D.4 THE HEART Flashcards
The heart is composed of
cardiac muscle cells which have specialised features that relates to their function
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 muscle has a longer period of
contraction and refraction, which is needed to maintain a viable heart beat
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
The heart contains a number of hear valves
which prevent the backflow of blood
This ensures the one-way circulation of blood around the body
There are two sets of valves located within the heart
Atrioventricular
Semilunar
Semilunar
valves (pulmonary and aortic) prevent blood in the arteries from flowing back into the ventricles
Atrioventricular
valves (tricuspid and bicuspid) prevent blood in the ventricles from flowing back into the atria
Heart sounds are made when
the 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
lubb
The second heart sound is caused by
the closure of the semilunar valves at the start of ventricular diastole
dubb
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 of cardiac cycle is measured
using a machine called an electrocardiograph to generate data called an electrocardiogram
Each normal heart beat should follow the same sequence of electrical 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)
Tachycardia
elevated resting heart rate = >120 bpm
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
heart conditions
Fibrillations
Arrhythmias
bradycardia
Tachycardia
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
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 can be affected by a number of conditions
including exercise, age, disease, temperature and emotional state
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
the body will attempt to compensate for any changes to stroke volume with
a corrective alteration to heart rate
The typical pulse rate for a healthy adult is between
60 – 100 beats per minute
Heart rate describes
he speed at which the heart beats, measured by the number of contractions per minute (or bpm)
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
Changes in stroke volume will affect the blood pressure
more blood or more resistance will increase the overall pressure
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
Blood pressure readings will vary depending on
the site of measurement (e.g. arteries have much higher pressure than veins)
Blood pressure can be affected by
posture, blood vessel diameter (e.g. vasodilation) and fluid retention or loss
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 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
thrombosis
(fat deposits) develop in the arteries and significantly reduce the diameter of the vessel (leading to hypertension)
The high blood pressure damages
thrombosis
the arterial wall, forming lesions known as atherosclerotic plaques
If a plaque ruptures
thrombosis
blood clotting is triggered, forming a thrombus that restricts blood flow
If the thrombus becomes dislodged
thrombosis
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
There are several risk factors for coronary heart disease (CHD), including:
Age – Blood vessels become less flexible with advancing age
Genetics – Having hypertension predispose individuals to developing CHD
Obesity – Being overweight places an additional strain on the heart
Diseases – Certain diseases increase the risk of CHD (e.g. diabetes)
Diet – Diets rich in saturated fats, salts and alcohol increases the risk
Exercise – Sedentary lifestyles increase the risk of developing CHD
Sex – Males are at a greater risk due to lower oestrogen levels
Smoking – Nicotine causes vasoconstriction, raising blood pressure
Mnemonic: A Goddess
An artificial pacemaker is
is a medical device that delivers electrical impulses to the heart in order to regulate heart rate
Modern pacemakers are externally programmable, allowing cardiologists to make adjustments as required
Artificial pacemakers are typically used to treat one of two conditions:
Abnormally slow heart rates (bradycardia)
Arrhythmias arising from blockages within the heart’s electrical conduction system
Fibrillation is
he rapid, irregular and unsynchronised contraction of the heart muscle fibres
This causes heart muscle to convulse spasmodically rather than beat in concert, preventing the optimal flow of blood
Fibrillation is treated by
applying a controlled electrical current to the heart via a device called a defibrillator
This functions to depolarise the heart tissue in an effort to terminate unsynchronised contractions
Once heart tissue is depolarised, normal sinus rhythm should hopefully be re-established by the sinoatrial node
