Cardiac Anatomy |
| The heart consists of 4 chambers: right atrium, right ventricle, left atrium, left ventricle |
- Right atrium receives blood from superior and inferior vena cava
- Blood flows from right atrium, across tricuspid valve, into right ventricle
- Muscle of right ventricle is not as thick as left ventricle
- Blood enters pulmonary artery from right ventricle. Backflow prevented by semilunar pulmonic valve
- Blood returns to heart from lungs via 4 pulmonary veins that enter left atrium
- Blood flows from left atrium, across mitral valve, into left ventricle
- Left ventricle has a thick muscular wall / generate high pressures during contraction
- Blood from left ventricle is ejected, across aortic valve, into aorta
- Tricuspid and mitral valves (atrioventricular AV valves)
- Have fibrous strands (cordae tendinae) that attach to papillary muscles
- The papillary muscles contract during ventricular contraction
- Generate tension on valve via cordae tendinae to prevent AV valves from bulging back into atria
- Semilunar valves (pulmonic and aortic) do not have these attachments
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The Cardiac Cycle |
- Atria receive blood from veins and store it prior to each heart beat
- Right atrium receives blood from main body veins called "vena cava"
- Superior vena cava SVC carries blood from head, upper chest and arms
- Inferior vena cava IVC carries blood from lower chest, abdomen and legs
- Left atrium receives blood from lungs via 4 separate pulmonary veins
- Systole refers to a period of contraction by heart muscle
- Diastole refers to a period of relaxation by heart muscle
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Atrial systole |
- Both atria contract and push stored blood across AV valves into ventricles, to help fill them
- Atrioventricular (AV) valves include
- Mitral valve located between left atrium and left ventricle
- and tricuspid valve which separates right atrium from right ventricle
- Reduces the volume of atria and increases pressure
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Ventricular systole |
- After atria contracts, ventricles begin to contract
- Pressure in ventricles increases, blood is forced against AV valves
- Valves close to prevent backflow → first heart sound
- Volume is reduced
- Blood is ejected into arteries through aortic and pulmonary valves
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Ventricular diastole |
- End of cardiac cycle, all chambers relax
- Aortic and pulmonary valves close (second heart sound) / prevents backflow into heart
- Atria begin to fill up again to start next cycle
- Volume increases and pressure decreases
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Electrophysiology |
- Sinus node is located at the top right atrium
- Also known as the "natural pacemaker" controlling heart rate
- Increases with physical activity and decreases when relaxing
- Electrical signal rapidly spreads from the Sinus node across the right atrium and left atrium
- Only one area where atria and ventricles are electrically connected
- Atrioventricular node or AV node deep in center of heart
- All electrical signals from atrium must pass through AV node in order to get to ventricles
- AV node is connected to the Bundle of His
- Branches into a right bundle (to right ventricle) and left bundle (to left ventricle)
- Fibers that branch out to distant ventricular tissues are called Purkinje Fibers
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Blood pressure |
- Baroreceptors near aorta and carotid arteries monitor blood pressure
- Abnormal blood pressure → signal send to medulla
- Cardiac center changes heart rate → cardiac output
- Vasomotor center changes diameter of blood vessels
- Shock: blood pressure too low
- Insufficient nutrients for cells with a high metabolism (heart, brain)
- Caused by excessive bleeding or extensive vasodilation
- Treated with vasoconstrictors such as epinephrine (adrenaline), atropine
- [NOTE] International name for adrenaline is now epinephrine
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Regulation of the Cardiac Cycle |
- Heart has unique ability to beat (contract) on its own
- Assisted by nerves and hormones in blood but functions without them
- Impulse leaves SA node and passes through both atria → causing them to contract
- From AV node impulse passes down to the Bundle of His
- Bundle of His branches and spreads through both ventricles via Purkinje fibers → ventricles contact
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Cardiac output as a function of stroke volume and heart rate |
- The volume of blood pumped by one ventricle during one beat is called the stroke volume
- Cardiac Output = Stroke Volume x Heart Rate (number of ventricular contractions/min)
- ↑Force of contraction → ↑Stroke volume → ↑Cardiac output
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Regulation of heart rate → Cardiac output is influenced by several factors |
| Autonomic Nervous System |
- Heart is innervated by the autonomic nervous system (ANS)
- The autonomic nervous system has 2 divisions
- These division are called parasympathetic and sympathetic
- Parasympathetic fibres decrease heart rate via the vagus nerve
- Sympathetic fibres increase heart rate
- Cardiac Inhibitory Centre
- Found in the medulla oblongata of the brain stem
- Sends signals via the vagus nerve, which is parasympathetic, to the heart
- Signal reaches SA and AV nodes to trigger a release of neurotransmitter acetylcholine
- This slows down the heart rate
- Cardiac Accelerating Centre
- Found in the medulla and the upper thoracic spinal cord
- Sympathetic fibres run towards the myocardium
- There they also innervate the SA and AV nodes, but also cardiac cells
- When stimulated, the sympathetic fibres cause a release of norepinephrine
- [NOTE] The international name for noradrenaline is now norepinephrine
- Norepinephrine increases heart rate and strength of ventricular and atrial contraction
- Cardiac centres balance stimulatory and inhibitory effects of the ANS
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Hormonal influence |
Stress releases epinephrine and norepinephrine from the adrenal medulla into the circulation.
Both hormones increase the heart rate. |
Electrolyte Balance |
- Excess K+ in the extracellular environment reduces heart rate and strength of contraction
- Only a fraction of a KCl infusion is required to kill a patient
- Spastic contraction of the heart results from excess Ca2+
- Heart can be defibrillated by applying an electrical current to the chest wall
- Stimulates depolarisation of all cardiac muscle fibres simultaneously
- As a result, all contractions cease
- If the SA node then begins to function, normal cardiac rhythm may be re-established
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Pressure Changes During the Cardiac Cycle |
- Ventricles begin to contract, intraventricular pressure rises causing AV valves to close. Ventricles are neither being filled with blood (AV valves are closed) nor ejecting blood (intraventricular pressure has not risen sufficiently to open semilunar valves). This is the phase of isovolumetric contraction.
- Pressure in the left ventricle becomes greater than pressure in the aorta, phase of ejection begins as semilunar valves open. Pressure in left ventricle and aorta rises to about 120 mm Hg when ejection begins and ventricular volume decreases
- Pressure in left ventricle falls below pressure in the aorta, back pressure causes semilunar valves to shut. Pressure in aorta falls to 80 mm Hg
- During isovolumetric relaxation AV and semilunar valves are closed. This phase lasts until pressure in ventricles falls below pressure in atria
- When pressure in ventricles falls below pressure in atria, ventricles are filled
- Atrial systole empties final amount of blood into ventricles immediately prior to next phase of isovolumetric contraction of ventricles
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