Heart 9 Flashcards
Central venous pool
corresponds approximately to the volume enclosed by the right atrium and the great veins in the thorax
Venous return
Rate at which blood returns to the thorax from the peripheral vascular beds, i.e. blood entering the central venous pool
Cardiac output
Rate at which blood leaves the central venous pool and is pumped out of the heart
Norma stead-state conditions
Venous return = cardiac output
Mean circulatory pressure
Mean pressure that exists in the circulatory system when cardiac output stops and the pressures within the vascular system redistribute
What does Pmc represent?
Relationship between the volume of blood in circulation compared to the functional capacity of the system
By what is Pmc influenced?
Volume of circulating blood and the smooth muscle venous tone
Pmc is normally . . .
7 mmHg
When central venous pressure equals mean circulatory pressure, . . .
there is no pressure gradient for venous return (blood flow ceases)
With normal heart function, and increase in cardiac output . . .
decreases CVP and increases the pressure gradient for venous return (increases venous return)
What happens at negative CVP?
Transmural pressure collapses the large veins, resulting in zero venous return
Four things that will influence venous return
Peripheral venous pressure, central venous pressure, venous valves, cardiac contraction
What would increase peripheral venous pressure?
- Increased sympathetic venoconstriction
- Increased blood volume
- Increased skeletal leg muscle pumping activity
What would decrease central venous pressure?
Respiratory pump activity
Cardiac suction
How do venous valves help regulate venous return?
Maintains pressure gradient between peripheral and central venous pools in the face of gravitational forces
How does cardiac contraction help regulate venous pressure?
Contractions generate peripheral venous pressure
How would hemorrhage or decreased venous tone change the vascular function curve?
Parallel shift down and to the left
How would transfusion or increased venous tone affect the vascular function curve?
Parallel shift up and to the right
Any shift in the vascular function curve will also change what?
Mean systemic circulatory pressure
Factors that influence the level of cardiac function and shift the cardiac function curve
- Sympathetic stimulation (elevates cardiac function curve)
- Inotropic drugs
- Heart failure (depresses cardiac function curve)
How do kidneys respond to decreased cardiac output?
By increasing blood volume (fluid retention)
When does fluid retention become an issue?
In severe heart failure, the heart’s contractility is reduced so much that cardiac output cannot be maintained, even with very elevated CVP.
What other problems does CHF cause?
Pulmonary congestion and pitting edema of the extremities.
After hemorrhage, several adaptive changes occur acutely. What are they?
Venoconstriction, arterial vasoconstriction, increased cardiac contractility
Why do post-hemorrhagic adaptive changes occur?
The low blood pressure in the capillaries shift the equilibrium of hydrostatic and oncotic pressures, resulting in net reabsorption of fluid from the tissues, partially restoring lost blood volume
Maximum survivable blood loss
40% of total blood volume (approximately 2 L)
Long-term post-hemorrhagic adaptive responses
Increased salt and water retention by the kidneys to increase blood volume, increased red blood cell synthesis to restore the blood’s oxygen carrying capacity
