Cardiac reserve.
The cardiac reserve is the difference between the work done at rest and work done at maximal exertion. The mechanisms that govern the cardiac reserve are:
- Heart rate (HR): sympathetic stimulation can cause HR to reach a maximum of 200 beats/min. Causing an increase of 3x in COP.
- Stroke volume (SV): dilation and sympathetic stimulation increase SV (through increased VR and contraction respectively) to a maximum of 200 ml/beat. Causing an increase of 3x in COP.
- Cardiac hypertrophy: increased cardiac muscle size due to exercise and hypertension can cause increased SV due to increased force of contraction. This mechanism has a limit which is reached when the size of cardiac muscle is disproportional to the blood flow it receives.
ABP.
- ABP: the force exerted by blood on the walls of arteries.
- Systolic blood pressure: the maximum ABP the vessels reach during the ventricular systole.
- Diastolic blood pressure: the minimum ABP the vessels reach at the end of the ventricular diastole during isometric relaxation phase.
- Pulse pressure: the difference between systolic (maximum ABP) and diastolic (minimum ABP) blood pressures.
- Mean systemic ABP: average pressure in systemic arteries during cardiac cycle. Equal to Diastolic BP + 1/3 the difference between Systolic and Diastolic BPs.
Mean ABP maintains pressure through systemic circulation to keep blood flowing, providing enough pressure inside capillaries for filtration.
Factors affecting ABP.
ABP is affected and maintained by multiple factors including COP, TPR, vessel elasticity, and TBV.
TPR (total peripheral resistance) effect on ABP.
The resistance to blood flow through blood vessels is affected by:
Diameter of arterioles specifically.
TPR is inversely proportional to diameter of arterioles, as arteriolar constriction causes markedly increased DBP and increased SBP. Because:
- VC of arterioles -⇒ blood leaves arterioles slowly -⇒ DBP is kept higher for longer.
Viscosity of blood.
TPR is proportional to blood viscosity, which is determined by # of RBCs and plasma protein content.
COP effect on ABP.
SV ⭐
Increased stroke volume affects COP and thus ABP. It causes increased SBP with decreased DBP because:
- Increased SV -⇒ more blood in arteries -⇒ increased SBP.
- Increased SV -⇒ more blood in arteries -⇒ extra blood escapes to venous side during diastole -⇒ unchanged DBP. The change in ABP is momentary though because:
- Increased ABP -⇒ baroreceptor stimulation -⇒ reflex peripheral VD -⇒ normal ABP.
HR ❌
Increased heart rate affects COP and thus ABP. It causes increased DBP with decreased SBP because:
- Reflex vasoconstriction.
- Shorter diastolic period.
- Decreased blood velocity.
Vessel elasticity effect on ABP.
Prevents further increase in SBP (because the vessels cant accommodate extra blood from systole) and prevents further decrease in DBP (because the vessels cant press against blood due to no elasticity.)