d4 heart

Question 1

how does the structure of cardiac muscle cells allow propagation of stimuli through heart wall?

Answer 1

• 1 of 3 main types of muscles present in the human body, besides skeletal and smooth muscles
• compared to skeletal muscles, cardiac muscles are shorter and have smaller diameter, but like skeletal muscles there is striation present
• not fused together but are connected to each other via intercalated discs
• cardiac muscle cells capable of generating own impulse, hence do not need to be stimulated by nerve impulses from central nervous system to work (unlike skeletal muscles)
• heavily branched, helps in signal propagation throughout heart muscles
• due to constant non-stop contractions in heart, cardiac muscle cells contain more mitochondria than skeletal muscle cells

Question 2

how do the signals from the sinoatrial node that cause contraction pass from atria to ventricles?

Answer 2

• signals from sinoatrial node that cause contraction cannot pass directly from atria to ventricles
• signals that propagate throughout heart, leading to heart contractions, start from sinoatrial node
• sinoatrial node is pacemaker of heart, responsible for heart rate in humans
• electrical impulses that leads to cardiac muscle contraction can propagate along cardiac muscle fibres
• cardiac muscle cells not fused, hence gap junctions present between cardiac muscles within intercalated discs
• technically each cardiac muscle is capable of independent contractions
• layer of connective tissue belonging to cardiac skeleton that separates atria and ventricles
• prevents spreading of impulse from atrial myocardial cells to ventricles
• only place where impulse can pass over to ventricles is at atrioventricular node
• after short pause, nerve impulses generated from AV node and propagates to rest of ventricular muscle cells

Question 3

why is there a delay between the arrival and passing on of a stimulus at the atrioventricular node?

Answer 3

• it is believed part of the reasons for delay is narrow diameter of cells in AV node which slows down impulse transmission
• reduced capacity in cells in AV node to conduct impulse between cells, leading to a delay
• delay approximately 100ms counting time taken for impulse to pass through entire AV node

Question 4

what is the purpose of the delay between the arrival and passing on of a stimulus at the atrioventricular node?

Answer 4

• delay allows for full contraction of atrial muscular walls to ensure maximum efficiency of pumping blood from atria to ventricles
• if delay is too short, ventricular contraction may start early, forcing AV valves to close and preventing any more blood from entering ventricles from atria

Question 5

how is coordinated contraction of the entire ventricle wall ensured? (describe how bundle of His and Purkinje fibres assist)

Answer 5

• conducting fibres ensure coordinated contraction of entire ventricle wall
• impulse conduction in heart carried out by heart muscle cells instead of nerve cells
• heart muscle cells are capable of generating action potentials like a nerve cell
• main form of impulse conduction is via all cardiac muscle cells, but there are specialised cardiac muscle cells that shows adaptation for specific paths of conduction in heart
• examples of such fibres include Bundle of His and Purkinje Fibres
• bundle of His connects sinoatrial node to Purkinje fibres at base of heart, and runs along interventricular septum
• bundle of His allows for better coordinated and synchronised ventricular contraction
• bundle of His branches into two fibres in septum
• Purkinje fibres made up of heart muscle cells larger in size, and also help in coordination and synchronization of ventricular contractions

Question 6

how to map the cardiac cycle other than mechanical perspective?

Answer 6

• normal cardiac cycle consists of systole (heart contraction) and diastole (heart relaxation)
• besides viewing the heart from mechanical perspective (changes in blood pressure, or checking on chamber size via ultrasound), it is possible to map a cardiac cycle in terms of electrical conductivity
• electrocardiograph can detect electrical impulses of the heart via electrodes and present data in form of an electrocardiogram

Question 7

what are the normal heart sounds caused by?

Answer 7

• normal heart sounds caused by atrioventricular valves and semilunar valves closing causing changes in blood flow
• atrioventricular valves refer to bicuspid (left) and tricuspid (right) valves that prevent backflow of blood from ventricles to atria
• semilunar valves in aorta and pulmonary artery prevents backflow of blood from arteries into ventricles
• valves ensure unidirectional flow of blood
• heart beat made up of sets of two sounds, often
  described as “lub – dub”
• 1st sound (lub) caused by closure of AV valves
• 2nd sound (dub) caused by closure of semilunar valves

Question 8

what is the equation of cardiac output?

Answer 8

cardiac output = heart rate (bpm) x stroke volume (ml/beat)

• heart rate: measured in beats per minute ; measure of number of cardiac cycles per minute
• stroke volume: capacity of heart ; volume of blood pumped out from heart per cycle

Question 9

how to interpret systolic and diastolic blood pressure measurements?

Answer 9

• blood pressure made up of 2 components, systolic pressure and diastolic pressure
• systolic pressure: shows pressure generated in blood vessels due to contraction of heart
• diastolic pressure shows pressure in between heart contractions
• in the arteries, this (diastolic) pressure maintained by semi-lunar valves of aorta and other factors (elasticity and smooth muscle contraction) in arterial walls
• average normal blood pressure for adults is around 120/80 mmHg (equated to pressure of a column of mercury)
• high systolic and diastolic pressures: indicative of hypertension
• low systolic and diastolic pressures (hypotension): indicative of problems like internal bleeding, shock, hormonal imbalances, etc

Question 10

what are the causes and consequences of hypertension?

Answer 10

• hypertension: condition characterized by abnormally high blood pressure
• often associated with sedentary lifestyle, or could be caused by underlying problems like kidney problems, substance abuse, gland or hormonal problems
• high pressures generated will cause heart to enlarge and weaken over time, leading to further health problems

Question 11

what are the causes and consequences of thrombosis?

Answer 11

• formation of clot within blood vessels
• thrombosis occurs in arteries when vessels are damaged from deposition of cholesterol (atherosclerosis)
• atheromas (fat deposits) develop in arteries and significantly reduce diameter of vessel (leading to hypertension)
• high blood pressure damages arterial wall, forming lesions known as atherosclerotic plaques
• if a plaque ruptures, blood clotting is triggered, forming a thrombus that restricts blood flow
• if thrombus becomes dislodged it becomes an embolus and can cause blockage at another site
• thrombosis in coronary arteries leads to heart attacks, while thrombosis in brain causes strokes

Question 12

what results are shown by the analysis of epidemiological data relating to the incidence of coronary heart disease?

Answer 12

through studies of data collected as part of epidemiology (studies of occurrences and distribution of disease), many factors that affect incidence of coronary heart disease (CHD) is discovered:

• age (older people more likely to have CHD)
• sedentary lifestyle and being overweight
• other correlated diseases like diabetes can increase risks of CHD
• smoking and other substance abuse can affect CHD rates
• genetic basis of having CHD
• males more likely to suffer from CHD

Question 13

how can artificial pacemakers be used to regulate heart rate?

Answer 13

• artificial pacemaker: small device placed next to heart via surgery
• consists of larger body which may contain a computer to collate data and send impulses
• electrodes that can either take data (heart rate, and even temperature and respiratory rate) or send impulses to trigger action potential and contraction of heart muscles

Question 14

how can life-threatening cardiac conditions be treated?

Answer 14

• defibrillation can be used to treat life-threatening cardiac conditions
• fibrillation refers to state where heart is not beating in a coordinated manner, and beats rapidly and irregularly
• often happens as part of cardiac arrest
• defibrillator is a device that sends a high voltage that
  produces an electric shock to heart
• aim of defibrillator is to shock the heart, causing a massive depolarisation to heart muscles, and gives a chance for SA node in heart to reset and reestablish a normal heart rhythm
• defibrillator cannot shock a non beating heart (in asystole) to start beating again