Heart

Question 1

Describe four structural features of cardiomyocytes and how each is related to heart function (2 marks)

Answer 1

• gap junctions between cells allow action potentials to be transmitted from one cell to another
• tight junctions between cells prevent cells from tearing apart
• contain actin and myosin, arranged in parallel, to be able to contract with force
• have T tubules to allow action potentials to travel deep into cells

Question 2

Explain what the skeleton of the heart is and iost two roles that it plays in heart function (2 marks).

Answer 2

• a layer of fibrous connective tissue between the atria and the ventricles
• it slows the rate of transmission of action potentials from atria to ventricles
• it anchors the heart valves

Question 3

Name five components of the conduction system, explaining the function and location of each (4 marks).

Answer 3

• sinoatrial node begins the signal for contraction (pacemaker)
• atrioventricular node causes signal to pause briefly between the atria and ventricles
• bundle of His carries action potentials from AV node into the interventricular septum
• right and left bundle branches carry impulses down the interventricular septum to the heart’s apex
• Purkinje fibers carry the action potentials upwards through the myocardium allowing them to spread out to cause contraction

Question 4

Explain the role of the prepotential (pacemeaker potential) in heart fun

Answer 4

• the prepotential is a gradually rising voltage caused by open sodium/potassium and calcium channels
• it automatically initiates the action potentials that eventually lead to contraction

Question 5

Sketch the action potential of conductile cardiomyocytes, referring to the
major ions and their movements (2 marks).

Answer 5

• first, draw a slowly increasing potential, label it prepotential (sodium/potassium and calcium channels open, allowing sodium and calcium to move IN)
• then draw a region with a steeper upwards slope and label it (L-type calcium channels open, allowing more calcium IN)
• then draw a downwards slope and label it (L-type calcium channels close and potassium channels open, allow potassium OUT)

Question 6

Sketch the action potential of contractile cardiomyocytes, referring to the
major ions and their movements (2 marks).

Answer 6

first draw a steep upwards rise and label it: voltage-gated sodium channels open and sodium moves IN

• then draw a long flat region that trails slowly downwards and label it: L-type calcium channels open, allowing calcium to move IN and potassium channels open allowing potassium to move OUT
• draw the downwards slope continuing down until it levels off at the starting level and label this region: calcium channels are closed and open potassium channels allow potassium to move OUT

Question 7

Compare the excitation-contraction of cardiac muscle with skeletal muscle (3
marks).

Answer 7

EXCITATION:

1) in skeletal muscle, action potentials begin in response to release of acetylcholine from a nerve; in cardiac muscle, action potentials are triggered by action potentials in neighbouring cardiomyocytes
2) in both skeletal muscle and cardiac muscle, the action potentials are transmitted along the cell membrane and down T-tubules
3) in both skeletal muscle and cardiac muscle, binding of actin and myosin is caused by increased concentration of calcium in the cytoplasm, caused by movement of calcium out of sarcoplasmic reticulum through ryanodine receptors
4) in skeletal muscle, action potentials that pass down T-tublules open ryanodine receptors; in cardiac muscle, calcium that enters the cytoplasm during the plateau phase causing opening of ryanodine receptors (calcium induced calcium release)

Question 8

Describe the feature of the heart that prevents tetanus from occurring.

Answer 8

he plateau phase of the action potential of contractile cardiomyocytes is prolonged, causing a very long refractory period.

Question 9

Explain what happens, in terms of blood flow through the heart, during

a) atrial systole
b) ventricular systole
c) atrioventricular diastole

Answer 9

a) blood is flowing into the ventricles
b) blood is leaving the ventricles and entering the pulmpnary trunk and aorta
c) blood is flowing through the atria into the ventricles

Question 10

(Paper Diagram) On the diagram below, label the waves of the ECG, opening and closing of semilunar valves, opening and closing of atrioventricular valves, atrial and ventricular diastole, atrial and ventricular systole, ventricular filling and ventricular emptying.

Answer 10

a) opening of semilunar valves
b) closing of semilunar valves
c) closing of AV valves
d) opening AV Valves
e) R
f) T
g) P
h) 0.0-0.1 sec atrial systole (ventricle filling)
i) 0.1-0.4 sec ventricular systole (vent. emptying)
j) 0.4 - 0.8 sec vent. and atrial diastole (vent. filling)

Question 11

Explain what causes the AV valves to open and close

Answer 11

AV valves close when the ventricles begin to contract; they open just after the ventricles stop contracting and blood begins to enter the ventricles (start of ventricular diastole)

Question 12

Explain what causes the semilunar valves to open and close.

Answer 12

semilunar valves open soon after the ventricles contract (pressure in ventricles increase) and close soon after they stop contracting (pressure in ventricles fall to become less than the pressures in the aorta and pulmonary trunk)

Question 13

Explain what heart sounds are and what causes them

Answer 13

heart sounds are caused by the percussion waves created by blood striking the newly closed AV valves and semilunar valves.

Question 14

Describe what cardiac output is and name its two major component

Answer 14

cardiac output is the volume of blood pumped by one side of the heart in one minute
- it depends on the volume pumped in one beat (stroke volume) and the number of beats per minute

Question 15

Explain what preload is and describe the effect of increasing preload on heart
function (Starling’s Law) (2 marks).

Answer 15

preload refers to the volume of blood returning to the atria

- the greater the preload, the greater the subsequent force of contraction (up to a certain point)

Question 16

Explain the difference between preload and afterload.

Answer 16

preload is the flow of blood into the atria and afterload is the flow of blood out of the ventricles

Question 17

Explain what positive and negative inotrophic agents are.

Answer 17

positive inotrophic agents increase the strength of contraction of the heart; negative inotrophic agents decrease the force of contraction of the heart

Question 18

Name the nerves that affect cardiac function, the neurotransmitters released
by each and two of the actions of each (4 marks).

Answer 18

the sympathetic nerve (cardioaccelerator nerve) releases norepinephrine that increases the heart rate and the strength of contraction; the vagus nerve releases acetylcholine that decreases the strength of contraction and rate of contraction

Question 19

Explain what beta-adrenergic receptors and muscarinic receptors are and
why they are important in heart function (2 marks).

Answer 19

beta adrenergic receptors and muscarinic receptors are located in the membranes of conductile and contractile cardiomyocytes

• beta-adrenergic receptors bind epinephrine and norepinephrine causing increased force of contraction and heart rate
• muscarinic receptors bind acetylcholine causing decreased force of contraction and heart rate

Question 20

xplain how beta-blockers and atropine act on the heart (2 marks)

Answer 20

beta-blockers are antagonists of epinephrine/norepinephrine and prevent these neurotransmitters from increasing heart rate and force of contraction, lowering blood pressure
- atropine is an antagonist of acetylcholine and prevents acetylcholine from lowering heart rate and force of contraction, increasing heart rate and force of contraction

Question 21

Explain how the intrinsic heart rate differs from the normal heart rate and
why this is.

Answer 21

• intrinsic heart rate (without nervous stimulation) is about 20 bpm higher than normal heart rate
• the vagus nerve is stronger at lowering the heart rate than the sympathetic nerve is at raising it so when the action of both nerves is removed, heart rate increases

Question 22

Describe the two major reflexes that affect cardiac function (the sensory
components, the stimuli that they respond to and the site of integration) (4
marks).

Answer 22

baroreceptor reflex
- blood pressure receptors in the aorta and carotid sinus
- send sensor information to the cardiovascular center of the medulla oblongata
- control heart rate and blood vessel diameter
chemoreceptor reflex
- chemical receptors in the aorta and carotid sinus
- send sensory information to the cardiovascular center of the medulla oblongata
- control heart rate and blood vessel diameter

Question 23

Describe how plasma potassium concentration influences cardiac function.

Answer 23

normal excitation of the heart requires plasma potassium concentration within a narrow range
- abnormally low plasma potassium and abnormally high plasma potassium concentration can seriously disrupt cardiac function and can be life threatening

Question 24

Describe the influence of body temperature on cardiac function.

Answer 24

abnormally low temperatures reduce heart rate and abnormally high temperatures increase heart rate

Question 25

Describe the influence of thyroid hormones on cardiac function.

Answer 25

thyroid hormones cause elevated rate and strength of contraction of the heart