The Heart as A Pump Flashcards
The heart
Two pumps acting in series
Systemic circulation = High Pressure
Pulmonary circulation = Low pressure
Output of left and right sides over time must be equal
Atria act as “priming pumps” for ventricles - the main pumps are the ventricles
Systole = Contraction and ejection of blood from ventricles
Diastole = Relaxation and filling of ventricles
At rest each ventricle pumps ~ 70 ml blood per beat (= Stroke volume)
At a heart rate of 70 bpm = 4.9 litres blood pumped per minute (i.e. the approximate volume of blood in the body) ~14,000 litre water tank
= 294 litres of blood pumped every hour
= ~7000 litres /day by each ventricle
= ~14,000 litres per day by both ventricles
By 60 years of age the total combined cumulative output of both ventricles is ~300 million litres…i.e. the capacity of this oil tanker!
Heart muscle
Specialised form of muscle
Discrete cells but interconnected Atrial electrical muscle
Cells contract in response to action potential in membrane
Action potential causes a rise in Ventricular intracellular calcium muscle
Cardiac action potential relatively long – lasts for durations of a single contraction of heart (~280 ms)
Action potentials are triggered by spread of excitation from cell to cell
Myoblasts connect to form long fibers of muscle fibers so cells are multinucleated - in cardiac muscle - connected by gap junctions between cells - they are electrically coupled - so when 1 AP originates in the SA node, it is able to cross all over the heart and give 1 synchronous beat (via the help of Purkinje fibres)
AP is a long duration in comparison (280ms) - needs this to be the case as this is how long it takes to fully contract the heart (for reference skeletal AP is 3-5ms)
Arrangement of muscle in heart - in a kinda figure of 8 - so when ventricles contract there is a squeezing twisting action to force the blood out of heart
Heart valves
4 valves - 2 separating atria from ventricles (Tricuspid (right) and mitral (left)), and 2 separating ventricles form respective blood vessels (output valves) (pulmonary (right) and aortic (left)) - these valves are operated by pressure difference across the two sides of the valve - i.e. the blood flows from high pressure to low pressure so, when the pressure in one chamber drops the valves open and blood flows through
Valve cusps are pushed open to allow blood flow and close together to seal and prevent backflow.
Cusps of mitral and tricuspid valves attach to papillary muscles via chordae tendineae.
Prevents inversion of valves on systole (contraction) (as there is less pressure in the atrium, without the papillary muscles and chordate tendineae, on systole of the ventricles, the blood could go back into the atria
When the aortic valve is closed the mitric valve is open and vice versa
Conduction system
Pacemaker cells in sinoatrial node generate an action potential
Activity spreads over atria – atrial systole (contraction)
We don’t want ventricles same time as the atria (want them contraction at then of atrial contraction)
Reaches the atrioventricular node and delayed for ~ 120 ms - allows for atria to complete contraction
From a-v node excitation spreads down septum between ventricles
Next, excitation spreads through ventricular myocardium from inner (endocardial) to outer (epicardial) surface via Purkinje fibers
Ventricle contracts from the apex up forcing blood through outflow valves
Cardiac cycle can be split into 7 phases
1) atrial contraction
2) isovolumetric contraction
3) rapid ejection
4) reduced ejection
5) isovolumetirc relaxation
6) rapid filling
7) reduced filling
When we excercise our systole (contraction) stays the same speed (constant) and diastole (relaxation) is the one that increases
Wiggers Diagram
Can compare pressure in aorta and ventricles - can see sound we hear with a stethoscope, can align this with electrocardiograph and ventricle volume and time
Usually done for the left side of the heart - could be done with right, would be similar but pressure would be less
Shows the 7 phases of the heart
Phase 1 of cardiac cycle - atrial contraction
Atrial pressure rises due to atrial systole. This is called the “A wave”
Atrial contraction accounts for final ~10% of ventricular filling. This value varies with age and exercise
P wave in ECG signifies onset of atrial depolarisation
Mitral/Tricuspid: Open Aortic/Pulmonary: Closed
At the end of Phase 1 ventricular volumes are maximal: termed the End-Diastolic Volume (EDV) (Typically ~120 ml)
Most of the blood flowing into the ventricle is passive - due to the ventricle wall relaxing - therefore pressure decreases therefore blood flows in, last bit of blood gets into ventricles by atrial kick (when atria contracts (gives you final 10% of ventricular blood volume))
Phase 2 of the cardiac cycle - Isovolumetric contraction
Mitral valve closes as intraventricular pressure exceeds atrial pressure
Rapid rise in ventricular /pressure as ventricle contracts
Closing of mitral valve causes the “C wave” in the atrial pressure curve
Mitral/Tricuspid: Closed Aortic/Pulmonary: Closed
Isovolumetric since there is no change in ventricular volume (all valves are closed)
QRS complex in ECG signifies onset of ventricular depolarisation.
Closure of the mitral and tricuspid valves results in the first heart sound (S1) (the lub part of lub dub)
Phase 3 - Rapid ejection
Ejection begins when the intraventricular pressure exceeds the pressure within the aorta.
This causes the aortic valve to open
Blood continues to flow into the atria from their respective venous inputs
Atrial pressure initially decreases as the atrial base is pulled downward as ventricle contracts. This is called the “X descent”
Rapid decrease in ventricular volume as blood is ejected into aorta
Mitral/Tricuspid: Closed Aortic/Pulmonary: Open
Phase 4 - reduced ejection
Repolarisation of ventricle leads to a decline in tension and rate of ejection begins to fall
Atrial pressure gradually rises due to the continued venous return from the lungs - called the V wave
Ventricular repolaristion depicted by T wave of ECG
Mitral/Tricuspid: Closed EDV Aortic/Pulmonary: Open
Phase 5 Isovolumetric relaxation
When intraventricular pressure falls below aortic pressure, there is a brief backflow of blood which causes the aortic valve to close
“Dicrotic notch” in aortic pressure curve caused by valve closure
Although rapid decline in ventricular pressure, volume remains constant since all valves are closed. Hence isovolumetric relaxation
Mitral/Tricuspid: Closed Aortic/Pulmonary: Closed
End systolic Volume (ESV)
EDV-ESV = Stroke volume ESV (Typically ~70-80ml)
Closure of the aortic and pulmonary valves results in the second heart sound (S2).
Phase 6 - rapid filling
Fall in atrial pressure that occurs after opening of mitral valve is called the Y-descent
When the intraventricular pressure falls below atrial pressure, the mitral valve opens and rapid ventricular filling begins
Ventricular filling normally silent - however, third heart sound (s3) sometimes present.
S3 heart sound is normal in children but can be sign of pathology in adults
Phase 7 reduced filling
Rate of filling slows down as ventricle reaches its inherent relaxed volume.
Further filling is driven by venous pressure - At rest ventricles are ~90% full by the end of phase 7
Abnormal valve function
Valve doesn’t open enough —> obstruction to blood flow when valve would normally be open —> Stenosis
Valve doesn’t close all the way —> Back leakage when valve should be close —> regurgitation
Aortic valve stenosis
Causes:
Degenerative (senile calcification/fibrosis)
Congenital (bicuspid form of valve)
Chronic rheumatic fever –inflammation- commissural fusion
So less blood is able to get through the valve, which leads to increased left ventricle pressure —> LV hypertrophy, and it can lead to left sided heart failure —> angina (blood supply to heart not enough)
Microangiopathic haemolytic anaemia - lack of RBC and Hb, - blood is forced through a restricted valve, therefore there is stress on the RBC causing lysis, —> MHA
