Cardiac Output Flashcards
Why is venous return directly proportional to cardiac output?
Because the cardiovascular system is a closed loop system
From an experimental aspect- easier to measure output from LV and input into the RA and the factors that affect them
What is mean circulatory filling pressure?
Represents the average equilibrated pressure throughout the circulatory system without blood flow
-It can be measured by stopping blood flow and allowing the pressures to equilibrate
As such, it is a function of the volume of the fluid within the system and the vascular tone
What is mean systolic filling pressure (MSFP)?
Represents the average equilibrated pressure throughout the systemic circulation without blood flow
-The MSFP creates the pressure gradient that allows for blood flow into the right atrium
MSFP can be affected by:
- blood volume
- venous resistance
What is venous return?
VR is the flow of blood into the right atrium (RA)
Fir blood to flow, there must be a pressure gradient
Flow= 🔼P/R
- MSFP is 7mmHg (upstream)
- RAP is 2 mmHg (downstream)
What factors that affect VR?
- mean systolic filling pressure (MSFP)
- Venous resistance
- Blood volume
- Right atrial pressure(RAP)
- Arteriole resistance
If RAP is increased(independently)- what happens to VR?
-VR will fall- because the pressure gradient between the peripheral venous compartment and the central compartment RA will be reduced
What is central venous pressure?
A measure of pressure in the vena cava, can be used as an estimation of central blood volume, preload and Right Atrial Pressure.
CVP is synonymous to RAP
Cardiac filling pressure is a crucial factor in determining how well the cardiovascular system is operating
What are the main factors that affect VR?
Blood volume
Venomotor tone (venous compliance
Arteriolar tone
How does blood volume affect VR?
Increased VR leads to increased VR
Central venous blood volume is influenced by
-total blood volume (regulated by kidney)
-rate of VR to central compartment- influenced by mechanical factors e.g. Muscle pump
What mechanical factors affect CVP?
Body posture: upright —> venous pooling and decreased venous right
Muscle pump: displaces blood from peripheral veins to central veins (increased VR)
Respiratory pump: during inspiration, increased blood into thoracic veins (-ve intrathoracic pressure—> increased venous flow into thoracic veins and increased VR)
How does venomotor tone affect VR?
Venomotor tone (venous compliance)
Venous compliance relates to state of vascular contraction. Contraction of the vascular smooth muscle. (E.g. sympathetic)
Venoconstriction(decreased compliance) leads to increased VR
Blood is displaced from peripheral to central veins
Increased sympathetic (decreased compliance): increased VR —> increased EDV
How does arteriolar tone affect VR?
Arteriolar vasoconstriction leads to decreased venous return
Arteriolar vasoconstriction tends to hold blood in arterial system- so decreased venous volume and venous return decreases
Arteriolar vasodilation leads to increased venous return
Since arteriolar tone is the major determinant of TPR
increased TPR —> decreased VR
decreased TPR —> increased VR
What is the function of vascular function curve (VR)?
This is the relationship between VR and RAP
As RAP increases- what happens to VR?
As RAP increases, 🔼P between the veins and RAP decreases and VR gets smaller
At a particular RAP (X) there will be NO venous return because there is NO pressure gradient driving the blood into the heart
Explain the parts of the vascular function curve
Curve us flat at the top/beginning, because RAP is <0, veins collapse so impeding blood flow back to heart
- VFC is very steep: veins are very distensible (at low pressures.)
- When RAP is +7 mmHg- no pressure gradient between veins and RA and VR is ZERO
Where does the heart normally function on the vascular function curve?
In a resting heart VR= about 5L/min
Point where heart generally operates (RAP= +2 mmHg(x-axis), when VR= 5 L/min)
