Control of Cardiac Output - Lecture 12 Flashcards
What is cardiac output?
Stroke volume x Heart rate
○ Stroke volume = volume of blood ejected by each ventricle per beat
○ Heart rate = number of heart beats per minute
What is the cardiac output, heart rate and stroke volume at rest?
○ CO = 5L
○ HR = 70 bpm
○ SV = 70ml
What is the cardiac output, heart rate and stroke volume during exercise?
○ CO = 20L
○ HR = 190-200 bpm
○ SV = 105ml
What is the cardiac output, heart rate and stroke volume at rest for an athlete?
○ CO = 5.5L
○ HR = 40 bpm (Lower)
○ SV = 140ml (Higher)
What is the cardiac output, heart rate and stroke volume during exercise for an athlete?
○ CO = 40L (Higher)
○ HR = 190 bpm
○ SV = 210ml (Higher)
Why do athletes have a higher stroke volume?
Physiological left ventricular hypertrophy = chamber sizes increase
What are factors affecting heart rate called?
Chronotrophic effects
What factors affect heart rate?
○ Autonomic innveration
○ Hormones
○ Venous return
How does the parasymphatetic system affect heart rate?
○ Cardioinhibitory centre is activated
○ Impulses increase in the vagus nerve as well as acetylcholine
○ Pacemaker cells slow down
○ Heart rate decreases
○ Negative chronotrophic effect
How does the sympathetic system affect heart rate?
○ Cardioacceleratory centre is activated
○ Noradrenaline is secreted from the sympathetic ganglia or from adrenal medulla
○ Pacemaker cells speed up
○ Increases heart rate
○ Positive chronotrophic effect
Where is the cardioregulatory system found?
Medulla obolongata
What does depolarisation at the SA node involve?
○ HCN channel opens to allow entry of Na+: Increases membrane potential
○ At threshold potential: calcium channel opens
What does repolarisation at the SA node involve?
○ K+ leaves the cell
○ Membrane potential decreases
How does parasympathetic system affect repolarisation?
○ K+ channel is sensitive to acetylcholine
○ More K+ leaves the cell
○ More -ve membrane potential
○ Takes longer to reach threshold potential so HR slows down
How does sympathetic system affect depolarisation?
○ Increases depolarisation
○ Reduces repolarisation
○ Threshold potential is reached quicker
○ HR increases
What is vagal tone?
○ Parasympathetic activity is higher than sympathetic activity at rest which reduced the rate of SA node
○ Athletes have higher parasympathetic activity so lower HR at rest
How does venous return affect heart rate?
If venous return increases:
○ Atria and pacemaker cells are stretched more
○ Stretch receptors activate sympathetic nervous system
○ Heart rates increases
What is the equation for stroke volume?
End diastolic volume - end systolic volume
What factors affect end diastolic volume?
Preload
What factors affect end systolic volume?
○ Preload
○ Contractility
○ Afterload
What is preload?
The degree to which ventricular muscle cells are stretched at the end of diastole
What is contractility?
The force produced by ventricular muscle cells during systole at a given period
What is afterload?
The force the ventricle needs to overcome to open the semilunar valve and eject blood
What factors affect preload?
○ Rate of venous return
○ Available ventricular filling time
○ Preload is directly proportional to EDV
What is Frank-Starling law?
The force developed in a muscle fibre is dependent on the extent it is stretched
What factors affect venous return?
○ Posture: blood pools in leg venins when standing due to gravity so venous return decreases
○ Skeletal muscle pump: movement of skeletal muscles constricts veins aiding venous return so venous return increases’
○ Respiratory pump: Inspiration reduces intrathoracic pressure whilst increasing intraabdominal pressure so increases venous return
○ Venous capacitance: SNS activity reduces compliance and increases central venous pressure so increases venous return
What factors affect contractility?
○ Autonomic innervation
○ Hormones
What are factors affecting contractility called?
Inotropic effects
How does sympathetic and parasympathetic system affect contractility?
○ Sympathetic: positive inotropic effects
- Increases force of contraction and velocity of conduction
○ Parasympathetic: negative inotropic effects
What factor affects afterload?
Vascular tone
What is vascular tone?
○ Vasodilation reduces afterload
○ Vasoconstriction increaseas afterload
How does increased afterload affect stroke volume?
○ Decreases stroke volume
○ Shorter ejection
○ Longer isovolumetric contraction
○ Increased ESV
○ Prolonged increases in afterload damage the myocardium and lead to heart failure
