6.22 The Heart Flashcards
The Heart Structure
Superior Vena Cava
Inferior Vena Cava
Pulmonary artery
Pulmonary vein
Right Atrium
Right Ventricle
Left Atrium
Left Ventricle
Valves:
1. Tricuspid valve (right)
2. Bicuspid valve (left)
3. Pulmonary valve (right)
4. Aortic valve (left)
Mechanism of the Heart Beat
A heart beat is myogenic (contraction initiated by the heart)
* Electrical signals are initiated by a sinoatrial (SA) node
* This pacemaker stimulates the atria to contract and also
relays signals to an atrioventricular (AV) node
* The AV node sends signals to ventricular Purkinje fibres
(via a Bundle of His within the wall of the septum)
* The Purkinje fibres cause the ventricular walls to contract
The SA node maintains a normal sinus rhythm (60-100 bpm)
* The pacemaker is regulated by the medulla oblongata
* Sympathetic nerves release noradrenaline (⇧ heart rate)
* Parasympathetic nerves release acetylcholine (⇩ heart rate)
* Heart rate may also be increased via hormonal action
(via the release of adrenaline / epinephrine)
* Adrenaline will cause a more sustained elevation in heart
rate than that achieved by the action of the brainstem
The Cardiac Cycle
The cardiac cycle describes the events of a heart beat
Systole (contraction)
* As atria contract, atrial pressure exceeds ventricular
pressure (AV valves open → blood flows to ventricles)
* As ventricles contract, ventricular pressure exceeds
atrial pressure (AV valves close → 1st heart sound)
* Pressure builds (isovolumetric contraction) until the
ventricular pressure exceeds the arterial pressure
* Semilunar valves open and blood flows into arteries
Diastole (relaxation)
* As blood flows into arteries, ventricular pressure drops
* Backflow closes semilunar valves → 2nd heart sound
* When ventricular pressure drops below atrial pressure,
the AV valves will open and cardiac cycle is repeated
Coronary Heart Disease
Coronary thrombosis is caused by clots within the coronary arteries
* Vessels are damaged by cholesterol deposition (atherosclerosis)
* The deposits reduce vessel diameter and increase blood pressure
* The stress damages arterial walls (and is repaired with fibrous tissue)
* The vessel wall loses elasticity and forms atherosclerotic plaques
* If a plaque ruptures, blood clotting is triggered, forming a thrombus
* If the thrombus blocks blood flow, a myocardial infarction results
* These events are collectively described as coronary heart disease
Risk factors
Risk factors for CHD include:
* Genetics (e.g. hypertension)
* Obesity (overweight = risk)
* Diseases (e.g. diabetes)
* Diet (e.g. ⇧ trans fats)
* Exercise (inactivity = risk)
* Smoking (⇧ blood pressure)
* Sex (males = higher risk)
