Cardiovascular Physiology – Exercise & Coronary Dynamics
Mean Arterial Pressure (MAP) and the Dominance of Diastole
Mathematically, mean arterial pressure is calculated as
Because only one‐third of the pulse pressure is added to diastolic pressure, the diastolic blood pressure (DBP) weighs more heavily in the final value.
Physiologically, this fits the time course of the cardiac cycle: we spend roughly one-third of each cycle in systole and two-thirds in diastole.
Therefore, any sizeable change in DBP exerts a proportionally larger effect on MAP than an equivalent change in systolic blood pressure (SBP).
Example: For a classic blood pressure of 120⁄80 mmHg:
• Pulse pressure =
• One-third of pulse pressure =
• MAP =
This numerical exercise highlights both the computation and the conceptual takeaway: DBP is the stronger determinant of mean pressure because the cardiovascular system dwells longer in diastole.
Redistribution of Blood Flow From Rest to Exercise
At rest (≈ 5 L·min⁻¹ cardiac output) blood is apportioned widely: the gastrointestinal (GI) tract receives a conspicuous share, followed by kidneys, brain, skin, bone, and a modest 15–20 % to skeletal muscle.
During dynamic exercise the pattern flips. Skeletal muscle now receives 80–85 % of total flow, while:
GI perfusion plummets (≈ 20 % → 3 %),
Renal, cutaneous, and even coronary shares drop proportionally,
Cerebral flow falls slightly in relative terms, though absolute flow is fairly protected.
The cardiovascular system accomplishes this by global sympathetic drive (vasoconstriction) superimposed on powerful local metabolic vasodilation within active muscles. The outcome is a dramatic shunting of blood toward working tissue so oxygen delivery can match metabolic demand.
Acute Functional Consequences of Flow Redistribution
Cognitive function immediately post-exercise often declines because cerebral perfusion is transiently reduced while muscular demands dominate.
Exercise-related nausea or vomiting after eating stems from curtailed GI blood flow; digestion competes unsuccessfully with exercising muscle for perfusion.
Stroke risk rises transiently during strenuous effort principally because arterial pressure (especially SBP) soars, subjecting fragile cerebral micro-vessels to higher wall tension; it is the pressure surge rather than absolute cerebral flow that underlies the danger.
Cardiac Output and Skeletal Muscle Flow During Exercise
Total cardiac output ((Q\dot_c)) may climb from roughly 5 L·min⁻¹ at rest to 20–30 L·min⁻¹ in vigorous activity.
Skeletal-muscle flow alone can expand from ≈ 1.2 L·min⁻¹ up to ≈ 22 L·min⁻¹, dwarfing distribution to all other beds.
Although other organ systems experience curtailed perfusion acutely, post-exercise hyperemia helps restore and sometimes elevate blood supply to those tissues, underscoring the net health benefit of exercise.
Coronary Circulation: Structure and Oxygen Extraction
The myocardium is a continuously active, highly aerobic organ. Blood reaches it via left and right coronary arteries, which branch from the aortic root.
Even at rest the heart extracts ≈ 60–70 % of the oxygen contained in coronary arterial blood—a far higher extraction ratio than skeletal muscle at rest—signifying heavy basal metabolic needs.
Because the heart already harvests most available O₂, increases in demand are met primarily by increasing coronary blood flow, not by increasing extraction.
Anatomical knowledge of the coronary tree becomes clinically pivotal when interpreting 12-lead ECGs: the site of ST-segment changes (inferior, anterior, lateral, etc.) maps to occlusion in specific coronary branches—content to be revisited in later lectures.
Regulation of Coronary Blood Flow
Flow hinges on vessel resistance (R), governed by Poiseuille’s law, so small changes in diameter markedly alter perfusion.
Key regulators:
• Metabolic demand (local PO₂ ↓, adenosine ↑, H⁺ ↑) → vasodilation.
• Sympathetic activation raises heart rate and inotropy but tends to constrict coronaries; however, metabolic vasodilators usually override (functional sympatholysis) this effect, preserving or boosting flow.
The term autoregulation denotes the coronary bed’s capacity to maintain fairly stable flow despite fluctuating perfusion pressure, while sympatholysis describes metabolic negation of sympathetic vasoconstriction.
Heart Rate (HR), Stroke Volume (SV), and Cardiac Output (CO)
Cardiac output is
• Sympathetic stimulation (positive inotropic and chronotropic effects) accelerates HR and intensifies contractility.
• A higher HR shortens the cardiac cycle, disproportionately truncating diastole, the prime filling phase. Shorter filling time can lower SV.
• Thus, in some circumstances HR ↑ leads to SV ↓, keeping CO constant.
During progressive exercise both HR and SV rise initially because elevated contractility offsets abbreviated filling. Nevertheless SV typically plateaus once HR reaches ~50–70 % of VO₂max, reflecting the finite stretch capacity of the ventricle.
Myocardial Metabolism and Oxygen Consumption (MVO₂)
The myocardium relies almost exclusively on aerobic metabolism. Abundant mitochondria support sustained ATP generation as long as oxygen supply keeps pace. Consequently, oxygen availability tightly couples to energy production: any impediment to flow (e.g., coronary stenosis) threatens contractile performance almost immediately.
Isovolumetric Contraction and its Cost
Roughly 50 % of total myocardial O₂ consumption occurs during the brief isovolumetric contraction phase, when the ventricle generates steep pressure without ejecting blood. The cost is high because force development peaks in milliseconds.
MVO₂ scales with afterload (arterial pressure):
Thus chronic hypertension burdens the heart not only mechanically but metabolically, escalating risk for ischemia and failure.
Summary Integration
Diastolic pressure dominates MAP owing to the temporal asymmetry of the cardiac cycle.
Exercise diverts blood to muscle, curtailing GI and renal flow, modestly trimming cerebral perfusion; short-term this aids performance, long-term it stimulates vascular and metabolic adaptations.
Elevated SBP during exertion heightens cerebrovascular risk, while reduced visceral flow explains exercise-induced GI distress when training too soon after meals.
The heart’s blood supply is self-regulated by a balance between autonomic drive and metabolic need—functional sympatholysis ensures that oxygen delivery matches work.
Myocardial oxygen demand surges with pressure work, peaking during isovolumetric contraction; sustaining supply is therefore crucial in hypertensive or ischemic states.
These interconnected principles provide a foundation for interpreting hemodynamic responses in health, exercise physiology, and cardiovascular pathology.