Cardiology: The Cardiovascular System Notes PDF

**Cardiology: The Cardiovascular System** The cardiovascular system is fundamental to the body\'s ability to maintain homeostasis, providing tissues with nutrients and oxygen while removing waste products. This unit will cover the anatomy and physiology of the heart, blood vessels, and blood flow d...

**Cardiology: The Cardiovascular System** The cardiovascular system is fundamental to the body\'s ability to maintain homeostasis, providing tissues with nutrients and oxygen while removing waste products. This unit will cover the anatomy and physiology of the heart, blood vessels, and blood flow dynamics, along with the mechanisms of electrical activity, circulation, and blood pressure regulation. **1. Introduction to the Cardiovascular System** The heart is the central organ of the cardiovascular system, responsible for pumping blood throughout the body. It is located in the mediastinum, a central compartment in the thoracic cavity, flanked by the lungs. The heart is roughly the size of a clenched fist and weighs approximately 350 grams. It pumps between 3,700 to 7,500 litres of blood per day. **Myocardium:** - The heart muscle, or myocardium, makes up 99% of the heart\'s tissue, with the remaining 1% consisting of nerve tissue that regulates heart contractions. **Pericardium:** - The heart is enclosed in a double-layered sac called the pericardium, which consists of a fibrous outer layer and a serous inner layer. The serous pericardium is further divided into the visceral layer (epicardium) that covers the heart and the parietal layer that lines the fibrous sac. **2. The Cardiac Conduction System** The heart\'s rhythmic contractions are controlled by its intrinsic conduction system, which generates and propagates electrical impulses. **Sinoatrial (SA) Node:** - Known as the pacemaker of the heart, the SA node is located in the right atrium and sets the heart rate at approximately 100 beats per minute (bpm) in the absence of neural regulation. **Atrioventricular (AV) Node:** - The AV node is located at the junction between the atria and ventricles. It delays the electrical impulse to allow the atria to complete their contraction and fill the ventricles with blood before the ventricles contract. This delay slows the heart rate to around 60 bpm. **Bundle of His and Purkinje Fibres:** - The Bundle of His transmits the impulse from the AV node to the bundle branches in the ventricles, which then carry the impulse through the Purkinje fibres to the ventricular myocardium, triggering ventricular contraction. **Action Potential and Cardiac Cycle:** - The cardiac action potential involves the rapid depolarisation and repolarisation of the heart\'s muscle cells, which is essential for coordinated contraction and relaxation during the cardiac cycle. **3. The Electrocardiogram (ECG)** An ECG is a graphical representation of the heart\'s electrical activity as it travels through the conduction system. **Sinus Rhythm:** - Sinus rhythm is the normal rhythm of the heart, initiated by the SA node. It is characterised by the presence of P waves, QRS complexes, and T waves on an ECG. **P Wave:** - The P wave represents atrial depolarisation, leading to atrial contraction. This phase moves blood from the atria into the ventricles. **QRS Complex:** - The QRS complex reflects ventricular depolarisation and the initiation of ventricular contraction. It consists of three waves: the Q wave (septal depolarisation), R wave (depolarisation of the main ventricular walls), and S wave (depolarisation of the Purkinje fibres). **T Wave:** - The T wave represents ventricular repolarisation, preparing the ventricles for the next cycle of contraction. **ST Segment:** - The ST segment is the flat section between the end of the QRS complex and the beginning of the T wave. Changes in the ST segment can indicate myocardial ischaemia or necrosis. **4. Peripheral Circulation and Blood Vessels** The peripheral circulation involves the systemic distribution of blood through arteries, veins, and capillaries. **Capillary Exchange:** - **Diffusion:** The movement of substances like oxygen, carbon dioxide, and nutrients across the capillary membrane, down their concentration gradients. - **Bulk Flow:** The movement of fluids and solutes together from an area of higher pressure to lower pressure. This includes filtration (movement from blood to interstitial fluid) and reabsorption (movement from interstitial fluid back to blood). **Venous Circulation:** - **Muscle Pump and Valves:** Venous return is assisted by the contraction of skeletal muscles, which compress veins and push blood towards the heart. Valves in the veins prevent backflow. - **Respiratory Pump:** During inspiration, the pressure in the thoracic cavity decreases, creating a suction effect that draws blood towards the heart. **Systemic Vascular Resistance (SVR):** - SVR, or total peripheral resistance (TPR), is influenced by factors such as blood volume, vessel diameter, and blood viscosity. These factors affect blood pressure and flow throughout the systemic circulation. **5. Regulation of Blood Pressure** Blood pressure is regulated by several reflex mechanisms involving the cardiovascular centre in the brain. **Baroreceptor Reflex:** - Baroreceptors are stretch-sensitive receptors located in the carotid arteries, aortic arch, and right atrium. They detect changes in blood pressure and send signals to the cardiovascular centre to adjust heart rate and vessel diameter accordingly. **Chemoreceptor Reflex:** - Chemoreceptors located in the carotid and aortic bodies respond to changes in blood pH, carbon dioxide, and oxygen levels. They help regulate blood pressure by modulating heart rate and vessel tone. **Proprioceptor Reflex:** - Proprioceptors in muscles and joints provide feedback about body position and movement, which can influence cardiovascular responses during physical activity. **6. Clinical Considerations: Understanding Heart Sounds and Murmurs** **Heart Sounds:** - Heart sounds, such as S1 and S2, are produced by the closing of heart valves. S1 corresponds to the closure of the atrioventricular (mitral and tricuspid) valves, while S2 corresponds to the closure of the semilunar (aortic and pulmonary) valves. **Murmurs:** - Murmurs are abnormal heart sounds that may indicate turbulent blood flow through a stenotic or regurgitant valve. They are classified based on their timing within the cardiac cycle (systolic or diastolic) and their intensity. **Korotkoff Sounds:** - These sounds are heard during blood pressure measurement with a sphygmomanometer. They reflect the resumption of blood flow as the cuff pressure is gradually released. **Multiple Choice Questions (MCQs)** 1. **Which structure in the heart acts as the primary pacemaker?** - a\) AV node - b\) SA node - c\) Bundle of His - d\) Purkinje fibres 2. **What does the QRS complex on an ECG represent?** - a\) Atrial depolarisation - b\) Ventricular depolarisation - c\) Ventricular repolarisation - d\) Atrial repolarisation 3. **Which of the following factors does not directly influence systemic vascular resistance (SVR)?** - a\) Blood viscosity - b\) Heart rate - c\) Vessel diameter - d\) Vessel length 4. **What causes the first heart sound (S1)?** - a\) Closure of the semilunar valves - b\) Opening of the atrioventricular valves - c\) Closure of the atrioventricular valves - d\) Opening of the semilunar valves 5. **Which reflex is responsible for adjusting heart rate in response to changes in blood pressure?** - a\) Chemoreceptor reflex - b\) Proprioceptor reflex - c\) Baroreceptor reflex - d\) Vestibular reflex **Clinical Cases** **Case 1: Myocardial Infarction (Heart Attack)** **Presentation:**\ A 55-year-old man arrives at the emergency department with severe chest pain radiating to his left arm and jaw. He is diaphoretic and short of breath. An ECG shows ST-segment elevation in several leads. **Discussion:** - **Question:** Explain the pathophysiology behind myocardial infarction and the significance of the ST-segment elevation on the ECG. - **Answer:** Myocardial infarction occurs due to the blockage of a coronary artery, leading to the death of heart muscle tissue (myocardial necrosis) due to a lack of oxygen. ST-segment elevation indicates ongoing myocardial injury, which can lead to permanent damage if not treated promptly. **Case 2: Congestive Heart Failure** **Presentation:**\ A 70-year-old woman presents with shortness of breath, fatigue, and swelling in her legs. She has a history of hypertension and coronary artery disease. Physical examination reveals distended neck veins and crackles in the lungs. **Discussion:** - **Question:** Discuss the mechanisms that lead to congestive heart failure and how these symptoms are related to the condition. - **Answer:** Congestive heart failure occurs when the heart is unable to pump blood effectively, leading to a build-up of fluid in the lungs and peripheral tissues. This can result from myocardial damage, chronic high blood pressure, or valve dysfunction. The symptoms of shortness of breath and leg swelling are due to fluid accumulation, while distended neck veins indicate increased central venous pressure.