13 Cardiac output II Flashcards
What does venous return effect?
Atrial filling
What effects stroke volume?
- ventricular filling rate
- EDV (large causes muscle fibres to stretch more, making them work harder and get stronger)
Starlings Law:
What goes into the heart, must come out of the heart
-the energy of the contraction is proportional to the initial length of the muscle fibre
What are the key factors in venous return?
- muscular pumps
- respiratory pumps
- posture
what does the preload volume effect?
EDV directly
ESV indirectly
How does posture effect venous return?
standing causes pooling in the feet and less venous return
How do muscle pumps facilitate venous return?
Skeletal muscle contractions constrict venous return and since valves prevents back flow the blood has to go up
How do respiratory pumps facilitate venous return?
- when you breathe in diaphragm flattens increasing abdominal pressure and decreasing thoracic pressure
- this causes blood to move up from the abdomen into the thorax towards the heart
What are the filling times?
times taken for atria and ventricle to contract - directly linked to contractility
Contractility
contractile ability of cardiac muscle at a given preload volume
What is an inotropic effect?
effects the contractility of the heart (force of contraction)
what does a positive inotropic do?
increases contractility and therefore stroke volume
How do you cause an increase in contractility?
Increase the levels of intracellular calcium (from intracellular stores) as this increases the binding between the actin and myosin filaments
What has positive inotropic effects?
SNS and adrenaline
How does the PNS effect the heart?
through the vaygus nerve which effects atria because it doesn’t reach the ventricles.
-it slows HR and decreases contractility and so stroke volume
What effects the filling times of the heart?
HR and contractility
What changes are there in diastole and systole times when the heart rate speeds up?
- with no SNS systole would be the same length, only diastole would be effected so the filling time would reduce reducing SV and CO
- SNS slightly slows down diastole and quickens systole allowing the atria/ventricles time to fill
What is the afterload?
the (aortic) pressure against which the heart has to work to eject blood from the ventricles. this is greater if the vessels are constricted.
What effects the afterload?
vasoconstriction (blood vessel tone)
blood pressure
What effects cardiac output?
Blood pressure
What determines blood pressure?
CO x total resistance of peripheral vessels
CO (HR and SV)
TPR (arterial radius)
How do we regulate blood pressure short term?
HR
Vasoconstriction and dilation
-regulated by baroreceptors (neural)
-and by chemoreceptors (o2 co2 h+)
How do we regulate blood pressure long term (hrs and days)
-changing extracellular fluid volume
-kidney NA/Water homeostasis
-Hormones (ADH, aldosterone, atrial natriuretic peptide)
ADH + ALD increased the sodium and water conc in the blood
ANP released from the atria reduces it
What is too low blood pressure called?
hypotension
what is too high blood pressure called?
hypertension
Where are baroreceptors for blood located?
- in walls of blood vessels
- aorta
- carotid artery
Where are the chemosensors located?
carotid body and in the CNS in the medulla oblongata
How do baroreceptors work?
They fire more as blood pressure increases. fail to respond 40mmHg or less blood pressure.
-respond more to pulsatile changes in blood pressure than smooth changes
Why are baroreceptors located in the aortic arch?
so they can detect every beat of the heart
What do the peripheral chemoreceptors (heart) detect?
-increase in O2
stimulates increase in respiratory rate
-Decrease in CO2 or increase in pH
stimulates vasomotor centre and cardioacceleratory centre
What do the central chemosensors detect? (in the cardiovascular system)
Not O2, just CO2 and pH.
- cause vasodilation of cerebral vessels and promote blood flow to the brain
- effect respiratory function
