Ch. 16 - The heart Flashcards
Describe the size of the heart and its location in the thorax.
The human heart is hollow and cone shaped, varying in size.
An average adult has a heart that is about 14cm/5in long, by 9cm/3.5in wide, and weighs approx. 300g.
The heart lies inside the thoracic cavity, resting on the diaphragm.
It is within the mediastinum, the medial cavity of the thorax, between the lungs.
Its posterior border is near the vertebral column, and it’s anterior border near the sternum.
It is not situated in the exact centre of the thorax, approx. 2/3 of the heart lies to the left of the midsternal line, and is partially obscured, laterally, by the lungs.
The base of the heart is actually the upper portion, where it attaches to several large blood vessels.
This wide portion of the heart lies beneath the 2nd rib.
The distal end of the heart extends downwards and left, ending in a blunt point called the apex, which lies even with the 5th intercostal space, pointing toward the left hip.
Just below the left nipple, between the 5th and 6th ribs, the apical impulse can be felt. This is caused by the apex touching the chest wall.
Identify the layers of the heart wall and the function of each.
(Outer)Epicardium:
- Protects the heart by reducing friction, and is the visceral portion of the pericardium on the hearts surface.
- Consists of connective tissue and some deep adipose tissue.
(Middle)Myocardium:
- Thick layer pumps blood out of the chambers of the heart.
- Consists mostly of cardiac muscle tissue organised in planes and richly supplied by blood capillaries, lymph capillaries, and nerve fibres.
(Inner)Endocardium:
- Consists of squamous epithelium and connective tissue with many elastic and collagenous fibres.
- Contains blood vessels, as well as specialised cardiac muscle fibers called Purkinje Fibres.
- Lines the chambers of the heart and covers the fibrous tissue that forms the heart valves.
- It is continuous with the endothelial linings of the heart’s blood vessels.
Identify the 4 chambers of the heart, and list its associated great vessels.
Upper chambers-Atria-recieve blood returning to the heart.
Lower chambers-Ventricles-recieve blood from the Atria which is pumped into arteries.
Right Atrium:
- Recieves low oxygen blood from the body tissues via 2 large veins called the Superior Vena Cava(returns blood from areas superior to the diaphragm) and the Inferior Vena Cava(returns blood from areas inferior to the diaphragm), and from a smaller vein called the Coronary Sinus(returns blood from myocardium).
Tricuspid ► Valve
Right Ventricle:
- Forms most of anterior surface of the heart, muscular wall 3 x thinner than left ventricle as it only pumps blood to the lungs for oxygenation(low resistance to blood flow), flatter than left ventricle and crescent shaped.
Pulmonary Trunk/Arteries/Lungs ► Lungs/Pulmonary Veins
Left Atrium:
- Recieves oxygenated blood from lungs through 4 Pulmonary veins, forms most of the Base(upper) of the heart
Bicuspid ► Valve
Left Ventricle:
- Thicker as it must pump oxygenated blood to all body parts through the Aorta(high resistance to blood flow), forms most of the apex and inferoposterior area of the heart, the cavity is almost circular.
Name the 4 heart valves, and describe the locations and functions of each.
Tricuspid/Right Atrioventricular valve:
- A-V/Atrioventricular valve
- located between Right Atrium and Right Ventricle
- 3 flexible projections called ‘cusps’ which are flaps of endothelium reinforced by a core of connective tissue
- The cusps are attached to to strong fibers called the Chordae Tendineae, which originate from small Papillary Muscles projecting inward from the ventricle wall and contracting as the ventricle contracts.
- When the tricuspid valve closes, the papillary muscles pull on the chordae tendoneae, preventing the cusps from swinging back into the atrium.
- during ventricular contraction, prevents backflow of blood from Right Ventricle into Right Atrium
Pulmonary Semilunar Valve:
- Semilunar valve
- located at the entrance to the Pulmonary trunk
- during ventricular relaxtion, prevents backflow of blood from the Pulmonary trunk into the Right Ventricle
Mitral/Bicuspid/Left Atrioventricular Valve:
- A-V/Atrioventricular valve
- located between the left atrium and the Left Ventricle
- 2 ‘Cusps’
- When the left atrium is filled with oxygenated blood, it oushes the mitral valve open, sneding the blood into the left ventricle
- during ventricular contraction, prevents backflow of blood from the Left Ventricle into the Left Atrium
Aortic Semilunar Valve:
- Semilunar valve
- located at the base of the Aorta
- 3 ‘Cusps’
- Near each cusp are sac-like structures called Aortic Sinuses, which prevent the cusps from sticking to the aortic wall as the valve opens
- during ventricular relaxation, prevents backflow of blood from the Aorta into the Left Ventricle
Describe the vascular supply to the heart.
The first 2 Aortic branches are called the Right and Left Coronary Arteries.
They supply blood to the heart tissues, with opening lying just beyond the Aortic Valve.
The Coronary Arteries deliver blood when the heart is relaxed and have less function while the ventricles are contracting as they are compressed by the myocardium.
Both Coronary Arteries enclose the heart in the Coronary Sulcus and provide the arterial supply of the Coronary Circulation.
The anterior Interventricular Artery/Left Anterior Descending supplies blood to the anterior walls of both ventricles and to the Interventricular Septum.
The Circumflex Artery supplies the posterior walls of the Left Ventricle and the Left Atrium.
Coronary Artery branches supply many capillaries in the myocardium. These arteries have smaller branches with connections called Anastomoses between vessels providing alternate blood pathways, known as collateral circulation.
The Coronary Veins join to form the enlarged Coronary Sinus, emptying into the Right Atrium.
The Coronary Sinus empties into the Great Cardiac Vein, Middle Cardiac Vein and Small Cardiac Vein.
Several Anterior Cardiac Veins empty into the Right Atriums anterior portion.
The Posterior Cardiac Vein also empties into the Great Cardiac Vein or the Coronary Sinus.
Identify the electrical events associated with a normal electrocardiogram.
Electrocardiogram(ECG) is used to record electrical changes in the myocardium during the cardiac cycle using a machine known as an electrocardiograph.
Because body fluids conduct electrical currents, changes in the cardiac cycle can be detected on the body’s surface.
A normal electrocardiographic pattern includes several waves or deflections during each cardiac cycle.
In between, muscle fibres remain polarised and the ECG will will indicate the Baseline reading.
- As the SA node triggers an impulse, atrial fibers depolarise, leading to atrial contraction and producing a P wave(lasts about 0.08secs)
- As the impulse reaches ventricular fibres, they quickly depolarise, showing a greater electrical change due to thicker ventricular walls. When the change ends, the result is called the QRS complex, consisting of the Q wave, R wave and S wave that correspond to the depolarisation of ventricular fibres before ventricular contraction. The QRS complex has an intricate shape because there is a constant change in the paths of depolarisation waves through the ventricular walls(also lasts about 0.08secs)
- As the ventricles repolarise a T wave is produced. Because repolarisation is slower than depolarisation, the T wave appears more spread out, with a lower amplitude height than the QRS complex. The T wave can be obscured by the larger QRS wave being recorded simultaneously(lasts about 0.16secs)
- Atrial repolarisation is missing from the pattern because atrial fibers repolarise at the same time as the ventrical fibers are depolarising.
- Another, U wave may also be present as an ECG picks up the repolarisation of the Purkinje fibres. However this is often hidden by T or R waves.
Draw a diagram of a normal electrocardiogram.
Define Cardiac Output, and describe the factors that influence this variable.
Cardiac Output is defined as the volume of blood discharged from the ventricle per minute.
It is calculated by multiplying the stroke volume by the heart rate, in beats per minute.
CO=HR x SV
Cardiac Output=Heart Rate x Stroke Volume
The factors that influence cardiac output include increases or decreases in stroke volume or heart rate.
Stroke Volume is affected by:
- Preload
- Contractility
- Afterload
Heart rate is affected by:
- Autonomic Regulation
- Chemical Regulation
- Age
- Body Temperature
- Exercise
- Gender
Describe Preload.
Preload is the degree to which the heart muscle can stretch just before contraction.
Preload controls stroke volume, and in normal conditions, the higher the preload, the higher the stroke volume.
The relationship between preload and stroke volume is called the Frank-Starling Law of the Heart.
Resting cardiac cells are shorter than their optimal length, so stretching can cause significant increases in contractile force.
Most important factor in preload is Venous Return-the amount of blood returning to the heart and distending its ventricles.
Atrial Reflex/Bainbridge Reflex concerns adjustments to heart rate as a response to increases in venous return.
Describe Contractility.
Contractility is the contractile strength acheived at a certain muscle length.
Extrinsic factors increasing contractility of the heart muscle can also enhance stroke volume.
Greater contractility results in more blood being ejected from the heart, which increases stroke volume and lowers end-systolic volume.
Contractility is increased when sympathetic stimulation is increased.
Various substances can also affect contractility.
Positive Inotropic Agents will increase contractility:
- Adrenaline
- Glucagon
- Thyroxine
- Digitalis
- extracellular Calcium Ions
Negative Inotropic Agents decrease contractility:
- Excessive Hydrogen Ions
- increased extracellular Potassium levels
- Calcium channel blockers
Describe Afterload.
Afterload is the back pressure exerted by the arterial blood on the aortic and pulmonary valves(the ventricles must overcome this pressure to eject blood).
This pressure is approx. 80mmHg in the aorta, and approx 10mmHg in the pulmonary trunk.
In healthy people afterload is normal quite constant, but in hypertensive people, afterload reduces the ventricles ability to eject blood. As a result the heart retains more blood after systole, increasing end-systolic volume and decreasing stroke volume.
Describe Cardiac Reserve.
The difference between the resting and the maximum cardiac output.
Usually 4-5 x the resting cardiac output in a nonathletic person.
This resting output is 20-25 liltres per minute.
In a trained athlete, cardiac output may reach 7 x the resting cardiac output, which is 35 litres per minute.
Describe Autonomic Regulation of Heart rate.
The most important extrinsic controls on heart rate occur because of the autonomic nervous system.
Anxiety, exercise, or fright activate the sympathetic nervous system, and related nerve fibers release noradrenaline at their cardiac synapses.
This binds to Beta1-adrenergic receptors in the heart, and threshold can be reached faster.
Therefore, the S-A node fires more quickly, and the heart beats faster.
Sympathetic stimulation also speeds relaxation by enhancing contractility.
This is accomplished by enhancing calcium ion movements in contractile cells.
A ventricular contractile cell’s resting potential of -90mV is similar to that of skeletal muscle’s resting potential of -85mV
Once the membrane of a ventricular muscle reaches threshold, or about -75mV, an action potential begins.
The action potential then proceeds in 3 steps:
1 - rapid depolarisation - using fast sodium channels
2 - plateau - slow down calcium channels then as these channels begin to close
3 - repolarisation - slow potassium ion channels begin opening
When resting, both divisions of the autonomic nervous system repeatedly send impulses to the S-A node, with mostly inhibitory effects.
The heart is descibed as having vagal tone, with the heart rate usually slower than if it was not innervated by the vagal nerves.
If the vagal nerves were cut, there would be a fast increase in rate of approx. 25 beats per minute.
This shows the 100 beats per minute inherent rate of the S-A node.
When various types of sensory input activate either Autonomic division strongly, the other division is inhibited temporarily.
Most of this sensory input comes from baroreceptors, which respond to systemic blood pressure changes.
Describe Chemical regulation of heart rate.
Heart rate is influenced by many chemicals, particularly when present in very high or very low amounts.
Hormones and Ions are implicated in this.
Hormones like Adrenaline and Thyroxine.
During sympathetic activation, adrenaline is released by the adrenal medulla.
This hormone produces equivalent cardiac effects to that of noradrenaline, when it is released by the sympathetic nerves.
Heart rate and contractility are both enhanced.
Thyroxine from the thyroid gland increases production of body heat as well as metabolic rate.
In large amounts the heart rate increases and remains sustained.
Thyroxine acts directly on the heart and enhances both the effects of adrenaline and noradrenaline.
Intracellular and extracellular ions, in normal levels, also maintain normal heart function.
When plasma electrolytes are out of balance, the heart may be afftected to a severe degree.
Decribe other regulatory factors of heart rate.
Heart rate can be influenced by age, body temperature, exercise and gender.
The resting heart rate of infants is highest, at 140-160bpm, but gradually declines throughout life.
A womans average heart rate is higher than a mans, at 72-80bpm, mens is 64-72bpm.
Via the sympathetic nervous system, exercise raises the heart rate and increases systemic blood pressure.
In a physically fit adult, resting heart rate will be much lower than in those who are out of shape.
By enhancing the metabolic rate of cardiac cells heat increases heart rate.
For example, during a high fever, the heart can be felt beating more rapidly, because the muscles are generating large amounts of heat.
Oppositely, heart rate is directly decreased by cold temperatures.
