Regional blood flow & exercise

Where the cardiac output goes at rest, how radically that redistributes in exercise, and the metabolic numbers that go with it.

Distribution of cardiac output at rest

Based on a resting cardiac output of about 5 L/min.

Organ	Flow at rest	% of CO	In exercise
Skeletal muscle	1200 mL/min	~20%	Light: 4500 mL/min (~47%) Heavy: 22 000 mL/min (~88%)
GI tract / splanchnic	1400 mL/min	~25%	Falls to ~300 mL/min
Kidney	1100 mL/min	~20%	Falls to ~250 mL/min in severe exercise
Brain	750 mL/min	~15%	Essentially unchanged
Coronary	250 mL/min	~5%	Rises to ~1000 mL/min
Skin	200–500 mL/min	~5%	Rises — thermoregulation

In exercise, flow is redistributed, not just increased. Muscle and skin gain; gut and kidney lose; brain is protected and unchanged; coronary flow rises in proportion to cardiac work.
Cardiac output itself rises from about 5 L/min to 20–25 L/min — so muscle takes both a bigger share of a much bigger total.
The kidney and gut are the organs sacrificed. That is exactly why prolonged hypoperfusion causes acute kidney injury and gut ischaemia before it damages the brain.

Organ	O₂ consumption (mL/100 g/min)	Extraction
Heart	10 at rest → ~70 in exercise	70% — the highest in the body
Kidney	5	Low — flow far exceeds metabolic need
Brain	3	~35%
Body as a whole	Total VO₂ ≈ 250 mL/min at rest	~25%

At rest the myocardium already extracts 70% of delivered oxygen, against about 25% for the rest of the body. Coronary sinus saturation is therefore only around 30%.
With so little extraction reserve, the only way to meet extra demand is to increase coronary flow. This is the physiological basis of angina: if flow cannot rise, ischaemia follows.

Mitochondrial PO₂ is only about 1–3 kPa — the bottom of the oxygen cascade.
The critical mitochondrial PO₂, below which oxidative phosphorylation fails and lactate is produced, is roughly 0.13 kPa (about 1 mmHg). Above that, ATP production is independent of oxygen tension.

Quantity	Value	Meaning
ATP from 1 mole of glucose — aerobic	38 ATP	Complete oxidation via glycolysis, Krebs and the electron transport chain
ATP from 1 mole of glucose — anaerobic	3 ATP	Glycolysis alone (2 net ATP from glucose, 3 from glycogen) — hence the enormous inefficiency of anaerobic metabolism
Respiratory quotient at rest	0.8	Mixed diet. Pure carbohydrate = 1.0, pure fat = 0.7, protein = 0.8
RQ above the anaerobic threshold	> 1.0	Extra CO₂ from bicarbonate buffering of lactate — this is what makes the threshold detectable
Basal metabolic rate	197 kJ/m²/hr ≈ 40 kcal/m²/hr	Roughly 2000 kcal/day for an average adult
Anaerobic threshold	45–65% of VO₂max up to ~80% in the trained	The workload at which lactate begins to accumulate

Your page has "50–70% anabolic threshold". The standard term for the 50–70% of VO₂max figure is the anaerobic (lactate) threshold — "anabolic threshold" is not standard physiology, and the two lines on your page (50–70% and 45–65%) are describing the same quantity. Quote it as the anaerobic threshold.
Your page also records "RSR at rest 0.8". That value is the respiratory quotient (RQ) — or the respiratory exchange ratio (RER) when measured at the mouth. Worth relabelling in your notes.

1 MET = resting oxygen consumption = 3.5 mL/kg/min.
4 METs — climbing a flight of stairs or walking up a hill — is the classic threshold for acceptable functional capacity.
CPET measures the anaerobic threshold directly. An anaerobic threshold below about 11 mL/kg/min predicts significantly increased perioperative morbidity and mortality.

Built from handwritten pages IMG_0987 and IMG_0988.