2015B09 Briefly outline the differences between the pulmonary circulation and the systemic circulation.

 

List:

�       Aims

�       Anatomy

�       Haemodynamics

�       Controls

 

Aims:

 

Systemic

Pulmonary

 

�  Perfusion of tissues

�  For nutrient/gas/waste exchange

�  Perfusion of lungs

�  For gas exchange

 

�  ↓Pulmonary venous pressure ->

�  ↓pulmonary oedema

�  ↓Systemic venous pressure ->

�  ↓peripheral oedema

 

�  Blood reservoir

�  Filtration, metabolism (e.g. ACE)

 

Anatomy:

 

Systemic

Pulmonary

Pump

�  LV

�  Larger

�  Thicker

�  Less compliant (LVEDP 6mmHg)

�  Higher MVO2 and CBF (� total)

�  Blood supply in diastole only

�  RV

�  Smaller

�  Thinner

�  More compliant (CVP 2mmHg)

�  Lower MVO2 and CBF (� total)

�  Blood supply throughout cycle

Vessels

�  Arteries: elastic media
(for Windkessel effect)

�  Arterioles: muscular media
(for control of flow)

�  Veins and venules: highly distensible

�  Arteries: less elastic

�  Arterioles: less muscular

�  Veins and venules: less distensible

Capillaries

�  Thicker endothelium

�  ↑Diffusion distance (variable)

�  Filtration pressure +ve arteriolar end, -ve venular end (in theory)

�  Transudation 4L/day

�  Thinner endothelium

�  ↓Diffusion distance 0.5μm

�  Filtration pressure -ve throughout (?)

�  Transudation minimal

 

Haemodynamics:

 

Systemic

Pulmonary

Volume (supine)

�  Total: 76% = 3500mL (+ LA,LV 4%)

�  Arteries: 14%

�  Capillaries: 6%

�  Veins: 56%

�  Total: 16% = 800mL (+ RA, RV 4%)

�  (↓to 9% when upright)

�  Arteries: 8%

�  Capillaries: 3%

�  Veins: 5%

Flow

�  5L/min

�  Continuous at capillary

�  ↑In spont vent expiration

(↑intrathoracic pressure)

�  5L/min

�  Pulsatile at capillary

�  ↑in spont vent inspiration

(↓intrathoracic pressure)

Pressure

�  High

�  SBP 120mmHg, DBP 80mmHg

�  mAP 90mmHg -> CVP 2mmHg

�  High pressure for a) uphill flow b) allocation

�  Major drop at arterioles

�  Low

�  PASP: 25mmHg, PADP 8mmHg

�  mPAP 15mmHg -> LVEDP 6mmHg

�  Low pressure minimizes transudation

�  Gradual drop across entire length

Resistance

�  High 720-1600 dyn.s.cm-5

�  9-20 Wood units

�  Mostly at arterioles

�  Low 20-160 dyn.s.cm-5

�  0.25-2 Wood units

�  Distributed arteries/capillaries/veins

Distribution

�  Local autoreg (metabolic myogenic)

�  Autonomic (SNS > PSNS)

�  West zones (gravity)

o Z1: PA > Pa > Pv (no flow)

o Z2: Pa > PA > Pv (flow in systole)

o Z3: Pa > Pv > PA (flow throughout)

�  Hypoxia -> HPV -> redirection

Shunt

�  L-R shunt (minimal in health)

o Bronchial arteries -> pulmonary arteries via pre-capillary anastomoses

�  R->L shunt (1.5%)

o Thebesian veins

o Bronchial veins -> pulmonary veins via post-capillary anastomoses

 

Control:

 

Systemic

Pulmonary

Most important

�  Local autoregulation (myogenic, metabolic)

�  SNS

�  PAO2 (i.e. hypoxic pulmonary vasoconstriction)

�  Gravity (West zones)

Nervous

�  SNS dense -> vasoconstriction (VC)

�  PSNS sparse -> vasodilatation (VD)

�  SNS sparse -> VC

�  PSNS sparse -> VD

Gaseous (note opposites!)

�  ↓PaO2 -> VD

�  ↑PaCO2 -> VD

�  ↓PvO2 -> VC

�  ↑PvCO2 -> VC

Acid-base

(note opposites)

�  ↓pH -> VD

�  ↓pH -> VC

Exercise

�  Active vasodilatation in skeletal muscles

�  Passive vasodilatation: recruitment/distension

 

 

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