D.4 The heart Flashcards
What is cardiac muscle?
A type of striated muscle found in the wall of the heart.
Describe the structure of cardiac muscle cells
-This muscle has thick and thin muscle fibres with myofibrils containing myofilaments similar to those found in skeletal muscle.
-The nucleus of cardiac muscle cells is found in the centre of the cell.
-The cells are rich in mitochondria and glycogen granules that are found adjacent to the myofibrils.
-Unlike skeletal muscle, which has multinucleate cells, the cardiac muscle has numerous short, cylindrical cells arranged end-to-end, resulting in long, branched fibres giving a characteristic Y-shape.
What does the structure of cardiac muscle cells allow?
The propagation of stimuli through the heart wall.
Diagram of cardiac muscle cells
What is an intercalated disc?
The attachment site between cardiac muscle cells. It appears as a linear structure transverse to the muscle fibre.
Explain how intercalated discs work
-Cardiac muscle contains transverse cross-bands called intercalated discs.
-These discs represent the attachment site between adjacent cells.
-Intercalated discs contain adhering junctions between cells that hold cells together and gap junctions, which allow communication between cells.
What are gap junctions?
-Arrays of densely packed protein channels that permit intercellular passage of ions and small molecules.
-Electrical activation of the heart requires cell-cell transfer of current via gap junctions.
What do the intercaled discs and gap junctions in cardiac muscle allow to happen?
This characteristic structure of cardiac muscle allows electrical impulses to pass rapidly from cell to cell, so the linked cells contract almost simultaneously.
Explain how contraction works in cardiac muscle cells
-These cells have an “all-or-none” effect: once a cell is activated, it produces maximum contraction.
-These cells also have a long refractory period, during which the cells cannot contract for a second time.
-This ensures that the contraction or systole of the heart is separated by a resting period or diastole.
In what way does a cardiac muscle cell differ from a skeletal muscle cell?
-Cardiac cells have intercalated discs while skeletal cells do not.
-This allows communication between cardiac cells, as the electrical activation needs to propagate from one cell to the other.
What is the refractory period?
Time for cardiac muscle cell membrane to be ready for a second stimulus.
What causes the heartbeat?
The spontaneous rhythmic contraction of cardiac muscle.
Explain how the heartbeat is created and regulated
-This contraction starts in embryonic tissues by specialized cells forming the pacemaker.
-Nodes and bundles that transmit the impulse to various parts of the cardiac muscle tissue regulate the heartbeat.
Diagram of cardiac conduction
Where is the heartbeat initiated?
In the sinoatrial node (SA), a group of specialised cardiac muscle cells found where the superior vena cava joins the right atrium.
Explain what happens after a heartbeat is initiated (until atrial systole occurs)
-Signals from the sinoatrial node that cause contraction cannot pass directly from atria to ventricles.
- Instead, the impulse spreads along the atria to the atrioventricular node (AV), from where it spreads to the ventricle.
-There is a delay between the arrival and passing on of a stimulus at the atrioventricular node.
-This delay allows time for atrial systole before the atrioventricular (tricuspid and bicuspid or mitral) valves close.
-The blood, therefore, leaves the atria to the ventricles when the atrial systole occurs.
The delay between the arrival and passing of a stimulus at the AV node gives time for ___
The atria to contract before the AV valves close.
Explain what happens after the blood enters the ventricles
-The blood is now in the ventricles and the atrioventricular valves are closed.
-In the interventricular walls, there are specialised fibres grouped to form a bundle called Bundle of His.
-This bundle splits into two branches that go to the left and right ventricles, leading to other specialised cells called Purkinje fibres.
-These fibres carry the impulse at a very high speed.
-Conducting fibres ensure coordinated contraction of the entire ventricle wall.
-The ventricular contraction is called the systole.
-Blood then flows out of the heart as the semilunar (pulmonary and aortic) valves open and give access to the aorta and pulmonary artery.
-The heart is now relaxed; this is called the diastole.
How are heart sounds generated?
-By the beating of the heart and the closing of the heart valves.
-Normal heart sounds are caused by the atrioventricular valves and semilunar valves closing causing changes in blood flow.
Describe the heart sounds of a healthy person
-When listening to the heart of a healthy person, a rhythmic lub-dub sound can be heard.
-The closing of the atrioventricular valves at the beginning of the ventricular contraction (systole) produces the first sound, the lub.
-Immediately afterwards, the closing of the semilunar valves just after the ventricular systole and beginning of the diastole causes the second sound, the dub sound.
What is a stethoscope?
-A tool used to listen to the heart sounds.
-This is an acoustic device that has a resonator that is placed on the patient’s chest and two tubes connected to earpieces.
What is the order of the cardiac conduction?
SA node, AV node, bundle of His, Purkinje fibres.
Why is there a delay between the arrival and passing on of a stimulus at the atrioventricular node?
It allows time for atrial systole before the atrioventricular valves close.
What is an action potential?
An electrical event associated with different concentrations of ions across membranes.
Explain how an action potential is generated in a cardiac muscle (contraction of muscle)
-The mechanism by which the cardiac muscle contracts is similar to that of the striated skeletal muscle.
-The membrane action potential leads to an increase in the calcium ions around the myofilaments, activating the myosin-ATPase.
-This leads to the sliding of the thick and thin filaments.
-The action potential is much longer (around 300 ms) than in nerve cells (around 2 ms) and skeletal muscle cells (around 4 ms).
-Cardiac muscle cells have a special disposition forming fibers and have gaps between the cells (gap junctions).
-This allows for a more rapid spread of the action potential.
What is the purpose of the plateau of the action potential in cardiac muscles?
This prevents the heart from contracting before time.
Diagram of action potentials in the sinoatrial node (SA) and atrioventricular node (AV) and in the atria and ventricles
What explains action potentials in muscle cells?
The changes in permeability of the membrane of the cardiac muscle cells to different ions.
Diagram showing membrane permeability during action potential in a cardiac muscle cell
How is membrane potential changed? (reword)
-Sodium ions are positive.
-When the signal reaches the cell, these ions come in, changing the overall membrane potential to positive.
-The plateau phase is given due to the balance between the potassium ions leaving the cell and the calcium ions coming in.
-This phase maintains depolarization for a longer time.
How do pacemaker cells work?
-They have their own intrinsic rhythms of activity; therefore, as one action potential is completed, immediately another action potential is generated in the membrane, even in the absence of nerve impulse.
-This is why its rhythm is considered myogenic (of muscle origin) rather than neurogenic (of neural origin).
-Pacemakers are myogenic, so the action potential is generated by the pacemaker itself.
What is the myogenic coordination of heartbeats?
Contraction initiated by cardiac muscle cell.
What occurs during the depolarisation phase of an action potential in the atrium?
Sodium ions enter the cells, increasing the membrane potential.
As sodium ions enter the muscle cells, the difference in membrane potential increases from around -70 mV to +30 mV, initiating the depolarization.
What is the heart rate?
-The number of times the heart contracts in one minute (beats per minute).
-A person’s heart rate can be obtained by taking their pulse.
How can a person’s heart rate be obtained?
By taking their pulse
What is a normal heart rate?
It can range between 50 and 100 beats per minute.
What factors increase heart rate?
-Gender
-Physical activity
-Body size
-Temperature
-Altitude
-Posture
-Stress
-Eating
-Sodium and calcium ions in the blood
-Drugs
How does gender affect heart rate?
Women have a slightly higher heart rate than men because they need more energy for metabolism.
How does physical activity increase heart rate?
More blood is required in muscles for oxygen supply.
How does body size increase heart rate?
Larger people have a higher heart rate than smaller people as blood needs to cover a larger area.
How does temperature increase heart rate?
-The vasodilation that occurs when it is hot increases heart rate.
-If a person is running a fever, the pulse is accelerated.
How does altitude increase heart rate?
The body requires a greater amount of oxygen, therefore cardiac output must increase.
How does posture increase heart rate?
-When one is standing the heart rate is higher than when lying down.
-This is related to the effect of gravity, as when standing blood needs to be pumped against this force.
How does stress increase heart rate?
The body will produce epinephrine, which will accelerate the rate.
Graph showing how heart rate decreases with age and increases with physical activity
Diagram of an artificial pacemaker with two leads
Picture of a patient connected to an ECG machine
Diagram of an ECG of a healthy person
Diagram of cardiac conduction and the heart cycle to explain the recording of an ECG
Diagram of how blood pressure is measured with a sphygmomanometer
Table showing the categories for blood pressure range in adults
Diagram showing the causes of high blood pressure (hypertension)
Presence of CHD around the world in recent years
-Over the past years, CHD mortality rates worldwide have declined, but in developed countries it is still a major cause of death and disability.
-Figure 1 shows epidemiological data relating to the incidence of ischemic heart disease in men and women worldwide measured as the disability adjusted life years (DALYs).
Maps showing epidemiological data relating to the incidence of ischemic heart disease in men and women worldwide measured as the disability adjusted life years (DALYs)
Picture of a
