Rob - The heart as a pump Flashcards
What are the 3 things the heart does?
- Delivers oxygen, sugars to respiring tissues and hormones to sites of action.
- Removes CO2 and metabolic products
- Maintenance of environment -> Homeostasis
How much blood and gases can a heart deliver and remove in a minute?
Each side = 5l/min of blood
- Delivers 250ml O2/min
- Removes 200ml CO2/min
How much blood is pumped and beat numbers/day?
Beats: 100,000/day
Pumps: 7,000L/day
How big is the heart?
Slightly bigger than a fist
Adaptations and effects of size etc
Larger SA-Vol ration = more heat loss
More metabolism = higher heart rate
What are the adaptations of the LHS of the heart?
- Higher pressure
- Thicker ventricle walls
- Why? Pumps blood to whole body
Different functions of LHS and RHS of heart
LHS = pumps blood to all round the body
RHS = pumps blood only to the lungs
Diastole vs Systole
Systole:
- Ventricles contract
- ~300ms
Diastole:
- Relaxed heart
- Heart fills with blood
- ~500ms at 70 bpm
How to calculate mean arterial pressure?
MAP = 1/3 systole + 2/3 diastole
How many valves are there?
4:
- Tricuspid
- Mitral
- Pulmonary
- Aortic
What does the tricuspid valve do?
Shut in systole
Open in diastole
What does the mitral value do?
Shut in systole
Open in diastole
What does the pulmonary valve do?
Open in systole
Shut in diastole
What does the aortic valve do?
Open in systole
Shut in diastole
How to calculate cardiac output?
cardiac output = stroke volume x heart rate
What is starling’s law?
Energy of contraction is a function of the length of cardiac muscle fibres
How does starling’s law relate to the heart?
- Output of heart has to be equal on all sides
- Stroke volume depends on filling and muscle stretching
- More blood volume = more stretched myocardium = more force to pump blood out.
Phases of the cardiac cycle
Filling
Isovolumetric contraction
Outflow
Isovolumetric relaxation
What happens in filling?
Filling:
- Ventricles fill with blood until the pressure is equal to vein pressure (diastole)
- Atria contract (last 20% of filling) + increase ventricular pressure to be higher than the atria -> mitral valve closes.
What happens in isovolumetric contraction?
Isovolumetric ventricular contraction:
- Start of systole
- All valves closed => causes increased pressure in the ventricles.
What happens in the outflow phase?
- Ventricular pressure exceeds pressure in aorta.
- Causes aortic valves to open and blood pumped into arteries.
- Ventricles then relax and decrease ventricular pressure to less than in the aorta so aortic valve closes.
What happens in isovolumetric relaxation?
- All valves are closed
- Ventricles relax and reduce pressure causing the AVs to open and starts ventricular filling.
What type of muscle is the heart?
Myogenic muscle -> muscle contractions initiated by heartbeat.
How are electrical impulses passed through the heart?
- Sinoatrial node: pacemaker of heart
- Atrioventricular node: impulses travel through atrial muscle to AVN.
- Purkinje fibres + bundle of His: electrical impulses conducted through fibres to His and branches.
What do these impulses do?
- Spread through myocardium
- Cause bottom to top contraction
Ion pacemakers what are they?
- Potassium = determines resting membrane potential
- Depolarisation in cardiac muscle greatly determined by calcium as opposed to sodium.
What does SAN pacemaker potential depend on?
- Slow influx of sodium before depolarisation
- Rapid influx of calcium causes depolarisation
- Potassium outflow causes repolarisation
Time between repolarisation via potassium and pre potential influx of sodium = pacemaker potential
Ventricular myocyte refractory period - what is it?
The period when no more contractions can be fired
Ventricular myocyte refractory period - how is it determined?
Via calcium ions:
- Causes a longer refractory period to prevent tetanus of cardiac muscle (increased muscle twitching).
What are myocytes?
- Branched muscle cells with nucleus
- Cylindrical
- Surround the heart
- Branched -> allows signals to propagate
How are the myocytes connected?
- Tight junctions and transmembrane proteins (connexions) join these.
- Their contraction activated by entering calcium from intra and extracellular stores.
What do myocytes do?
- Action potentials can propagate through the connections.
- Current flow allows ECG to form.
What is an ECG?
- Spread of heart beat across heart.
Stages of ECG:
T = ventricular repolarisation
QT = systole
RR-QT = diastole
HR = ((number of r waves -1)/r-r interval) * 60
ECG what do the stages look like?
P = hump before QRS complex
Q = dip before big spike
R = peak
S = trough after peak
T = hump after QRS complex
How long is a cardiac cycle?
From beginning of P to start of next P
