Intro.
First, some definitions:
- EDV: volume of blood in ventricle at the end of diastole. ⭐
- ESV (residual blood): volume of blood in ventricle at the end of systole. ⭐
- SV: volume of blood pumped by the ventricle / beat. (EDV - ESV) ⭐
- EF: is the percentage of SV to EDV. (SV / EDV = EDV - ESV / EDV)
- COP: is the volume of blood the ventricle pumps / min. (SV * HR) ⭐
- CI: volume of blood pumped by each ventricle / meter2.
Variations in COP.
COP may vary from person to person and in a single individual under different circumstances due to posture, physical activity, temperature, eating, pregnancy, hormones, etc…
Factors affecting COP. ⭐
The factors that affect cardiac output can be deduced from the formula for COP which is: and since SV is affected by EDV and ESV, and those are affected by afterload, preload, and contractility (ESV): Therefore, COP is affected by: Preload, Afterload, Heart Rate, and Contractility.
Preload / Venous return. (main)
- Increased venous return -⇒ increased ventricular filling -⇒ increased EDV -⇒ increased force of contraction -⇒ Increased SV-⇒ increased COP. (Frank-Starling law.)
Afterload / Arterial blood pressure.
- Sudden rise in ABP -⇒ first systole isn’t strong enough -⇒ increased ESV -⇒ increased next beat EDV -⇒ increased force of contraction -⇒ increased SV -⇒ increased COP. (Frank-Starling law.) (This means that the next beat’s EDV will be increased by the amount of this beat’s ESV. The reverse happens during a sudden drop in ABP.)
Heart rate.
Heart rate can affect COP in different ways according to whether venous return is constant or changing with heart rate.
- Increased HR with constant VR -⇒ decreased SV to balance HR -⇒ No change in COP.
- Increased HR with variable VR -⇒ increased EDV -⇒ increased force of contraction -⇒ increased SV -⇒ increased COP.
Contractility.
- Increased VR -⇒ increased elongation of the heart -⇒ increased EDV -⇒ increased force of ventricular contraction. (Frank-Starling law.) -⇒ increased SV -⇒ increased COP.
Factors affecting venous return.
These are the factors that determine the amount of blood returning to the heart’s right atrium through the SVC and IVC. They are:
- Right atrial pressure: exerts backwards pressure on incoming blood away from the heart.
- Mean systemic (filling) pressure: forces the blood towards the heart. (The pressure inside the systemic circulation ONLY once heart is stopped.)
- Resistance of vessels to blood flow: determines how easily blood will flow to the heart.
