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 gradient aids local Q control

·   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

·   <1% L-R shunt: bronchial->pulmonary veins

·   1.5% R->L shunt: Thebesian, bronchial, pleural veins

 

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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