  • Modulated by changing CO or TPR or both
  • CO increases with:
  • Sympathetic activity (increases HR and contractility)
  • Exercise (increase venous return, increase preload)
  • Depends on the O2 needs of the body
  • TPR is mainly regulated by:
  • Arteriolar tone (vasoconstriction increases TPR, vasodilation decreases TPR)
  • Can vary 20-fold due to smooth muscle control
  • Blood pressure meds often target arteriolar tone (e.g., vasodilators)
• How do the autonomic nervous system and humoral factors control blood pressure?

  • Sympathetic Nervous System:
  • increases HR and contractility → increases CO
  • Arteriolar vasoconstriction → increase TPR
  • Parasympathetic Nervous System:
  • reduce HR (mainly via vagus nerve)
  • Little effect on contractility or arteriolar tone
  • Baroreceptors (carotid sinus, aortic arch):
  • Sense stretch → send signals to the brainstem
  • Rapid reflex control of HR and vascular tone
  • Hormones:
  • Adrenaline/noradrenaline (increase HR, vasoconstriction)
  • Renin-Angiotensin-Aldosterone System (RAAS):
  • Angiotensin II → vasoconstriction
  • Aldosterone → Na⁺/H₂O retention → increase blood volume → increase MAP
  • ADH (vasopressin): water retention + vasoconstriction
• What are the consequences of vasoconstriction and vasodilation in arteries and veins?

  • Arteries:
  • Vasoconstriction → increases TPR → increases MAP
  • Vasodilation → reduces TPR → reduces MAP
  • Veins (high compliance):
  • Venoconstriction → increase venous return → increase preload → increase CO
  • Venodilation → decrease preload → decrease CO
  • Local control (e.g., metabolic demand) and systemic control (e.g., SNS) coordinate tone.
  • Hyperaemia:
  • Functional hyperaemia: increases blood flow during activity (e.g., muscles during exercise)
  • Reactive hyperaemia: rebound increase flow after temporary occlusion