Lecture 5: control of cardiac output Flashcards
Describe the experiment by Guyton et al, 1957
What was observed?
What did they conclude
The hearts of dogs were replaced by high output pumps
Pumping capacity was reduced below normal CO
However, inceasing pumping capacity did not increase CO
- Heart rate is necessary to maintain CO
- The heart does not normally limit CO
Why doesn’t an increase in HR increase CO?
The circulation is a closed system and so the heart cannot increase the arteriovenous pressure gradient (PA-PV) beyond the point at which PV becomes negative because this will collapse veins and limit CO
What is theh central venous pressure in a healthy heart?
Almost 0 (even at rest)
What must also increase with HR to cause an increase in CO?
Venous return
What is the main determinant of cardiac output?
Mean systemic filling pressure (MSFP)
How can PA be increased if PV =0?
Increase MSFP
If PV was 0 then it would become negative when heart pumped causing venous collapse
What does the mean pressure in a system determine according to Darcy’s law?
The maximum flow rate for a given resistance
What is mean systemic filling pressure (MSFP)?
The mean pressure in a system (the pressure that would eventually exist everywhere if the heart stopped pumping)
How can MSFP be increased?
- Blood transfusion
- Drinking isotonic fluid
- Venoconstriction
How is the total blood volume calculated?
70-80ml/Kg of body mass
What is the approximate total volme of blood for a 70Kg person?
5L
Why can large amounts of blood be added to empty vessels without causing an increase in pressure?
Pressure does not change until vessel wall begins to stretch
What is the unstressed volume of a circulation?
What percentage of the total volume does this acount for?
The volume of blood that just fills the circulation without stretching the vessel walls
80%
How is compliance of a vessel calculated?
ΔV/ΔP
When may a vein become stiff?
If it is overstretched
observed in a saphenous vein graft used to replace a blocked coronary artery