Heart Flashcards
Position of the heart
- Posterior to sternum
- from 3rd costal cartilage to 5th costal space
~ 7.5 cm from midline - within mediastinum (area between lungs)
- surrounded by pericardium
> visceral layer (serous membrane that adheres to heart)
> Parietal layer = fibrous outer layer
(with pericardial fluid between to allow smooth contraction)
Structure of heart
> Vena Cava (inf from LL, sup from UL) > right atrium > Tricuspid valve > right ventricle > pulmonary valve (semi-lunar) > pulmonary artery > pulmonary vein > left atrium > Bicuspid (Mitral) valve > Left ventricle > Aortic valve > Aorta
- Chordae tendineae make sure tricuspid and mitral valves don’t fold back on themselves
- Papillary muscles keep chordae tendineae in place
- Moderator band = involved in transmission of impulse
- Fossa Ovalis = closed foramen ovale
- trabeculae carneae = muscular ridges
Cardiac Muscle
- muscular pump with actin + myosin
- cells connected by intercalated discs/gap junctions (specialist sites where myofibrils lock together + transfer ions - co-ordinating efficient contraction)
- auto rhythmic fibres - SA/AV nodes excite contraction
- heart = 2 syncytium (interconnecting networks that contract independently - atria + ventricles)
- syncytia = electrically insulated from one another but co-ordinate as a whole syncytium
- 70 beats per minute
- 2.5 billion times in a life
Contraction cycle of cardiac muscle
- Depolarisation
- resting potential of -90mV
- AP in adjacent fibres - via gap junction - opens Na+ channels - Plateau (maintains 0mV)
- Na+ = pumped out
- Ca2+ slowly enters cell
- K+ stays in cell - Repolarisation
- Ca2+ channels slowly close
- k+ channels open - K+ leaves cell - hyperpolarisation
- resting potential is restored by active ion pumps - Refractory period
- time before another contraction can take place
- lasts longer than contraction
- ensures co-ordination of heart contraction
- AP lasts 0.3 seconds vs 0.001 of skeletal muscle
Cardiac cycle
- Relaxation
- all 4 chambers in diastole - Atrial systole (lasts 100msec)
- Atria contract forcing ~25ml into ventricles (AV valves)
- End diastolic volume (EDV) = ~130ml (70% = passive in flow)
- Atria enter diastole (semi-lunar valves = shut) - Ventricular Systole (lasts ~270msec)
- Ventricular contraction starts - pressure increases and pushes AV valves closed when exceeds atrial pressure
- Isovolumetric contraction - builds pressure further - semi-lunar valves are pushed open
- Isotonic contraction ejects blood from ventricles (stroke volume = amount ejected)
(ejection fraction = % of EDV)
- pressure drops and semi-lunar valves close
- end systolic volume - 50ml - Ventricular diastole (~430msec)
- atria fill with blood and AV valves open when pressure is greater than relaxing ventricles = passive filling until ~3/4 full
Conducting system of the heart
- SA node
- 100 AP’s per minute - vagus nerve brings down to 70
- triggers atrial activation - Reaches AV node via intermodal pathways in atrial wall (50msec)
- atrial contraction
- delay of 100msec - allows atria to empty
- Contracts at own rate of 40-60 AP’s - will take over if SA node is damaged - Impulse travels along interventricular septum (150msec)
- via bundle of hiss (AV bundle) - Reaches purkinje fibres + moderator band (175msec)
- to papillary muscles of right ventricle
- 20-40 AP’s per minute to ventricles only - if both SA+AV = damaged still some blood pumped but lack of O2 to brain means pacemaker = required - Ventricular contraction (225msec)
- impulse = relayed throughout ventricular myocardium
Regulation of heart rate
- Innervation from cardiac plexus = automatic (signals from CV centre of medulla oblongata)
a) sympathetic - cardiac accelerator nerves
- increase SA+AV rate + contractility of atria/ventricles
- heart rate + output increases
b) parasympathetic - vagus nerve (cranial nerve 10)
- Decreases SA+AV rate - decreased heart rate - Hormones
- adrenaline/noradrenaline increase HR + contractility
- thyroid hormones do the same (hyperthyroidism can lead to tachycardia = high hr) - Ions
- high K+/Na+ - decreases hr + contractility
- high Ca2+ = increases hr + contractility
Electrocardiogram
> P wave = atrial depolarization
PR segment = end of atrial depolarization as impulse is delayed at AV node
QRS complex = ventricular depolarization (masks atrial repolarization)
ST segment = ventricular depolarization = complete
T wave = Ventricular repolarization
define Stroke Volume + what it depends on
> amount of blood ejected in ventricular systole
affected by:
1. Preload (stretch on heart before contraction)
- Frank starling law: larger stretch (greater fill in diastole) = more forceful contraction (overstretch will weaken contraction)
2. Contractility (strength of contraction- given preload)
- Increased by: Ca2+ / hormones/ sympathetic nervous
- decreased by: Parasympathetic/ state of acidosis in blood/ lack of O2/ Ca2+ channel + Beta blockers
3. Afterload (pressure in blood vessels to overcome)
- increased afterload = decreased stroke volume
Define Cardiac output
Total blood volume passing through lungs + systemic system in one minute
- stroke volume x heart rate
- close to total blood volume (if more = needed we use cardiac reserve)
Blood supply to heart
Via coronary arteries
> Right coronary artery
- feeds right atria + ventricle as well as SA/AV nodes
- branch = right marginal artery
> Left coronary artery
- feeds left atrium + ventricles
- branches = left anterior descending artery
- left circumflex artery
*Usually the arteries replaced in coronary artery bypass graft = CABG
