WEEK TWO - Overview and anatomy of heart

Question 1

Define and distinguish between the pulmonary and systemic circuits

Answer 1

LEFT side of heart = SYSTEMIC unit
= supplies oxygenated blood –> all organs via aorta
= brings back deoxygenated blood from tissues –> right atrium via sup. + inf. vena cava

RIGHT side of heart = PULMONARY unit
= carries deoxygenated blood –> lungs via LHS RHS pulmonary arteries for gas exchange
= carries oxygenated blood –> LEFT atrium via 4 pulmonary veins

Question 2

Describe the general location, size, and shape of the heart

Answer 2

located between lungs
9cm width
13cm height
6cm sagittal plane
300 grams

shape
- broad sup. portion tapers to left at apex

Question 3

Describe the pericardium that encloses the heart

Answer 3

double walled sac encloses heart = allowing beat w/o friction, allows expansion and prevents unnecessary expansion

parietal pericardium = OUTER LAYER
- fibrous CT

pericardial cavity - BETWEEN layers - filled with pericardial liquid

visceral pericardium = INNER LAYER
- thin, smooth, moist

Question 4

Name & describe the 3 layers of the heart wall

Answer 4

epicardium [visceral pericardium] - OUTER layer
- serous membrane

myocardium - middle layer
- thick muscular layer
- fibrous skeleton [network of collagen and elastic fibres = structural support and attachment]

endocardium - INNER layer
- smooth inner lining of epithelial tissue

Question 5

Name, describe, identify & state the function, of the 4 heart chambers

Answer 5

RHS ATRIUM
receives oxygen-poor blood from the body and pumps → right ventricle

LHS ATRIUM
receives oxygen-rich blood from the lungs and pumps → left ventricle.

atria = superior, posterior chambers

RHS VENTRICLE
right ventricle pumps the oxygen-poor blood → lungs.
Pump blood into PULMONARY trunk

LHS VENTRICLE
pumps blood –> ascending aorta

ventricles = inferior larger chambers

Question 6

Name, identify & deduce function of the 4 heart valves

Answer 6

Atrioventricular [AV] valves
- Tricuspid Valve [made up of three cusps, located between right atrium and right ventricle]

• Bicuspid/Mitral Valve [made up of two cusps - located between left atrium left ventricle]

Semilunar valves [SL] valves
- Pulmonary valve [separates right ventricle from pulmonary trunk]

• Aortic valve [separates left ventricle from ascending aorta]

function = prevents back flow of blood into ventricles

Question 7

Trace the flow of blood through the chambers, valves and vessels of the heart

Answer 7

1. blood enters via right atrium from sup.inf. vena cava
2. blood flows through right AV valve –> right ventricle
3. contraction of right ventricle –> forces pulmonary valve open
4. blood flow through pulmonary valve –> pulmonary trunk
5. blood distributed by right and left pulmonary arteries –> lungs
   - unloads Co2 and loads Co2
6. blood returns to lungs via pulmonary veins via LEFT atrium
7. blood flows through left AV valve –> left ventricle
8. contraction of L ventricle –> forces aortic valve open
9. blood flows through aortic valve –> ascending aorta
10. blood in aorta = distrubted to every organ [unloads O2 and loads Co2]
11. returns to heart via sup.inf.vena cava

Question 8

Name, identify & describe the arteries that feed the myocardium and the veins that drain it

Answer 8

LEFT coronary artery [LCA]= supplies blood to LHS of heart

RIGHT coronary artery [RCA] = supplies blood to RHS of heart

venous drainage =
most blood returns to RIGHT ATRIUM via coronary sinus [THREE main inputs]
1. great cardiac vein
2. middle cardiac vein
3. left marginal vein

Question 9

Describe the external nerve supply to the heart

Answer 9

Sympathetic nerves
From upper thoracic spinal cord through sympathetic chain → cardiac nerves

–> To sinoatrial [SA] node, atrioventricular [AV] node and ventricular myocardium

Can raise HR to 230 BPM

Question 10

Describe the internal electrical system of the heart

Answer 10

SA node = pacemaker, modified cells, initiates heartbeat, sets HR, located in RIGHT ATRIUM

AV node = electrical gateway to ventricles in RIGHT ATRIUM
AV bundle = pathway for signals from AV node → ventricle

Purkinje fibres - upward from apex spread throughout ventricular myocardium to excite cardiomyocytes

Question 11

Relate the structure of cardiac muscle to its function

Answer 11

Intercalated discs join cardiomyocytes end to end [three features: interdigitating folds, mechanical junctions, and electrical junctions]

• Interdigitating fold - increases SA for contact

-gap/electrical junctions connect contractile myocyte cells = allow flow of ions = transport AP signal through atria

Desmosomes—mechanical linkages that prevent contracting cardiomyocytes from being pulled apart from each other

Question 12

Explain how the SA node fires spontaneously and rhythmically

Answer 12

sinus rhythm/ normal HR = controlled by SA node between 60-100 bpm
-adult rest = 70-80 bpm [vagal inhibition]

SA node = no stable resting membrane potential due to leaky Na+ ion channels

Starts at −60 mV and drifts upward due to slow Na+inflow

• When it reaches threshold of −40 mV, voltage-gated fast Ca2+and Na+channels open
• Faster depolarization occurs peaking at 0 mV
  -K+channels then open and K+leaves the cell
  = Causing repolarization
• Once K+channels close, pacemaker potential starts over
  [When SA node fires it sets off heartbeat]

Question 13

Describe the action potentials of cardiac muscle and relate to function

Answer 13

1. Na+ channels open –> rapid depolarisation
2. Na+ channels close and voltage peaks at +30mV
3. Slow Ca2+ channels OPEN = prolonging repolarization and creating plateau
4. Ca2+ channels close, K+ channels OPEN = efflux of K+ and return to RMP

Function
- Long absolute refractory period of 250 ms (compared to 1 to 2 ms in skeletal muscle)
–Prevents wave summation + tetanus [would stop the pumping of heart]

Question 14

Interpret a normal electrocardiogram and relate to function

Answer 14

P wave
SA node fires, atrial depolarisation + atrial systole

QRS complex
Ventricular depolarisation AND
Atrial repolarization and diastole - signal cannot be observed as it is dominated by ventricular depo.
- spike shape = difference in thickness and shape of two ventricles

ST segment - ventricular systole

T Wave
Ventricular repolarisation + relaxation

Question 15

Explain how pressure and resistance determine the flow of a fluid

Answer 15

Pressure causes a fluid to flow
- Pressure gradient needed for flow
[flow increases with decreased resistance]

Resistance opposes flow
- Ventricular pressure must rise above this resistance for blood to flow into great vessels

Question 16

Describe the cardiac cycle and relate its phases to pressure changes, volume changes, heart sounds and ECG

Answer 16

The cardiac cycle = one complete systole + diastole of all four chambers [typically lasting 0.8 sec [[HR 75bpm]]]

phases
1. ventricular filling
2. isovolumetric contraction of ventricles
3. ventricular ejection
4. isovolumetric relaxation of ventricles

phase ONE [three parts
1. ventricular filling [AV valves open]

2. diastasis
3. atrial systole
   - filling completed by atrial contraction [ventricles now contain end-diastolic volume -EDV]

phase TWO - isovolumetric contraction of ventricles [QRS complex in ECG]
- atria repolarise and relax
- ventricles depolarise and contract –> rising pressure = closes AV valves

phase THREE - ventricular ejection [ T wave in ECG]
- rising pressure –> opens SL valves –> rapid ejection of blood then reduced ejection

phase FOUR - isovolumetric relaxation of ventricles [T wave continues]
- ventricles repolarise and relax [begin to expand]
- SL valves close // AV valves remain closed
- ventricles relax/expand but do NOT fill

Question 17

State and describe 2 homeostatic imbalances due to unequal output from both ventricles

Answer 17

pulmonary edema
LEFT ventricle pumps less blood than the right = BP backs up into lungs
- Can be caused by left ventricular failure

systemic edema
RIGHT ventricle pumps less blood than left - BP backs up in systemic circulation
- Can be caused by right ventricular failure

Question 18

Define cardiac output and state its typical values at rest and exercise

Answer 18

Amount [litres] ejected by ventricle in one minute

Cardiac output = heart rate X stroke volume

• CO = ~ 4-6L / min at rest
• CO = up to ~ 22L/min for untrained male during exercise and up to ~35L/min for athletes

Question 19

Discuss nervous factors that affect cardiac output (CO)

Answer 19

Sympathetic Nervous System
- Cardioacceleratory centre [in medulla] sends signals via sympathetic cardiac nerves –> SA + AV node + myocardium = nerves secrete norepinephrine = contractility HR INCREASES

Parasympathetic Nervous System
- Cardioinhibitory centre in medulla stimulates vagus nerves
R vagus n. = SA node
L vagus n. = AV node
- Secretes ACh
- Nodal cell hyperpolarised = HR SLOWS

Question 20

Discuss chemical factors that affect cardiac output

Answer 20

chemoreceptors
- Sensitive to blood pH, CO2 and O2
- increase in CO2 –> hypercapnia
- decreased pH –> acidosis
- hypercapnia + acidosis stimulates cardiac centre –> INCREASE HR

electrolytes
- Elevated blood levels of potassium [hyperkalemia] –> DECREASES HR [inhibiting repolarisation]

Question 21

Define and then discuss how preload, afterload and contractility affect cardiac output

Answer 21

preload - Amount of tension in ventricular myocardium before contraction
- Increased preload tension = increased force of contraction
–> increases CO
[more filled = more tension -like a balloon]

afterload = amount of resistance left ventricle must overcome to open aortic SL valve –> circulate blood
- V pressure must be > than aortic pressure to force valve open and eject blood
- ^ afterload can come from valve damage eg stenosis
- ^ afterload –> ^ cardiac workload

• Any impedance in arterial circulation increases afterload
  –> Increased afterload = increased cardiac workload

contractility - Contraction force for given preload
- Factors that INCREASE contractility [positive inotropic agents] = hypercalcemia, catecholamines

Factors that DECREASE contractility [NEGATIVE inotropic agents] = hyperkalemia, hypocalcemia