Unit 1 Cardiovascular System

Structure of the Cardiovascular System

• The Heart as a Double Pump System: The cardiovascular system functions through a double-pump mechanism.     * Right Side: The chambers on the right side of the heart supply blood at low pressure to the lungs via the pulmonary arteries, arterioles, and capillaries. This is where gaseous exchange occurs. Blood returns to the left side via capillaries, venules, and veins.     * Left Side: When the left-side chambers are full, they contract simultaneously with the right side. The left side acts as a high-pressure pump, supplying oxygenated blood via the arteries, arterioles, and capillaries to body cells (such as muscle cells). Oxygen passes into the cells, while carbon dioxide and waste products of aerobic respiration are taken on board. Blood returns to the heart via the capillaries, venules, and veins.

• Anatomical Components of the Heart:     * Atria (Left and Right): These are the upper chambers of the heart. They receive blood from either the lungs or the body.         * Right Atrium: Receives deoxygenated blood from the body.         * Left Atrium: Receives oxygenated blood from the lungs.     * Ventricles (Left and Right): These are the pumping chambers of the heart.         * Right Ventricle: Facilitates pulmonary circulation to the lungs.         * Left Ventricle: Facilitates systemic circulation to the rest of the body.     * Septum: The muscular wall that separates the left and right sides of the heart.     * Valves: Essential for ensuring one-way blood flow and preventing backflow.         * Tricuspid Valve: Located between the right atrium and right ventricle.         * Bicuspid (Mitral) Valve: Located between the left atrium and left ventricle.         * Semi-Lunar Valves: Includes the Aortic Valve (between the left ventricle and aorta) and the Pulmonary Valve (between the right ventricle and the pulmonary artery).     * Major Blood Vessels:         * Aorta: The body's main artery; originates in the left ventricle and carries oxygenated blood to all parts of the body except the lungs.         * Superior Vena Cava: A vein receiving deoxygenated blood from the upper body (excluding lungs).         * Inferior Vena Cava: A vein receiving deoxygenated blood from the lower body to empty into the right atrium.         * Pulmonary Vein: Carries oxygenated blood from the lungs to the left atrium.         * Pulmonary Artery: Carries deoxygenated blood from the heart to the lungs. It is noted as the only artery that carries deoxygenated blood.     * Coronary Arteries: There are two (left and right). These vessels supply oxygenated blood directly to the heart muscle itself.

Blood Vessel Classification and Characteristics

• Arteries:     * Function: Carry blood away from the heart to body tissues.     * Location: Found deeper within the body.     * Structure: More muscular than veins with significantly more elastic fibres. They do not contain valves.     * Pressure: Contain blood under high pressure.

• Arterioles: Serve as the functional link between arteries and capillaries.

• Capillaries:     * Structure: Extremely thin walls (typically one cell thick).     * Function: Allow for gaseous exchange between blood and tissues.

• Venules: Serve as the functional link between capillaries and veins.

• Veins:     * Function: Carry blood from body tissues back to the heart.     * Location: Usually found just beneath the skin.     * Structure: Less muscular walls compared to arteries. They contain valves to prevent the backflow of blood.     * Pressure: Contain blood under low pressure.

Composition and Functions of Blood

• Blood Composition Components:     * Red Blood Cells (Erythrocytes):         * Main function is carrying oxygen to living tissues.         * Contain haemoglobin, a protein that gives blood its red colour and combines with oxygen to form oxyhaemoglobin.         * Feature a flattened disc shape for free movement through plasma and a large surface area for oxygen attachment.         * Comprise approximately $41\%$ of blood volume.     * Plasma:         * A straw-coloured liquid comprising approximately $90\%$ water.         * Provides suspension for blood cells to move through.         * Carries carbon dioxide dissolved as carbonic acid ($H_2CO_3$).         * Comprise approximately $55\%$ of blood volume.     * White Blood Cells (Leucocytes):         * Produced in bone marrow.         * Protect the body from infection by identifying and destroying pathogens (viruses/bacteria).         * Produce antibodies and antitoxins.     * Platelets (Thrombocytes):         * Disc-shaped fragments responsible for blood clotting to prevent loss (e.g., forming scabs).         * White blood cells and platelets together comprise approximately $4\%$ of blood volume.

• Primary Functions of the Cardiovascular System:     * Delivering Oxygen and Nutrients: Nutrient delivery includes blood glucose (carbohydrates), amino acids (proteins), fatty acids (fats), vitamins, and minerals. Oxygen is used for cellular respiration/energy production.     * Removing Waste Products: Includes $CO_2$ and lactate. Waste is taken to the kidneys and liver; $CO_2$ is returned to the lungs for exhalation.     * Thermoregulation: Maintaining stable body temperature.         * Vasodilation: Relaxation of muscle fibres and widening of vessels to release heat at the skin surface (decreases body temperature).         * Vasoconstriction: Decrease in vessel diameter; blood is moved away from the skin surface to maintain heat (increases body temperature).     * Fighting Infection: Mediated by Leucocytes.     * Clotting Blood: Platelets form a "plug" at damaged vessel sites; fibrin strands strengthen the clot.

Nervous Control of the Cardiac Cycle

• The Cardiac Cycle: Defined as the process of the heart filling with blood followed by contraction to pump blood out. This is regulated by the heart's electrical system.

• Electrical Impulse Pathway:     1. Sinoatrial Node (SAN): Located in the right atrium near the vena cava. Known as the "heart's pacemaker." It triggers atrial systole (atria contract, pushing blood into ventricles).     2. Atrioventricular Node (AVN): Located in the centre of the heart (septum near atria). It acts as a buffer to delay signals from the SAN, allowing atria to contract fully while ventricles remain relaxed to receive blood.     3. Bundle of His: Specialized heart muscle cells in the septum that transport impulses from the AVN to the ventricles.     4. Purkinje Fibres: Located at the end of the Bundle of His in the ventricle walls. They allow synchronized contraction of the ventricles at paced intervals.

• Cardiac Cycle Phases:     * Atrial Systole: Both atria contract; blood pushed into ventricles through AV valves.     * Iso-volumetric Contraction: Pressure pushes AV valves closed (the "Lub" sound); pressure forces semi-lunar valves open.     * Ventricular Ejection: Both ventricles contract; blood is ejected into the Aorta/Pulmonary artery.     * Iso-volumetric Relaxation: Semi-lunar valves close (the "Dub" sound); filling occurs passively as blood returns to the heart.

• Autonomic Nervous System (ANS) Control:     * Sympathetic Nervous System (Excitatory):         * Prepares the body for intense activity ("Fight or Flight").         * Secretes adrenaline and noradrenaline.         * Increases Heart Rate (HR), breathing rate, blood pressure, and contractile force of cardiac muscle.         * Benefit: Greater Cardiac Output ($Q = HR \times SV$) and faster oxygen supply.     * Parasympathetic Nervous System (Inhibitory):         * Relaxes the body ("Rest or Digest").         * Decreases HR, blood pressure, and Cardiac Output ($Q$).         * Benefit: Reduces time spent in stressful states to avoid chronic stress symptoms like arterial hardening or type $2$ Diabetes.

Responses and Adaptations to Exercise

• Short-Term Responses (Initial/Single Session):     * Anticipatory Rise: Slight increase in HR prior to exercise due to the nervous system preparing the body.     * Increased Heart Rate: Direct response to exercise demand mediated by the sympathetic nervous system.     * Increased Cardiac Output ($Q$): Higher volume of blood pumped from the left ventricle ($Q = HR \times SV$).     * Increased Blood Pressure: Systolic pressure increases to empty ventricles with more force.     * Redirection of Blood Flow (Vascular Shunt): Controlled by the medulla oblongata. Blood is shunted toward active muscles via vasodilation and away from non-essential organs (e.g., gut) via vasoconstriction.

• Long-Term Adaptations:     * Cardiac Hypertrophy: The heart (especially the left ventricle wall) gets bigger and thicker, leading to stronger contractions.     * Increased Stroke Volume ($SV$): The heart holds and ejects more blood per beat.     * Decreased Resting HR: Reduced workload on the heart due to increased efficiency.     * Reduction in Resting Blood Pressure: Lowers stress on the body and reduces the chance of hypertension.     * Capillarisation: Increase in the number of capillaries in skeletal muscle and alveoli, making gas exchange more effective.     * Increased Blood Volume: Occurs as a result of capillarisation to support oxygen delivery.

Additional Factors Affecting the Cardiovascular System

• Blood Pressure (BP):     * Systolic: Pressure on artery walls when the heart contracts.     * Diastolic: Pressure on vessel walls when the heart is relaxed.     * Values:         * Ideal: between $90/60\,mmHg$ and $120/80\,mmHg$.         * Hypertension (High): $140/90\,mmHg$ or higher.         * Hypotension (Low): $90/60\,mmHg$ or lower.     * Hypotension Effects: Slow blood movement can restrict oxygen to vital organs, causing dizziness, fainting, and tiredness.

• Temperature Extremes:     * Hyperthermia: Prolonged increase in body temperature. The body dilates vessels and sweats to dissipate heat. High-risk environments or incorrect clothing can trap heat.     * Hypothermia: Body core temperature drops below $35^{\circ}C$ (Normal is $37^{\circ}C$). Symptoms include shivering, confusion, and risk of the heart stopping.

• Sudden Arrhythmic Death Syndrome (SADS):     * A genetic heart condition causing sudden death in apparently healthy young people.     * Result of the heart's normal rhythm becoming disrupted, causing it to stop beating despite no structural disease.

• Blood Conditions:     * Anaemia: Low RBC or haemoglobin count, limiting oxygen delivery and increasing fatigue.     * Polycythaemia: Abnormal increase in RBCs; causes thickened blood and sluggish circulation, increasing cardiovascular strain.

Questions & Discussion

• Describing the Journey of Blood (8 Marks):     * Deoxygenated blood enters the Right Atrium via the Superior and Inferior Vena Cava.     * Moves through the Tricuspid Valve into the Right Ventricle.     * Exits through the Pulmonary Valve into the Pulmonary Artery to the lungs.     * Blood becomes oxygenated in the lungs.     * Oxygenated blood enters the Left Atrium via the Pulmonary Vein.     * Moves through the Bicuspid Valve into the Left Ventricle.     * Exits through the Aortic Valve into the Aorta to be distributed to the body.     * Blood becomes deoxygenated at the tissues and returns to the Vena Cava.

• Describing the Process of Blood Clotting (5 Marks):     * Platelets attach together at the wound site to form a plug (clot).     * Platelets release fibrin.     * Fibrin seals the wound to stop further bleeding.     * The fibrin network traps red blood cells, plasma, and white blood cells.     * This structure remains to repair the wound (forming a scab).

• Application to Sport (Boxing Example):     * RBCs: Needed for energy to last $12\,rounds$; oxygen reacts with haemoglobin.     * WBCs: Ensure the boxer stays healthy during training camps by fighting infection.     * Platelets: Help clot blood if the boxer receives a cut during a match or training.     * Plasma: Maintains blood viscosity so nutrients and oxygen flow to working muscles.