Special circulations

Local control vs systemic control

In brain and heart, intrinsic flow-regulating mechanisms are dominant.

In skin, systemic control dominant (role in BP control AND temperature control)

In skeletal muscles

• Resting - neural control (vasoconstriction) dominant
• Active - local control (vasodilation) dominant

Special circulation

• Bone
• Placenta

Summary of different circulations

Some baseline facts:

• Cardiac output = 5L/min
• O2 content of arterial blood = 20mL per 100mL blood
• O2 content of venous blood = 15mL per 100mL blood
• Oxygen flux = 1L O2/min
• Oxygen consumption = 250mL O2/min
• CO2 production = 200mL/min

Heart

• 300g by weight
• 5% of cardiac output
• 12% of total basal oxygen consumption
  * or 10% [KB]

Thus,

• Coronary blood flow
  = 250 mLs/min
• MVO2
  = 30 mLs/min
  = 10 mLs O2/100g/min
• Oxygen extraction ratio
  = 60%
  = 55-65% [KB2:p52]
  = 70-80% [WG21:p625]

Brain

• 1400g by weight
• 15% of cardiac output
• 20% of total basal oxygen consumption

Thus,

• CBF
  = 750 mLs blood/min
  = 50-54 mLs/min/100g
• CMRO2
  = 50 mLs O2/min
  = 3-3.5mLs O2/100g/min
• Oxygen extraction ratio
  = 33%

Kidney

• 300g by weight
  * ~0.5% of body weight
  * same as heart
• 24% of cardiac output
  * or 25% [KB]
• 7.2% of body oxygen consumption

Thus,

• Renal blood flow
  = 1200 mLs/min (or 1250mLs/min [KB])
  = 400 mLs/min/100g
• Renal oxygen consumption
  = 18mLs O2/min
  = 6 mLs O2/min/100g
• Oxygen extraction ratio
  = 7.5%

Liver

• ~2.5kg by weight
• 30% of cardiac output
• 20% of body oxygen consumption

Thus,

• Hepatic blood flow
  = 1500 mLs/min
  = 60 mLs/min/100g
• Hepatic oxygen consumption
  = 50 mLs O2/min
  = 2 mLs O2/min/100g
• Oxygen extraction ratio
  = 16.7%

NB:

Contribution to blood supply

• Portal vein : Hepatic artery
  = 3:1, or
  = 2:1 [KB]

Contribution to oxygen supply

• Portal vein : Hepatic artery
  = 1:3, or
  = 3:2 [KB]

Others

[KB]

• Blood flow to bone = 300mL/min
  * Blood flow = 200-400mL/min [WG21:p387]
• Blood flow to skin = 300mL/min
  * Blood flow to skin ~450mL/min [WG21:p615]
  * ~8% of cardiac output

Other notes

SVC vs IVC

PvO2 from IVC is normally higher (SatO2 = 77%)
* Due to low O2 extraction by kidney

PvO2 from SVC is lower (SatO2=71%)

Basal O2 consumption

• Normal O2 consumption = 3-3.5mL O2/kg/min
  * Or 250mL O2/min
• Foetal O2 consumption = 5 mL O2/kg/min
• Neonatal O2 consumption = 6-7 mL O2/kg/min
  * i.e. double that of adult

Alveolar ventilation

• Adult = 60-70 mLs/kg/min
• Neonate = 120-140 mLs/kg/min
  * i.e. double that of adult

Cardiac output and O2 consumption

	% of C.O.	% of VO2	Flow/ 100g	VO2/ 100g
Brain	15	20	50-54	3-3.5
Heart	5	10-12	~80	10
Liver	28-30	20	60	2
Kidney	24 (20-25)	7.2	400	6
Skin	9	5
Muscles	16	20	2-4
Bone
Bowel

NB:

[WG21:p615]

• Liver has the highest oxygen consumption as an organ (20.4%)