F5iii / 17A07: Compare and contrast the systemic circulation with the pulmonary circulation

17A07: Exam Report

Compare and contrast the systemic circulation with the pulmonary circulation.

26% of candidates passed this question.

This question encompasses a wide area of cardiovascular physiology. As a compare and contrast question this question was well answered by candidates who used a table with relevant headings. Comprehensive answers included: anatomy, blood volume, blood flow, blood pressure, circulatory resistance, circulatory regulation, regional distribution of blood flow,
response to hypoxia, gas exchange function, metabolic and synthetic functions, role in acid base homeostasis and filter and reservoir functions.

A frequent cause for missing marks was writing about each circulation separately but comparing. For example: many candidates stated ‘hypoxic pulmonary vasoconstriction’, but did not contrast this to ‘hypoxic vasodilation’ for the systemic circulation. Frequently functions of the circulations were limited to gas transport / exchange.

F5iii / 17A07: Compare and contrast the systemic circulation with the pulmonary circulation

  • Pulmonary circulation = flow from RV → PA → P capillaries → pulmonary vessels → P veins → LA
  • Systemic circulation = flow from LA → LV → systemic arteries → capillaries → veins → RA → RV

Differences

Pulmonary

Systemic

Anatomy

Circuit

Pulmonary

RV → LA

Systemic

LA → RV

Vessels

Pulmonary

Arterial vessels also have same  3 layers

  1. Adventitia
  2. Media
  3. Intima

 BUT

Thin walled

Less smooth m.

Compressible & distensible

Rapid subdivision over short distance

Subject to Recruitment & Distention

Systemic

Thick walled

Main arteries have 3 layers

  1. Adventitia
  2. Media
  3. Intima

Minimally distensible

Innervation

Pulmonary

Sympathetic, but less effect 2° minimal smooth m.

Systemic

Abundant smooth m.

ANS inn. → SVR highly regulated by vascular tone

Blood Vol

Pulmonary

500mL

Systemic

4.5L

Functions

Pulmonary

Oxygenate blood

Excrete CO2

Filter, immune, blood reservoir, metabolic (inactivate, BK, 5HT, convert Angiotension I → Angiotensin II)

Systemic

Deliver O2 to tissue

Remove CO2 for transport

Excretion mechanisms

Pressures & Flow

Pressures

Pulmonary

25/8 (MAP 15)

Systemic

120/80 (MAP 100)

Pressure Drop; Inlet/Outlet

Pulmonary

10mmHg (low R circuit)

Systemic

100mmHg (high R circuit)

Atrial P

Pulmonary

5mmHg

Systemic

2mmHg

Major Resistance

Pulmonary

Evenly distributed

Systemic

Arterioles – very muscular arterioles which direct BF to various organs according to need

Resistance

Pulmonary

80 dynes/s/cm5

Systemic

800 – 1600 dynes/s/cm5­­

Flow

Pulmonary

Undampened, highly pulsatile

↓

Because of low resistance

↓

Low PA resistance does not damp the pressure waveform produced by RV

Systemic

Dampened, linear

Factors Affecting Flow & Resistance

ANS

Pulmonary

Minimal effect on vessel tone

Systemic

Principle determinant of SVR, esp in arterioles which is main site of Resistance

↑CO2

Pulmonary

VC

Systemic

VD

↓O2

Pulmonary

HPV C

Systemic

Local VD

Lung Volume

Pulmonary

PVR lowest @ FRC

Higher R with ↑/↓ lung volume

Systemic

Minimal effect

Posture – Hydrostatic Force

Pulmonary

Lung apex → base

30 – 40cm H2O

Low R circuit ∴gravity has a large effect of on circulation

Culminating in the…

West’s Zones 1 → 4

Systemic

Large hydrostatic force from head →  ~170cm H2O, but MAP is high (high resistance circuit, so minimal effect)