Regulation of CO Flashcards
CO=
CO= 5L.min CO = HR x SV HR = 70bpm SV = 70ml
SV =
SV = EDV - ESV
How can cardiac output be altered?
By changing the HR or SV (ESV or EDV)
How is HR changed?
Via autonomic innervation
How can SV be increased?
Increase EDV and/or Decrease ESV
How does cardiac muscle contract?
Uses excitation contraction coupling.
Ca induced Ca release.
Extracellular Ca enters the cell via L-type channels to induce Ca release from SR via RyR channels. Ca regulated the number of cross bridges formed and therefor the force of contraction.
How can the force of cardiac muscle contraction be increased?
Increase Ca sensitivity (Starling’s law)
Increase Ca concentration (inotropy)
What is EDV and what determines it?
The amount of blood in the ventricles after diastolic filling. It is determine by VR and CVP.
How does VR and CVP cause an increase in EDV?
Increasing filling during diastole causes the muscle to stretch. The stretch allows the formation of more cross bridges so the muscle can contract with greater force, despite the same [Ca]. = LENGHT DEPENDENT increase in Ca sensitivity
What factors affect VR?
PRELOADS - any factor that influences the stretch of the muscle before contraction.
- Blood volume
- Skeletal muscle pump
- Respiratory pump
- Gravity
- Venous tone
Why might blood volume change?
More blood within the venous circulation = increase VR
Increased volume from renal failure
Decreased volume from dehydration, haemorrhage
What is the affect of the skeletal muscle pump on VR?
VR is aided by contraction of postural muscles around venous vessels. Valves prevent retrograde movement by closing distally to the pressure
How does the respiratory pump aid VR?
Inspiration causes a decease in thoracic pressure but an increase in abdominal, pulling the blood along the gradient
How does venous tone affect VR?
Increased sympathetic activity causes vasoconstriction to reduce the capacitance and so blood which cannot be held is forced back to the heart
What effect does gravity have on VR?
When supine there is uniform distribution of blood which maintains VR and CVP. When standing, blood redistributes due to gravity and causes venous pooling in the lower extremities and a reduced thoracic blood volume.
Which 2 factors affect EDV directly?
Atrial contraction: Sympathetic activity can increase the force of contraction. Important at high HR when the filling phase is reduced. Contributes more blood.
HR >180bpm: Atrial contraction cannot compensate for this increase in HR so EDV will be REDUCED.
Which 2 factors affect ESV?
Increased contractility / inotropy
Afterload
How is inotropy changed?
Increasing sympathetic activity or circulating NAd/Ad stimulates beta1 receptors. It is a GsPCR that increases AC, PKA to phosphorylate L-type Ca channels so that Ca influx increases for more Ca induced Ca release. The raised [Ca] allows more cross bridges to form so the force of contraction increases = LENGTH INDEPENDENT increase in Ca sensitivity
What happens to a proportion of the Ca that is influxed into a cell?
Some is sequestered into the SR store to increase its reserve
What is an afterload?
The load that the heart must contract against to eject. It is determined by aortic and PA pressure.
In what condition will an afterload be present?
Hypertension - high pressure makes ejection less efficient
How does a high pressure change the afterload?
An increase in TPR will increase pressure upstream due to reduced flow, and therefore increasing the pressure in the aorta and PA in which the ventricle must contract against.
How does the heart compensate for an afterload?
As cardiac filling is unaltered by afterload but SV falls as less can be ejected against the pressure, so the muscle contracts with GREATER force to return the SV to normal.
What problem results from chronic afterload?
The muscle will HYPERTROPHY to contract harder but eventually leads to ventricular failure due to the excess amount of work
