🤾🏻‍♂️Human Physiology Engineering Unit 6 – Cardiovascular System

Cardiovascular System Basics

• Consists of the heart, blood vessels, and blood that work together to transport oxygen, nutrients, hormones, and waste products throughout the body
• Plays a crucial role in maintaining homeostasis by regulating blood flow, pressure, and distribution to various organs and tissues
• Cardiovascular system is a closed system, meaning blood remains within the blood vessels and does not directly mix with interstitial fluid
• Divided into two main circuits: pulmonary circulation (between heart and lungs) and systemic circulation (between heart and rest of the body)
• Heart acts as a double pump, simultaneously receiving deoxygenated blood from the body and pumping oxygenated blood to the body
• Blood vessels form a complex network of arteries, capillaries, and veins that facilitate blood flow and exchange of substances between blood and tissues
• Cardiovascular system is closely linked to other body systems, such as the respiratory system for gas exchange and the lymphatic system for fluid balance and immune function

Heart Structure and Function

• Heart is a four-chambered, muscular organ located in the thoracic cavity between the lungs
• Composed of specialized cardiac muscle tissue (myocardium) that contracts rhythmically to pump blood
• Four chambers: right atrium, right ventricle, left atrium, and left ventricle
  • Atria receive blood from the body (right) and lungs (left) and pump it into the ventricles
  • Ventricles pump blood to the lungs (right) and body (left)
• Heart valves ensure unidirectional blood flow and prevent backflow
  • Atrioventricular valves (tricuspid and mitral) between atria and ventricles
  • Semilunar valves (pulmonary and aortic) at the outlets of the ventricles
• Coronary arteries supply the heart muscle with oxygenated blood to meet its high metabolic demands
• Pericardium, a double-layered sac, surrounds and protects the heart, reducing friction during heart contractions

Blood Vessels and Circulation

• Arteries carry blood away from the heart and have thick, elastic walls to withstand high pressure
  • Aorta is the largest artery, originating from the left ventricle and branching into smaller arteries
  • Arterioles are small arteries that regulate blood flow to capillary beds
• Capillaries are the smallest blood vessels, forming a network for exchange of substances between blood and tissues
  • Capillary walls are thin and semipermeable, allowing for efficient diffusion of gases, nutrients, and waste products
• Veins carry blood back to the heart and have thinner walls and valves to prevent backflow
  • Venules collect blood from capillaries and merge into larger veins
  • Vena cava (superior and inferior) are the largest veins, returning blood to the right atrium
• Pulmonary circulation transports deoxygenated blood from the right ventricle to the lungs for oxygenation and returns oxygenated blood to the left atrium
• Systemic circulation carries oxygenated blood from the left ventricle to the body tissues and returns deoxygenated blood to the right atrium

Blood Composition and Properties

• Blood is a connective tissue composed of plasma (liquid portion) and formed elements (cells and cell fragments)
• Plasma makes up about 55% of blood volume and consists of water, proteins, electrolytes, nutrients, and waste products
  • Plasma proteins include albumin (maintains osmotic pressure), globulins (antibodies), and fibrinogen (clotting factor)
• Formed elements include red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes)
  • Red blood cells contain hemoglobin, an iron-containing protein that binds to oxygen for transport
  • White blood cells are part of the immune system and defend against infections and foreign substances
  • Platelets are cell fragments that play a crucial role in blood clotting and hemostasis
• Blood has several important properties, such as viscosity (thickness), osmolarity (solute concentration), and pH (acidity or alkalinity)
• Hematocrit is the percentage of blood volume occupied by red blood cells and is an indicator of blood's oxygen-carrying capacity

Cardiac Cycle and Electrical Activity

• Cardiac cycle refers to the sequence of events that occur during one complete heartbeat, consisting of systole (contraction) and diastole (relaxation)
• Atrial systole: atria contract, pumping blood into the ventricles
  • Followed by atrial diastole, where atria relax and fill with blood
• Ventricular systole: ventricles contract, pumping blood to the lungs (right) and body (left)
  • Followed by ventricular diastole, where ventricles relax and fill with blood from the atria
• Heart's electrical activity is initiated by the sinoatrial (SA) node, the natural pacemaker located in the right atrium
• Electrical impulses spread through the atria, causing contraction, and reach the atrioventricular (AV) node
  • AV node delays the impulse, allowing time for atrial contraction and ventricular filling
• Impulses then travel through the bundle of His, bundle branches, and Purkinje fibers, causing ventricular contraction
• Electrocardiogram (ECG or EKG) records the electrical activity of the heart, providing valuable diagnostic information
  • P wave represents atrial depolarization, QRS complex represents ventricular depolarization, and T wave represents ventricular repolarization

Hemodynamics and Blood Pressure

• Hemodynamics is the study of blood flow and the forces involved in circulation
• Blood flow is determined by the pressure gradient (difference in pressure between two points) and the resistance of the blood vessels
  • Poiseuille's law: $Flow = \frac{\Delta P \pi r^4}{8 \eta L}$, where $\Delta P$ is pressure gradient, $r$ is vessel radius, $\eta$ is blood viscosity, and $L$ is vessel length
• Blood pressure is the force exerted by blood against the walls of blood vessels
  • Systolic pressure: maximum pressure during ventricular contraction
  • Diastolic pressure: minimum pressure during ventricular relaxation
• Mean arterial pressure (MAP) is the average pressure throughout the cardiac cycle and is a key determinant of organ perfusion
  • $MAP \approx \frac{1}{3} (Systolic Pressure) + \frac{2}{3} (Diastolic Pressure)$
• Pulse pressure is the difference between systolic and diastolic pressures and reflects the elasticity of arteries
• Baroreceptors in the aortic arch and carotid sinuses detect changes in blood pressure and send signals to the cardiovascular center in the brainstem for regulation

Cardiovascular Regulation and Homeostasis

• Cardiovascular system is regulated by neural, hormonal, and local mechanisms to maintain homeostasis
• Autonomic nervous system plays a central role in cardiovascular regulation
  • Sympathetic nervous system increases heart rate, contractility, and vasoconstriction, leading to increased blood pressure
  • Parasympathetic nervous system decreases heart rate and contractility, leading to decreased blood pressure
• Hormones, such as epinephrine, norepinephrine, and angiotensin II, also influence cardiovascular function
  • Epinephrine and norepinephrine (catecholamines) increase heart rate, contractility, and vasoconstriction
  • Angiotensin II is part of the renin-angiotensin-aldosterone system (RAAS) and promotes vasoconstriction and sodium retention
• Local factors, such as nitric oxide and endothelin, regulate vascular tone and blood flow at the tissue level
  • Nitric oxide is a vasodilator released by endothelial cells in response to shear stress and other stimuli
  • Endothelin is a potent vasoconstrictor that helps maintain vascular tone
• Cardiovascular system adapts to changes in posture, exercise, and other physiological challenges to ensure adequate perfusion and oxygenation of tissues

Clinical Applications and Engineering Perspectives

• Cardiovascular diseases (CVDs) are a major cause of morbidity and mortality worldwide, including coronary artery disease, heart failure, and hypertension
• Diagnostic tools, such as ECG, echocardiography, and angiography, are used to assess cardiovascular function and detect abnormalities
  • ECG records the heart's electrical activity and can identify arrhythmias, ischemia, and other disorders
  • Echocardiography uses ultrasound to visualize the heart's structure and function, assessing valve function, chamber size, and contractility
  • Angiography involves injecting contrast media into blood vessels to visualize the vascular anatomy and detect blockages or abnormalities
• Therapeutic interventions for CVDs include medications, surgical procedures, and medical devices
  • Medications, such as beta-blockers, ACE inhibitors, and statins, are used to manage hypertension, heart failure, and hyperlipidemia
  • Surgical procedures, such as coronary artery bypass grafting (CABG) and valve replacement, are used to treat advanced CVDs
  • Medical devices, such as pacemakers, implantable cardioverter-defibrillators (ICDs), and ventricular assist devices (VADs), are used to support or replace heart function
• Tissue engineering and regenerative medicine approaches are being explored to develop novel therapies for CVDs
  • Stem cell therapy aims to regenerate damaged cardiac tissue and improve heart function
  • Engineered heart valves and blood vessels are being developed to replace diseased or damaged structures
• Computational modeling and simulation techniques are used to study cardiovascular physiology, hemodynamics, and device performance
  • Finite element analysis (FEA) is used to model the mechanical behavior of heart valves, blood vessels, and other cardiovascular structures
  • Computational fluid dynamics (CFD) is used to simulate blood flow and analyze the effects of vascular geometry, stenosis, and other factors on hemodynamics
• Wearable and implantable sensors are being developed to monitor cardiovascular function and detect abnormalities in real-time
  • Wearable ECG monitors and photoplethysmography (PPG) sensors can continuously track heart rate and rhythm
  • Implantable pressure sensors can monitor intracardiac pressures and provide early warning of heart failure exacerbations