Week 7 - Circulation Of Blood / Cardiac Output Flashcards
What is the function of the C/V (cardiovascular) system?
- maintain an adaptable supply of blood to tissues in order to supply nutrients + signalling molecules and remove waste products
- achieved through generating pressure differentials across tissues to enable capillary exchange
What are some of the components of the C/V system?
- heart
- brain
- adrenal glands
- kidney
- blood vessels
What two factors is the flow (F) of blood from A to B determined by?
- pressure difference between A and B (deltaP)
- resistance to flow (R)
What are some factors affecting vascular resistance?
- vessel length (L) - internal SA and flow are inversely proportional
- blood viscosity (n) - higher viscosity = higher resistance
- vessel radius (r) - flow proportional to r^4
What is Darcy’s Law?
Flow (F) = ΔP/ R
ΔP = pressure difference
R = resistance to flow
What is the Hagen-Poiseulle formula?
F = (ΔP x πr^4) / 8Ln
- F = flow
- P = pressure
- r = resistance
- n = blood viscosity
- L = vessel length
- r = vessel radius
Which side of the heart generates more pressure?
- left side generates more pressure - pumps blood all around body via systemic circuit
- right side generates less pressure - pumps blood to lungs via pulmonary circuit
Outline the cardiac conduction system
- impulse generated at sino-atrial node
- impulse carried to atrioventricular node (atrial contraction occurs)
- impulse carried to bundle of His
- impulse is carried via the purkinje fibres to the ventricular myocardium (ventricular contraction)
What does the P wave on an ECG denote?
Atrial contraction
(Atrial depolarisation)
what does the QRS complex denote on an ECG?
- ventricular contraction (depolarisation)
- atrial relaxation + repolarisation also occurs but is masked by larger ventricular electrical events
What does the T wave on an ECG denote?
- ventricular relaxation (repolarisation)
What is cardiac output?
Volume of blood ejected by each ventricle per minute
What is heart rate?
Number of heartbeats per minute
What is stroke volume?
Volume of blood ejected by each ventricle per beat
How is cardiac output calculated?
Cardiac output (CO) = Heart rate (HR) X Stroke Volume (SV)
what factors affect heart rate?
- autonomic innervation
- hormones
What factors affect stroke volume?
Preload
- filling time
Contractility
- autonomic innervation
- hormones
Afterload
- vascular tone
What 3 things occur to decrease heart rate?
- activation of cardioinhibitory centre in medulla oblongata
- impulses along vagus nerve
- acetylcholine release
What 3 things occur to increase heart rate?
- activation of cardioacceleratory centre in medulla oblongata
- impulses along accelerator + sympathetic nerves
- noradrenaline
How can you calculate stroke volume?
End diastolic volume (EDV) - End systolic volume (ESV)
What two things can occur to increase stroke volume?
- increase end diastolic volume (EDV)
- decrease end systolic volume (ESV)
Vice versa for decreasing stroke volume
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 preload
What is afterload?
The force the ventricle needs to overcome to open the semilunar valve + eject blood
What is the Frank-Starling law?
The force developed in a muscle fibre is dependent on the extent it is stretched
What are some factors affecting venous return & preload?
- posture - blood pools in leg while standing (therefore venous return decreases)
- skeletal muscle pump - skeletal muscle movement constricts veins - forces blood through veins (increases venous return)
- respiratory pump - inspiration reduces intrathoracic pressure whilst increasing intraabdominal pressure (increases venous return)
What happens if Venous return is increased?
- preload is increased
- end diastolic volume (EDV) is increased
- therefore a higher stroke volume is attained, and a greater cardiac output occurs.
What is the effect of the sympathetic nervous system on contractility?
- SNS increases force of contraction and velocity of conduction
- also allows longer for diastole so increased filling of blood in the heart can occur
What is the effect of afterload on stroke volume and cardiac output?
Increased afterload reduces SV & CO
How do vasodilation and vasoconstriction affect afterload?
- vasoconstriction increases afterload
- vasodilation decreases afterload
