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
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%)