D4 - The Heart Flashcards
Cardiac muscle specialised features
Cardiac muscle cells contract without stimulation by the central nervous system (contraction is myogenic)
Cardiac muscle cells are branched, allowing for faster signal propagation and contraction in three dimensions
Cardiac muscles cells are not fused together, but are connected by gap junctions at intercalated discs
Cardiac muscle cells have more mitochondria, as they are more reliant on aerobic respiration than skeletal muscle
Cardiac tissue unique functional properties
Cardiac muscle has a longer period of contraction and refraction, which is needed to maintain a viable heartbeat
The heart tissue does not become fatigued (unlike skeletal muscle), allowing for continuous, life-long contractions
The interconnected network of cells is separated between atria and ventricles, allowing them to contract separately
Function of gap junctions at intercalated discs
This means that while electrical signals can pass between cells, each cell is capable of independent contraction
The coordinated contraction of cardiac muscle cells is controlled by specialised autorhythmic cells (‘pace makers’)
Sinoatrial Node - function at atrial systole
This cluster of cells is collectively called the sinoatrial node (SA node or SAN)
The sinoatrial node acts as a primary pacemaker, controlling the rate at which the heart beats (i.e. pace ‘making’)
It sends out electrical signals which are propagated throughout the entire atria via gap junctions in the intercalated discs
In response, the cardiac muscle within the atrial walls contract simultaneously (atrial systole)
Signal passes to AV node
This connective tissue functions to anchor the heart valves in place and cannot conduct electrical signals
The signals from the sinoatrial node must instead be relayed through a second node located within this cardiac skeleton
This second node is called the atrioventricular node (or AV node) and separates atrial and ventricular contractions
The AV node propagates electrical signals more slowly than the SA node, creating a delay in the passing on of the signal
The separation of atrial and ventricular contraction is important as it optimises the flow of blood between the heart chambers
The delay in time following atrial systole allows for blood to fill the ventricles before the atrioventricular valves close
Ventricular contraction
Ventricular contraction occurs following excitation of the atrioventricular node (located at the atrial and ventricular junction)
The AV node sends signals down the septum via a specialised bundle of cardiomyocytes called the Bundle of His
The Bundle of His innervates Purkinje fibres in the ventricular wall, which causes the cardiac muscle to contract
This sequence of events ensures contractions begin at the apex (bottom), forcing blood up towards the arteries
Heart diastole
After every contraction of the heart, there is a period of insensitivity to stimulation (i.e. a refractory period)
This recovery period (diastole) is relatively long, and allows the heart to passively refill with blood between beats
This long recovery period also helps prevent heart tissue becoming fatigued, allowing contractions to continue for life
Function of valves
The heart contains a number of heart valves which prevent the backflow of blood
This ensures the one-way circulation of blood around the body
Valve locations
Atrioventricular valves (tricuspid and bicuspid) prevent blood in the ventricles from flowing back into the atria
Semilunar valves (pulmonary and aortic) prevent blood in the arteries from flowing back into the ventricles
Heart sounds
Heart sounds are made when these two sets of valves close in response to pressure changes within the heart
The first heart sound is caused by the closure of the atrioventricular valves at the start of ventricular systole
The second heart sound is caused by the closure of the semilunar valves at the start of ventricular diastole
Cardiac cycle
The cardiac cycle describes the series of events that take place in the heart over the duration of a single heart beat
It is comprised of a period of contraction (systole) and relaxation (diastole)
The cardiac cycle can be mapped by recording the electrical activity of the heart with each contraction
Activity is measured using a machine called an electrocardiograph to generate data called an electrocardiogram
ECG sequence of events
The P wave represents depolarisation of the atria in response to signalling from the sinoatrial node (i.e. atrial contraction)
The QRS complex represents depolarisation of the ventricles (i.e. ventricular contraction), triggered by signals from the AV node
The T wave represents repolarisation of the ventricles (i.e. ventricular relaxation) and the completion of a standard heart beat
Between these periods of electrical activity are intervals allowing for blood flow (PR interval and ST segment)
Possible heart conditions detected by ECG
Tachycardia (elevated resting heart rate = >120 bpm) and bradycardia (depressed resting heart rate = < 40 bpm)
Arrhythmias (irregular heart beats that are so common in young people that it is not technically considered a disease)
Fibrillations (unsynchronised contractions of either atria or ventricles leading to dangerously spasmodic heart activity)
Cardiac output
Cardiac output describes the amount of blood the heart pumps through the circulatory system in one minute
It is an important medical indicator of how efficiently the heart can meet the demands of the body
Factors effecting cardiac output
There are two key factors which contribute to cardiac output – heart rate and stroke volume
Equation: Cardiac Output (CO) = Heart Rate (HR) × Stroke Volume (SV)
Heart rate
Heart rate describes the speed at which the heart beats, measured by the number of contractions per minute (or bpm)
Each ventricular contraction forces a wave of blood through the arteries which can be detected as a pulse
The typical pulse rate for a healthy adult is between 60 – 100 beats per minute
Factor affecting HR
Heart rate can be affected by a number of conditions – including exercise, age, disease, temperature and emotional state
Additionally, the body will attempt to compensate for any changes to stroke volume with a corrective alteration to heart rate
HR control
An individual’s heart rate is controlled by both nervous and hormonal signals:
Heart rate is increased by the sympathetic nervous system and decreased by parasympathetic stimulation (vagus nerve)
Heart rate can also be increased hormonally via the action of adrenaline / epinephrine
Stroke volume
Stroke volume is the amount of blood pumped to the body (from the left ventricle) with each beat of the heart
It is affected by the volume of blood in the body, the contractility of the heart and the level of resistance from blood vessels
Factors affecting BP
Changes in stroke volume will affect the blood pressure – more blood or more resistance will increase the overall pressure
Blood pressure measurements typically include two readings – representing systolic and diastolic blood pressures
Systolic blood pressure is higher, as it represents the pressure of the blood following the contraction of the heart
Diastolic blood pressure is lower, as it represents the pressure of the blood while the heart is relaxing between beats
Variance in BP measurement
Blood pressure readings will vary depending on the site of measurement (e.g. arteries have much higher pressure than veins)
A typical adult is expected to have an approximate blood pressure in their brachial artery of 120/80 mmHg to 140/90 mmHg
Blood pressure can be affected by posture, blood vessel diameter (e.g. vasodilation) and fluid retention or loss
BP changed in circulatory system
Hypertension
Hypertension is defined as an abnormally high blood pressure – either systolic, diastolic or both (e.g. > 140/90 mmHg)
Common causes of hypertension include a sedentary lifestyle, salt or fat-rich diets and excessive alcohol or tobacco use
High blood pressure can also be secondary to other conditions (e.g. kidney disease) or caused by some medications
Hypertension itself does not cause symptoms but in the long-term leads to consequences caused by narrowing blood vessels
Thrombosis
Thrombosis is the formation of a clot within a blood vessel that forms part of the circulatory system
Thrombosis occurs in arteries when the vessels are damaged as a result of the deposition of cholesterol (atherosclerosis)
Atheromas (fat deposits) develop in the arteries and significantly reduce the diameter of the vessel (leading to hypertension)
The high blood pressure damages the arterial wall, forming lesions known as atherosclerotic plaques
If a plaque ruptures, blood clotting is triggered, forming a thrombus that restricts blood flow
If the thrombus becomes dislodged it becomes an embolus and can cause blockage at another site
Thrombosis in the coronary arteries leads to heart attacks, while thrombosis in the brain causes strokes
