Brief Summary
This video provides a detailed explanation of the heart's electrical activity, blood flow, and factors influencing heart rate. It covers the roles of different heart structures, the cardiac cycle, EKG interpretation, and common heart conditions.
- The heart generates its own electrical signals, influenced by hormones and the autonomic nervous system.
- The cardiac cycle involves coordinated contraction and relaxation of the atria and ventricles.
- Factors like sympathetic activity, hormones, carbon dioxide levels, and body temperature affect heart rate.
Heart's Electrical Activity
The heart can generate electrical signals independently, though hormones and the autonomic nervous system can alter its pace. Sympathetic activity increases heart rate, while parasympathetic activity slows it down. The sinoatrial (SA) node initiates atrial contraction, and the atrioventricular (AV) node delays the signal to allow complete atrial contraction before ventricular contraction begins. Specialised muscle fibres, including the His bundle and Purkinje fibres, facilitate ventricular contraction. The AV node slows down the transmission between the SA node, the His bundle, and the Purkinje fibres, giving the upper chambers a chance to complete their contraction.
Cardiac Cycle
Atrial contraction lasts 0.15 seconds, ventricular contraction lasts 0.30 seconds, and the heart rests for 0.40 seconds, making the average cardiac cycle 0.85 seconds. Blood flows from the lungs via pulmonary veins, delivering oxygen-rich blood to the heart. When all heart chambers are relaxed, blood passively flows from the atria to the ventricles due to gravity. The atria contract to push any remaining blood into the ventricles. During this passive phase and atrial contraction, the atrioventricular (AV) valves (tricuspid and bicuspid) are open, allowing blood to fill the ventricles.
Valve Function During Contraction
During ventricular contraction, blood is pumped into the aorta and pulmonary artery through the semilunar valves, which must be open. The AV valves must be closed to prevent backflow, which can cause a heart murmur. The tricuspid valve is on the right side of the heart, and the bicuspid valve is on the left. Valves ensure unidirectional blood flow. If the ventricles are relaxed and the atria are contracting, the AV valves (tricuspid and bicuspid) are open.
Blood Flow Mechanics
When the ventricles contract, blood is directed into the aorta and pulmonary arteries via the open semilunar valves, while the AV valves remain closed. Most blood passively flows from the atria to the ventricles when all chambers are relaxed, with the AV valves open. The remaining blood is pushed into the ventricles during atrial contraction. Valves cannot be stimulated electrically; their function is purely mechanical. Deoxygenated blood enters the right atrium via the superior and inferior vena cava, while oxygenated blood returns to the left atrium via the pulmonary veins. Ventricular contraction then propels blood into the pulmonary artery and aorta.
EKG and Heart Rate Factors
An EKG (electrocardiogram) shows the heart's rhythm, with variations indicating tachycardia (fast heart rate) and bradycardia (slow heart rate). Factors affecting heart rate include sympathetic activity (increasing rate), parasympathetic activity (decreasing rate via the vagus nerve and acetylcholine), hormones like adrenaline and thyroxine (increasing rate), and carbon dioxide levels (increased CO2 increases rate). Increased body temperature and stimulants like caffeine also increase heart rate.
Blood Flow Recap and Key Terms
Pulmonary veins deliver oxygenated blood to the left atrium, while the superior and inferior vena cava deliver deoxygenated blood to the right atrium. Blood then flows to the ventricles and is pumped into the pulmonary artery (from the right ventricle) and aorta (from the left ventricle). Pulse is the rhythmic expansion of arteries due to heart contractions. Blood pressure is the force of blood against artery walls, with systolic pressure during ventricular contraction and diastolic pressure during ventricular relaxation, measured using a sphygmomanometer. Systole refers to contraction, and diastole refers to relaxation.
Heart Conditions and Cardiac Output
Arteriosclerosis is the hardening of arteries, thrombosis is the formation of blood clots, and embolism is the displacement of a clot. With age, blood clots can form in the heart, potentially leading to strokes or pulmonary embolisms. Defective heart valves cause murmurs due to backflow of blood. Myocardial infarction (heart attack) results from blocked coronary arteries, leading to tissue damage. Cardiac output is calculated as heart rate multiplied by stroke volume (the amount of blood pumped per beat). Normal blood pressure is around 120/80 mmHg, but individual variations exist.
