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 (¾ total)

·  Blood supply in diastole only

·  RV

·  Smaller

·  Thinner

·  More compliant (CVP 2mmHg)

·  Lower MVO2 and CBF (¼ 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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