Coronary circulation and myocardial ischemia !
- Concept
- Exam priority: medium
The heart feeds mostly in diastole and already extracts almost all its oxygen at rest: if it needs more, it can only get more flow.
Updated 2026-11-01
See it in 3D: Left main coronary artery · LAD (anterior descending artery) · Circumflex artery · Right coronary artery · Posterior descending artery (PDA) · Coronary sinus
What it is
The coronary arteries arise from the aortic root and carry about 4–5% of cardiac output to the myocardium. The left ventricle squeezes its own vessels in systole, so it gets most of its flow in diastole. Because oxygen extraction is already very high at rest, extra demand is met by metabolic vasodilation; when a plaque limits that reserve, ischemia appears.
Key facts
- Adenosine, a breakdown product of ATP, is the main coronary metabolic vasodilator.
- Ischemic cascade: relaxation fails first, then contraction, then the ECG changes, and pain comes last.
- Myocardial venous blood drains mostly into the coronary sinus, which opens into the right atrium.
- Collaterals grow when a stenosis progresses slowly; a sudden occlusion leaves no time to form them.
Electrocardiogram: a normal beat in lead II
HR 100 /minRR 0.60 sPR 0.16 sQRS 0.09 sQT 0.33 sQTc 0.43 s
Tap a wave (P, QRS, T, U), an interval (PR, ST, QT) or the RR.
- ECG trace (lead II)
- Small box: 0.04 s · 0.1 mV
- Big box: 0.20 s · 0.5 mV
- Selected part
Equations
- Aortic DBP
- aortic diastolic pressure
- LVEDP
- left ventricular end-diastolic pressure
Diastolic hypotension or a high LVEDP reduce subendocardial perfusion.
- P
- intraventricular pressure
- r
- radius
- h
- wall thickness
A dilated ventricle spends more oxygen to generate the same pressure.
Reference values (resting adult)
| Parameter | Value | Note |
|---|---|---|
| Coronary blood flow | ≈225–250 mL/min | ≈70 mL/min per 100 g |
| Share of cardiac output | ≈4–5% | |
| Myocardial O₂ extraction | ≈70–75% | |
| Coronary sinus saturation | ≈25–30% |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Clinical notes
- Stable angina: a fixed plaque caps the reserve; pain comes with exertion and eases with rest.
- Infarction: plaque rupture forms an occlusive thrombus; without reperfusion necrosis spreads from subendocardium to epicardium over hours.
- Aortic stenosis causes angina without coronary disease: more muscle mass, higher systolic pressure and lower diastolic perfusion pressure.
Test yourself
Think of the answer, then tap the question to see it.
Why can't the heart raise oxygen extraction much when demand rises?
At rest it already extracts about 70–75% of the oxygen; extra demand is met almost only by more flow, through metabolic vasodilation.
Why can tachycardia trigger ischemia in a patient with coronary stenosis?
It raises oxygen demand and, by shortening diastole, cuts the time the LV receives flow: demand goes up and supply goes down.
Why is the subendocardium the first layer to become ischemic?
It takes the highest compressive pressure in systole, depends almost entirely on diastolic flow and sits at the end of the branching from the epicardial vessels.
How does the law of Laplace explain why a dilated ventricle uses more oxygen?
Wall tension rises with radius; to generate the same pressure a large ventricle needs more tension and therefore more oxygen.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
- Left main coronary artery
- LAD (anterior descending artery)
- Circumflex artery
- Right coronary artery
- Posterior descending artery (PDA)
- Coronary sinus
Related topics
- Local and humoral control of blood flow
- The ECG in ischemia and infarction
- Muscle blood flow and the cardiovascular response to exercise
- The cardiac cycle
- Physiology of hypertension
Related conditions
- Myocardial Infarction (Heart Attack)
- Stable angina
- Atherosclerosis and dyslipidaemia
- Aortic Valve Stenosis
In Biochemistry
- Cardiac troponin as a lab test
- Creatine kinase (CK) and CK-MB as lab tests
- ATP (adenosine triphosphate)
- Lactate
In Pharmacology
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 19.1 Heart Anatomy (OpenStax (Rice University), 2022); Anatomy, Thorax, Heart Left Anterior Descending (LAD) Artery (StatPearls (NCBI Bookshelf), 2026); Myocardial Infarction (StatPearls (NCBI Bookshelf), 2024); Stable Angina (StatPearls (NCBI Bookshelf), 2026); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 21). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.