A&P II Unit 2: Cardiovascular System Study Flashcards

Anatomy and Structural Features of the Heart

• Epicardium (Visceral Pericardium): The outermost muscle layer of the heart wall. It contains thick deposits of adipose tissue in certain areas and serves as a layer of protection located directly beneath the pericardium.

• Myocardium: The thick middle muscular layer of the heart wall. Muscle fibers spiral continuously around the heart chambers, generating a wringing motion during contraction. It provides essential electrical insulation between the atria and ventricles, enabling coordinated and appropriate contractile timing.

• Endocardium: The innermost layer of the heart wall. It forms a smooth lining throughout the internal heart chambers and blood vessels, covers the surfaces of the heart valves, and is continuous with the vascular endothelium.

• Superior Vena Cava: A major systemic vein that returns deoxygenated blood from the head, neck, upper limbs, and thoracic regions back to the right atrium of the heart.

• Inferior Vena Cava: A major systemic vein that returns deoxygenated blood from the trunk, abdomen, and lower extremities back to the right atrium of the heart.

• Right Atrium: The upper right receiving chamber that collects deoxygenated blood from the superior vena cava, inferior vena cava, and coronary sinus, emptying its contents into the right ventricle during atrial contraction.

• Tricuspid Valve (Right Atrioventricular Valve): The valve situated between the right atrium and right ventricle. It opens when the right atrium fills to capacity to permit filling and closes tightly during ventricular contraction to prevent backflow into the atrium.

• Chordae Tendineae (Tendinous Cords): Fibrous collagenous cords connecting the cusps of the atrioventricular valves to the papillary muscles on the ventricular floor. They prevent valve cusps from prolapsing or inverting into the atria during powerful ventricular contractions.

• Papillary Muscles: Specialized muscular projections extending from the internal ventricular walls that serve as anchoring structures for the chordae tendineae, preventing valve inversion during ventricular systole.

• Right Ventricle: The lower right muscular chamber that pumps deoxygenated blood through the pulmonary semilunar valve into the pulmonary trunk and arteries toward the lungs.

• Septum (Interatrial and Interventricular Septa): Muscular and fibrous walls separating the left and right sides of the heart, preventing the mixing of oxygen-poor blood in the right heart with oxygen-rich blood in the left heart.

• Pulmonary Semilunar Valve: A three-cusp valve positioned between the right ventricle and the pulmonary trunk. It opens during ventricular contraction to allow deoxygenated blood flow to the lungs and closes during ventricular relaxation to prevent backflow into the right ventricle.

• Pulmonary Trunk: The arterial trunk arising directly from the right ventricle that bifurcates into the right and left pulmonary arteries.

• Pulmonary Arteries (Right and Left): Blood vessels that transport deoxygenated, carbon dioxide-rich blood away from the heart to the lungs for gas exchange.

• Pulmonary Veins: Four blood vessels (two from each lung) that transport newly oxygenated blood from the lungs into the left atrium.

• Left Atrium: The upper left receiving chamber that collects oxygen-rich blood returning from the pulmonary veins.

• Bicuspid Valve (Mitral Valve / Left Atrioventricular Valve): The dual-cusp valve located between the left atrium and left ventricle. It opens as the atrium reaches capacity and snaps closed during ventricular contraction to prevent regurgitation into the left atrium.

• Left Ventricle: The thickest, highly muscular lower chamber that fills with oxygenated blood during diastole and forcefully ejects it through the aortic semilunar valve into the aorta during systole.

• Aortic Semilunar Valve: The three-cusp valve located between the left ventricle and the aorta. It opens during ventricular contraction to allow oxygenated blood into systemic circulation and closes during ventricular relaxation to prevent blood from backflowing into the left ventricle.

• Aorta: The primary systemic artery arising from the left ventricle that distributes oxygen-rich blood throughout the entire body.

• Apex: The pointed, inferior extremity of the heart oriented toward the left side of the chest.

• Coronary Arteries: Arterial branches originating at the base of the ascending aorta that deliver fresh oxygenated blood directly to the myocardium.

• Cardiac Veins: Venous channels that collect deoxygenated metabolic waste products and blood from the myocardium, draining into the coronary sinus.

• Coronary Sinus: A large venous channel on the posterior surface of the heart that collects drainage from all cardiac veins and empties directly into the right atrium.

• Auricles (Right and Left): Small, pouch-like muscular extensions on the anterior surface of each atrium that increase total atrial volume capacity.

Sequential Blood Circulation Through the Cardiovascular System

• Deoxygenated Inflow: Carbon dioxide-rich, oxygen-poor blood returns from systemic circulation and enters the right atrium via the superior vena cava, inferior vena cava, and coronary sinus.

• Atrioventricular Passage (Right): As the right atrium contracts, blood passes through the open tricuspid valve into the right ventricle.

• Pulmonary Ejection: During right ventricular contraction (systole), the tricuspid valve closes, and deoxygenated blood is pumped through the open pulmonary semilunar valve into the pulmonary trunk and through the right and left pulmonary arteries.

• Pulmonary Gas Exchange: Blood reaches the alveolar capillary beds of the lungs, where carbon dioxide ($\text{CO}_2$) is released and oxygen ($\text{O}_2$) is loaded into blood cells.

• Oxygenated Return: Freshly oxygen-rich blood exits the lungs via the four pulmonary veins and enters the left atrium.

• Atrioventricular Passage (Left): As the left atrium contracts, oxygen-rich blood flows through the open bicuspid (mitral) valve into the left ventricle.

• Systemic Ejection: During left ventricular contraction (systole), the bicuspid valve closes, and oxygenated blood is forcefully driven through the aortic semilunar valve into the ascending aorta and aortic arch.

• Systemic Distribution: The aorta and its branching arterial network distribute oxygenated blood throughout the upper and lower extremities, head, and abdominal organs.

• Regional Functional Division:

  • Right Side of Heart (Right Atrium and Right Ventricle): Ejects oxygen-poor, carbon dioxide-rich blood to the pulmonary circuit.

  • Left Side of Heart (Left Atrium and Left Ventricle): Ejects oxygen-rich, carbon dioxide-poor blood to the systemic circuit.

Congenital Heart Defects

• Atrial Septal Defect (ASD): An abnormal persistent opening in the interatrial septum between the left and right atria. While essential for fetal blood circulation in utero, this bypass should close immediately following delivery. Failure to close allows blood shunting between atria and may require surgical closure.

• Ventricular Septal Defect (VSD): An abnormal structural opening in the interventricular septum separating the left and right ventricles, leading to blood shunting between the high-pressure left side and lower-pressure right side.

• Patent Ductus Arteriosus (PDA): A congenital defect where the temporary fetal vascular channel connecting the aorta to the pulmonary artery fails to close after birth. This persistent connection forces oxygen-rich blood from the higher-pressure aorta back into the pulmonary artery, causing excessive pulmonary blood flow, heart strain, and elevated pulmonary arterial blood pressure.

• Tetralogy of Fallot: A complex cyanotic congenital heart condition characterized by four concurrent anatomical abnormalities:

  • Pulmonary Valve Stenosis: Pathological narrowing of the pulmonary valve outlet.

  • Ventricular Septal Defect (VSD): A large hole in the interventricular septum.

  • Overriding Aorta: An abnormally displaced aorta that shifts to the right, sitting directly above the VSD, which causes it to receive a mixture of oxygenated and deoxygenated blood from both ventricles simultaneously.

  • Right Ventricular Hypertrophy: Pathological muscular thickening of the right ventricular wall due to increased workload.

The Cardiac Conduction System

• Conduction Overview: The cardiac conduction system coordinates cardiac contraction through an internal pacemaker and nerve-like specialized pathways embedded within the myocardium, generating and conducting rhythmic action potentials.

• Sequential Pathway of Excitation:

  1. Sinoatrial (SA) Node: Composed of specialized, modified cardiomyocytes located in the upper wall of the right atrium near the opening of the superior vena cava. It acts as the primary intrinsic pacemaker, initiating each heart cycle and setting the resting heart rate. Excitation waves spread rapidly across both atrial myocardium layers.

  2. Atrioventricular (AV) Node: Situated near the lower portion of the interatrial septum adjacent to the right AV valve. Serves as the essential electrical gateway to the ventricles; the heart's fibrous skeleton acts as an electrical insulator, preventing signals from reaching the ventricles via any alternative route.

  3. Atrioventricular (AV) Bundle (Bundle of His): Originates at the AV node and forks into right and left bundle branches, which travel down through the interventricular septum toward the cardiac apex.

  4. Subendothelial Conducting Network (Purkinje Fibers): Large, nerve-like conductive processes that turn superiorly at the apex and spread extensively throughout the ventricular myocardium. Signals are transferred from Purkinje fibers to surrounding cardiomyocytes, which pass electrical impulses cell-to-cell via intercalated disc gap junctions.

Electrocardiography (ECG / EKG)

• Definition: An electrocardiogram (ECG or EKG) is a composite recording of all surface action potentials produced by nodal and myocardial cells during each cardiac cycle, picked up and amplified by skin electrodes.

• ECG Waveforms, Intervals, and Segments:

  • P Wave: Represents SA node firing and the resulting depolarization of the left and right atria. Atrial contraction (systole) begins roughly $100\,\text{ms}$ after the onset of the P wave.

  • PR Interval: The time elapsed from the beginning of the P wave to the start of the QRS complex. Represents signal conduction through the AV node, AV bundle, and bundle branches before ventricular depolarization.

  • QRS Complex: A sharp spike representing ventricular depolarization. Its multi-peaked shape (Q, R, and S waves) reflects the differing wall thicknesses and geometry of the two ventricles. Atrial repolarization occurs simultaneously during this complex but is completely masked by the high voltage of ventricular depolarization.

  • ST Segment: The elevated or flat plateau line between the QRS complex and the T wave. Corresponds to the prolonged plateau phase of ventricular action potentials during complete ventricular systole.

  • QT Interval: Represents the total duration of ventricular depolarization and repolarization. It shortens substantially during physical exercise as heart rate increases.

  • T Wave: Represents ventricular repolarization immediately preceding ventricular relaxation (diastole).

The Cardiac Cycle

• Phases of the Cardiac Cycle:

  1. Ventricular Filling (Diastole):

     • Ventricles expand, dropping internal ventricular pressure below atrial pressure.

     • Atrioventricular (AV) valves open, allowing blood to flow rapidly into the ventricles.

     • Subdivided into three sequential phases:

       • Rapid Ventricular Filling: The initial one-third of diastole, where blood surges in rapidly.

       • Diastasis: The middle one-third of diastole, characterized by slower passive filling. The P wave occurs at the end of this period.

       • Atrial Systole: The final one-third of diastole; atria contract to add the final volume of blood.

     • Achieves an End-Diastolic Volume (EDV) of approximately $130\,\text{mL}$ of blood in each ventricle.

  2. Isovolumetric Contraction (Systole):

     • Atria repolarize and relax, staying in diastole for the rest of the cycle.

     • Ventricles depolarize (generating the QRS complex) and begin contracting.

     • Ventricular blood surges upward, snapping the AV valves shut.

     • The First Heart Sound ($S_1$) occurs at the beginning of this phase due to AV valve closure.

     • Termed "isovolumetric" because although muscle fibers contract, no blood is ejected yet—pressures in the aorta and pulmonary trunk still exceed ventricular pressures, keeping all four heart valves firmly closed.

  3. Ventricular Ejection (Systole):

     • Begins when ventricular pressures exceed arterial pressures, forcing the semilunar valves open.

     • Peak pressure reaches approximately $120\,\text{mmHg}$ in the left ventricle and $25\,\text{mmHg}$ in the right ventricle.

     • Consists of Rapid Ejection (initial high-velocity outflow) followed by Reduced Ejection (slower outflow under decreasing pressure).

     • Ejection lasts about $200\text{--}250\,\text{ms}$, corresponding to the plateau phase of the cardiac action potential. The T wave occurs late in this phase.

     • Stroke Volume (SV): The volume of blood ejected, averaging about $70\,\text{mL}$.

     • Ejection Fraction: The percentage of EDV ejected, typically around $54\%$ ($70\,\text{mL}$ out of $130\,\text{mL}$).

     • End-Systolic Volume (ESV): The remaining blood left in a ventricle post-contraction, calculated as $\text{ESV} = \text{EDV} - \text{SV} = 60\,\text{mL}$.

  4. Isovolumetric Relaxation (Diastole):

     • The T wave ends and ventricles begin expanding.

     • Blood in the aorta and pulmonary trunk briefly surges backward, catching the semilunar valve cusps and snapping them shut.

     • This backflow rebound creates a noticeable dip in arterial pressure graphs known as the Dicrotic Notch.

     • The Second Heart Sound ($S_2$) occurs during semilunar valve closure.

     • Termed "isovolumetric" because semilunar valves are closed and AV valves have not yet opened, resulting in no volume changes.

     • When ventricular pressure drops below atrial pressure, AV valves open and filling restarts.

• Cycle Timing (Resting Heart at $75\,\text{bpm}$):

  • Atrial Systole: Lasts about $0.1\,\text{second}$.

  • Ventricular Systole: Lasts about $0.3\,\text{second}$.

  • Quiescent Period: Duration when all four heart chambers are simultaneously in diastole, lasting about $0.4\,\text{second}$.

  • Total Duration: The entire cardiac cycle completes in $0.8\,\text{seconds}$.

Heart Rate and Coronary Pathologies

• Heart Rate Abnormalities:

  • Bradycardia: A resting adult heart rate below $60\,\text{bpm}$. Common during deep sleep, hypothermia, or in highly trained endurance athletes.

  • Tachycardia: A resting adult heart rate exceeding $100\,\text{bpm}$. Caused by emotional stress, anxiety, fever, stimulant drugs, myocardial damage, heart disease, or excessive blood loss.

• Angina Pectoris: Chest pain resulting from partial obstruction of coronary arterial blood flow. Obstruction causes localized tissue hypoxia (ischemia); forced to undergo anaerobic fermentation, the myocardium produces lactic acid, which stimulates localized pain receptors.

• Coronary Thrombosis: Complete blockage of one or more coronary arteries by a blood clot, most frequently triggered by underlying atherosclerosis (cholesterol and lipid accumulation in vessel walls).

  • Risk Factors: High LDL cholesterol, cigarette smoking, sedentary lifestyle, and chronic hypertension.

  • Prevention: Regular physical exercise, high-fiber diet, weight management, moderate alcohol intake, blood glucose control, and stress reduction.

• Myocardial Infarction (MI): Sudden necrosis (death) of a region of myocardium resulting from sustained obstruction of coronary circulation by an atheroma or blood clot.

  • Pathophysiology: Tissue downstream of the occlusion dies. Symptoms include intense chest pressure or crushing pain radiating down the left shoulder and arm.

  • Variations: Painless or "silent" heart attacks occur frequently in elderly individuals and patients with diabetes due to autonomic neuropathy. Electrical pathway disruptions can cause ventricular fibrillation and cardiac arrest.

  • Risk Factors: Diabetes mellitus, hypertension, coronary artery disease, elevated LDL cholesterol, and smoking.

  • Prevention: Balanced nutrition, regular exercise, maintaining low LDL and blood pressure levels, and managing systemic chronic diseases.

• Congestive Heart Failure (CHF): A progressive condition where the heart muscle fails to pump blood efficiently enough to satisfy systemic organ demands. Results from pressure/volume overloads, structural valvular damage, or loss of functional myocardium.

  • Risk Factors: Coronary artery disease, previous myocardial infarction, valvular heart disease, chronic hypertension, arrhythmias, congenital defects, and diabetes.

  • Prevention: Dietary control, physical activity, weight maintenance, early treatment of hypertension/heart disease, and smoking cessation.

Hemodynamic Parameters and Calculations

• Blood Pressure (BP): The force exerted by circulating blood against the walls of arterial vessels.

  • Normal Value: Values lower than $120/80\,\text{mmHg}$ are considered healthy.

  • Abnormal Value: Readings persistently above $120/80\,\text{mmHg}$ indicate hypertension, elevating cardiovascular risk.

• Systolic Blood Pressure: The peak pressure exerted against arterial walls during ventricular contraction (top number of a reading).

  • Normal: Below $120\,\text{mmHg}$.

  • Abnormal: Readings at or above $120\,\text{mmHg}$.

• Diastolic Blood Pressure: The baseline pressure exerted against arterial walls during ventricular relaxation between beats (bottom number of a reading).

  • Normal: Below $80\,\text{mmHg}$.

  • Abnormal: Readings at or above $80\,\text{mmHg}$.

• Blood Volume: The total volume of blood within an individual's circulatory system.

  • Normal Value: Approximately $5\,\text{L}$ in an average adult.

  • Abnormal Value: Volumes falling below $4\,\text{L}$ or exceeding $6\,\text{L}$.

• Stroke Volume (SV): The volume of blood ejected by the left ventricle with each contraction.

  • Normal Value: Approximately $70\,\text{mL}$ per beat.

  • Abnormal Value: Volumes below $50\,\text{mL}$ or above $100\,\text{mL}$ at rest.

• Heart Rate (HR): The frequency of cardiac contractions measured per minute.

  • Normal Range: $60\text{--}100\,\text{bpm}$.

  • Abnormal: Below $60\,\text{bpm}$ (bradycardia) or above $100\,\text{bpm}$ (tachycardia).

• Cardiac Output (CO): The total volume of blood ejected by each ventricle per minute, calculated as:     $\text{CO} = \text{HR} \times \text{SV}$

  • Normal Value: $4\text{--}6\,\text{L/min}$ at rest.

  • Abnormal Value: Values falling outside the $4\text{--}6\,\text{L/min}$ resting range.

• End-Diastolic Volume (EDV): The total blood volume contained inside a ventricle immediately before contraction begins (average $130\,\text{mL}$).

• End-Systolic Volume (ESV): The blood volume remaining inside a ventricle immediately after contraction ends (average $60\,\text{mL}$).

• Preload: The passive tension and myocardial stretch produced by the volume of blood in the ventricles immediately before contraction.

• Afterload: The total resistance or opposing arterial force the heart must overcome to eject blood during ventricular contraction.

• Contractility: The intrinsic force of myocardial muscle fiber shortening at a given preload.

Frank-Starling Law of the Heart

• Law Definition: The Frank–Starling law of the heart states that stroke volume is directly proportional to end-diastolic volume:     $\text{SV} \propto \text{EDV}$

• Physiological Mechanism: Ventricles automatically eject a volume of blood matching what they receive. As venous return increases, ventricular walls stretch further; this increased stretch aligns actin and myosin filaments into a more optimal length–tension relationship, causing cardiomyocytes to contract with greater force.

Anatomy and Layers of Blood Vessels

• Principal Categories of Blood Vessels:

  • Arteries: High-pressure vessels that carry blood away from the heart.

  • Arterioles: Microscopic, highly muscular branches of arteries that regulate flow into capillary networks.

  • Capillaries: Microscopic exchange vessels connecting arterioles to venules.

  • Venules: Small venous channels that collect blood from capillary networks.

  • Veins: Low-pressure conduits that return blood toward the heart.

  • Circulatory Sequence: Heart $\rightarrow$ Arteries $\rightarrow$ Arterioles $\rightarrow$ Capillaries $\rightarrow$ Venules $\rightarrow$ Veins $\rightarrow$ Heart.

• Layers of the Vessel Wall (Tunics):

  • Tunica Interna (Tunica Intima): The innermost layer facing the vessel lumen. Composed of:

    • Endothelium: Simple squamous epithelium acting as a selectively permeable barrier, producing vasoactive agents (vasodilators/vasoconstrictors) and repelling platelets to prevent unwanted blood clotting. Under inflammation, endothelial cells express cell-adhesion molecules to capture leukocytes.

    • Basement Membrane: Underlying extracellular matrix.

    • Internal Elastic Lamina: Layer of elastic tissue found in medium/distributing arteries.

  • Tunica Media: The middle tunic, consisting of smooth muscle, collagen, and elastic fibers. Strengthens walls to prevent pressure rupture; smooth muscle contraction (vasoconstriction) and relaxation (vasodilation) regulate vessel diameter.

  • Tunica Externa (Tunica Adventitia): Outermost layer of loose connective tissue anchoring vessels to adjacent organs. Contains nerves, lymphatic vessels, and Vasa Vasorum (tiny blood vessels supplying outer walls of larger blood vessels).

• Capillary Structure: Capillaries consist solely of endothelium and a thin basal lamina. They are absent or extremely scarce in dense connective tissues (tendons, ligaments), epithelia, and the cornea and lens of the eye.

Venous Return and Varicose Veins

• The Venous Pump (Skeletal Muscle Pump): Venous return against gravity relies on skeletal muscle contractions surrounding deep veins. Muscle contraction squeezes veins, driving blood upward toward the heart, while one-way venous valves prevent retrograde flow during relaxation.

• Pathophysiology of Varicose Veins: When venous valves become stretched, weakened, or damaged, they fail to close properly. Blood flows backward and pools in superficial veins, stretching the vessel walls and resulting in enlarged, tortuous varicose veins.

Structural Mapping of Systemic Arteries and Veins

• Key Arteries to Identify: Aorta, Aortic arch, Brachiocephalic trunk, Common carotid artery, External carotid artery, Internal carotid artery, Vertebral artery, Subclavian artery, Axillary artery, Brachial artery, Deep brachial artery, Radial artery, Ulnar artery, Radial collateral artery, Superior ulnar collateral artery, Interosseous arteries, Palmar arches, Superficial temporal artery, Facial artery, Internal thoracic artery, Subscapular artery, Celiac trunk, Common hepatic artery, Splenic artery, Renal arteries, Superior mesenteric artery, Gonadal artery, Inferior mesenteric artery, Common iliac artery, Internal iliac artery, External iliac artery, Femoral artery, Deep femoral artery, Popliteal artery, Anterior tibial artery, Posterior tibial artery, Fibular artery, Dorsal artery.

• Key Veins to Identify: Superior vena cava, Inferior vena cava, External jugular vein, Internal jugular vein, Brachiocephalic vein, Subclavian vein, Axillary vein, Cephalic vein, Basilic vein, Median cubital vein, Brachial veins, Median antebrachial vein, Radial veins, Ulnar veins, Venous palmar arches, Dorsal venous network, Hepatic vein, Renal vein, Gonadal veins, Common iliac vein, Internal iliac vein, External iliac vein, Femoral vein, Deep femoral vein, Popliteal vein, Great saphenous vein, Small saphenous vein, Anterior tibial veins, Posterior tibial veins, Fibular veins, Dorsal venous arch, Plantar venous arch, Cardiac veins.

Specialized Circulatory Pathways

• Pulmonary Circuit:

  • Function: Reoxygenates deoxygenated blood and expels excess carbon dioxide.

  • Sequence: Right ventricle $\rightarrow$ Pulmonary trunk $\rightarrow$ Right/Left pulmonary arteries $\rightarrow$ Lobar arteries (superior, middle, inferior) $\rightarrow$ Pulmonary arterioles $\rightarrow$ Alveolar capillaries (gas exchange at alveolar sacs) $\rightarrow$ Pulmonary venules $\rightarrow$ Pulmonary veins (right and left) $\rightarrow$ Left atrium.

• Hepatic Portal System:

  • Function: Directs nutrient-rich, deoxygenated blood from the gastrointestinal tract and spleen to the liver for metabolic processing and detoxification prior to systemic recirculation.

  • Tributaries: Gastric veins, gastro-omental veins, pancreatic veins, cystic vein, superior mesenteric vein, and inferior mesenteric vein. The superior mesenteric vein joins the splenic vein to form the Hepatic Portal Vein.

  • Sequence: Abdominal aorta $\rightarrow$ Digestive tract capillaries $\rightarrow$ Hepatic portal vein $\rightarrow$ Liver hepatic sinusoids $\rightarrow$ Hepatic veins $\rightarrow$ Inferior vena cava $\rightarrow$ Right atrium. (Oxygenated blood reaches liver tissue via: Abdominal aorta $\rightarrow$ Proper hepatic artery $\rightarrow$ Liver).

• Coronary Circulation:

  • Arterial Supply: Ascending aorta branches into Left and Right Coronary Arteries.

    • Left Coronary Artery: Branches into Circumflex artery and Anterior Interventricular artery.

    • Right Coronary Artery: Branches into Marginal artery and Posterior Interventricular artery.

  • Venous Drainage: Great cardiac vein, middle cardiac vein, and small cardiac vein drain deoxygenated blood into the Coronary Sinus, which empties into the Right Atrium.

• Collateral Circulation and Anastomoses:

  • Collateral Circulation: An underlying network of small bypass microvessels. When primary coronary arteries narrow, collateral vessels enlarge and activate to bypass obstructions and protect myocardial tissue.

  • Vascular Anastomosis: Point of convergence between two blood vessels that forms an alternative pathway for blood flow during vessel occlusion.

Etiology and Dynamics of Edema

• Definition: Pathological accumulation of excess interstitial fluid in tissues occurring when fluid filtration out of capillaries exceeds reabsorption back into capillaries.

• Three Primary Causes:

  1. Increased Capillary Filtration: Driven by conditions such as renal failure, histamine release, advanced age, or elevated venous pressure due to poor venous return.

  2. Reduced Capillary Reabsorption: Results from reduced colloid osmotic pressure, caused by hypoproteinemia, liver failure (impairing albumin synthesis), or severe dietary protein deficiency.

  3. Obstructed Lymphatic Drainage: Occurs when lymphatic vessels are blocked or lymph nodes are surgically removed, preventing normal return of interstitial fluid.

Cardiovascular Regulation and Hemodynamics

• Factors Governing Arterial Blood Pressure:

  • Cardiac Output (CO): Higher output increases arterial pressure; lower output decreases pressure.

  • Heart Rate (HR): Elevated heart rate increases total blood flow into arteries per unit time, elevating blood pressure.

  • Peripheral Resistance: The friction blood encounters against vessel walls. Increasing resistance elevates blood pressure.

  • Arterial Wall Elasticity: Rigid, non-compliant arterial walls resist expansion during systole, elevating blood pressure.

  • Vessel Radius: Narrowing of vessel lumens drastically increases peripheral resistance and raises blood pressure.

  • Vasoconstriction vs. Vasodilation: Vasoconstriction narrows lumens and increases pressure; vasodilation widens lumens and lowers pressure.

  • Blood Viscosity: Thicker, more viscous blood increases flow resistance and elevates pressure.

  • Blood Volume: Increased blood volume raises hydrostatic pressure on arterial walls.

  • Hormones:

    • Antidiuretic Hormone (ADH / Vasopressin): Promotes renal water conservation and induces vasoconstriction, elevating blood pressure.

    • Epinephrine: Triggers systemic vasoconstriction and enhances cardiac output, raising blood pressure.

Neural Control and Autonomous Reflexes

• Medullary Vasomotor Center: Located in the medulla oblongata, this center exerts sympathetic control over systemic blood vessels, stimulating general vasoconstriction while dilating coronary vessels during stress or exercise.

• Integrates Three Autonomic Reflexes:

  1. Baroreflex:

     • Automatic negative feedback response to rapid blood pressure changes, governing short-term posture adjustments (ineffective for chronic hypertension due to set point resetting within 2 days).

     • Mechanism: Increased blood pressure stretches carotid sinus baroreceptors, sending signals via the glossopharyngeal nerve to the brainstem. This inhibits sympathetic vasomotor neurons and excites vagal parasympathetic fibers, slowing heart rate and inducing vasodilation to lower blood pressure. Decreased blood pressure triggers inverse responses.

  2. Chemoreflex:

     • Response to changes in blood chemistry, specifically $\text{pH}$, $\text{O}_2$, and $\text{CO}_2$ concentrations.

     • Receptors: Aortic bodies (in aortic arch) and Carotid bodies (in external carotid arteries).

     • Function: Primarily adjusts respiratory rate. Secondarily causes widespread vasoconstriction in response to hypoxemia, hypercapnia, or acidosis to elevate blood pressure, enhancing lung perfusion and gas exchange.

  3. Medullary Ischemic Reflex:

     • Emergency response to brain ischemia (reduced perfusion).

     • Mechanism: The medulla monitors its own blood flow. If ischemic, cardiac and vasomotor centers send powerful sympathetic output to the heart and blood vessels, raising heart rate, contractility, and inducing intense systemic vasoconstriction to restore cerebral blood flow. Higher brain centers (stress, anger) can also act on the vasomotor center to elevate blood pressure.

Vascular Pathology and Circulatory Shock

• Aneurysm: A weak, bulging sac in an arterial or cardiac wall that pulsates with each heart beat and risks sudden rupture.

  • Dissecting Aneurysm: Blood splits arterial tunics, accumulating between layers due to degeneration of the tunica media.

  • Common Sites: Abdominal aorta, renal arteries, and arterial circle at the base of the brain.

  • Etiology: Congenital vascular wall weakness, trauma, bacterial infection, atherosclerosis, and hypertension.

• Hypotension: Chronic or acute abnormally low blood pressure, commonly resulting from severe dehydration, hemorrhage, or anemia.

• Arteriosclerosis: Age-related stiffening and reduced elasticity of arterial walls.

• Atherosclerosis: A form of arteriosclerosis characterized by lipid and cholesterol deposits forming arterial plaques.

• Circulatory Shock: Any state in which cardiac output fails to meet systemic cellular metabolic needs.

  • Cardiogenic Shock: Caused by inadequate pumping performance of the heart (e.g., severe myocardial infarction).

  • Low Venous Return (LVR) Shock: Results from insufficient blood return to the heart, occurring in three main forms:

    • Hypovolemic Shock: Most common form; caused by massive loss of blood volume (hemorrhage, severe burns, dehydration).

    • Obstructed Venous Return Shock: Venous compression by a tumor or aneurysm restricting blood flow back to the heart.

    • Venous Pooling (Vascular) Shock: Caused by long periods of standing/sitting or widespread vasodilation.

  • Neurogenic Shock: Sudden loss of vasomotor tone leading to severe vasodilation; caused by emotional shock or brainstem injury.

  • Septic Shock: Bacterial endotoxins trigger widespread vasodilation and elevated capillary permeability.

  • Anaphylactic Shock: Severe allergic reaction causing massive systemic histamine release, widespread vasodilation, and increased capillary permeability.

• Body's Response to Shock:

  • Compensated Shock: Homeostatic mechanisms restore baseline perfusion (e.g., fainting lowers the body horizontally, allowing gravity to restore cerebral blood flow).

  • Decompensated Shock: Occurs when compensation fails, triggering life-threatening positive feedback loops that worsen cardiac damage and lead to organ failure.