Overview of the Cardiovascular System The Heart Blood Vessels Arteries Arterioles Capillaries Venules Veins

Blood Diagrams: Germann and Stanfield, Principles of Human Physiology Dr. Áine Kelly [email protected] https://medicine.tcd.ie/physiology/student/

Overview of Cardiovascular System Functions Transport oxygen & carbon dioxide absorbed products of digestion metabolic wastes delivered (to liver and kidneys) hormones, immune cells, clotting proteins Regulation

Hormones Thermoregulation (skin blood flow)

Protection

Blood clotting (protects against haemorrhage) Pathogens (immune system)

Exchange between blood and tissue takes place in capillaries Blood gases: Pulmonary capillaries Blood entering lungs is deoxygenated Oxygen diffuses from tissue to blood (CO2 from blood to tissue) Blood leaving lungs is oxygenated Systemic capillaries Blood entering tissues is oxygenated Oxygen diffuses from blood to tissue (CO2 from tissue to blood) Blood leaving tissues is deoxygenated

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Blood On average, 5L of blood (approx 8% body weight) Cellular portion of blood (45% blood volume) Erythrocytes (red blood cells): oxygen transport Leukocytes (white blood cells): immune function Platelets: Blood clotting

Plasma (55% blood volume) Water Dissolved solutes eg. ions Plasma proteins Other components eg. metabolites, hormones, enzymes, antibodies.

Path of blood flow through cardiovascular system Cardiovascular system is a closed system Flow through systemic and pulmonary circuits are in series Left ventricle  systemic circuit  right atrium  right ventricle pulmonary circuit  left atrium  left ventricle Flow within systemic (and pulmonary) circuit is in parallel: allows independent regulation of blood flow to organs

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Anatomy of the Heart Size of fist; weighs approximately 250 – 350 grams

Location of the Heart Located in thoracic cavity Diaphragm separates abdominal cavity from thoracic cavity

Internal anatomy of the heart

Walls of ventricles thicker than walls of atria Left ventricle wall thicker than right ventricle wall Cardiac muscle: gap junctions

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Function of Cardiac Muscle Contraction and relaxation generates pumping action Contraction pushes blood into vasculature Relaxation allows heart to fill with blood

Heartbeat Wave of contraction through cardiac muscle Atria contract as a unit Ventricles contract as a unit Atrial contraction precedes ventricle contraction

Valves and Unidirectional Blood Flow Pressure within chambers of heart vary with heartbeat cycle Pressure difference drives blood flow: High pressure to low pressure Normal direction of flow: Atria to ventricles, then ventricles to arteries Valves prevent backward flow of blood All valves open passively based on pressure gradient Atrioventricular valves = AV valves R: tricuspid valve; L: bicuspid (mitral) Papillary muscles and chordae tendinae keep AV valves from everting

Semilunar valves Aortic Valve Pulmonary Valve

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Electrical Activity of the Heart Autorhythmic cells generate their own rhythm

Conduction System Pacemaker cells: Coordinate and provide rhythm to heartbeat The Sinoatrial (SA) node is the pacemaker of the heart Conduction fibers: Rapidly conduct action potentials initiated by pacemaker cells to myocardium Atrioventricular (AV) node Bundle of His Purkinje fibers

Spread of Excitation (NB gap junctions)

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The Electrocardiogram A non-invasive technique used to record the electrical activity of the heart Tests for clinical abnormalities in conduction of electrical activity in the heart Body is conductor: currents in body can spread to surface (ECG, EMG, EEG) Distance & amplitude of spread depends on size of potentials and synchronicity of potentials from other cells Heart electrical activity is synchronized

Standard ECG Trace

P wave: atrial depolarization QRS complex: ventricular depolarization T wave: ventricular repolarization

Abnormal Heart Rates “Sinus rhythm”: generated by SA node Abnormal Heart Rates: Tachycardia- fast Bradycardia- slow

Ventricular Fibrillation Loss of coordination of electrical activity. Can be corrected by defibrillation Atrial fibrillation - weakness Ventricular fibrillation - death within minutes Damage to heart muscle

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Cardiac Cycle Events associated with the flow of blood through the heart during a single complete heartbeat

Mechanical Events Systole - Ventricular contraction and blood ejection Diastole - Ventricular relaxation and filling

Opening of Valves Valves open passively due to pressure gradients AV valves open when P atria > P ventricles Semilunar valves open when P ventricles > P arteries

Cardiac Cycle

volume of blood ejected from heart each cycle

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Volume of blood pumped by each ventricle per minute Cardiac Output = CO = SV x HR Equal on both sides of the heart Average CO = 5 litres/min at rest (70ml/beat x 70beat/min) Can increase 5-fold during exercise

Regulation of Cardiac Output We regulate CO by regulating heart rate and stroke volume These can change from moment to moment Regulation by nerves and hormones

Neural and hormonal input to the Heart Neural: Nerves can increase or decrease (a) heart rate and (b) contractility of the myocardium (hence stroke volume)

Hormonal: Hormones such as adrenaline can increase (a) heart rate and (b) contractility of the myocardium (hence stroke volume)

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Blood Flow and Blood Pressure Physical laws governing blood flow: Pressure Gradients & Resistance in the Cardiovascular System Pressure gradients: Flow occurs from high pressure to low pressure Heart creates the pressure gradient for flow of blood A gradient must exist throughout circulatory system to maintain blood flow Resistance: systemic circuit is high pressure, high resistance; pulmonary circuit is low pressure, low resistance

Flow = ΔP/R = pressure gradient/resistance

Poiseuille’s Law R=

8ηL r4

Flow = ΔP/R =

ΔP r4 8ηL

Factors Affecting Resistance to Flow Length of vessel (normally doesn’t change) Viscosity of fluid = η (normally doesn’t change) Radius of vessel In arterioles (and small arteries) - can regulate radius

RADIUS IS THE MOST IMPORTANT FACTOR

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Blood Vessel anatomy

Arteries Rapid transport pathway: large diameter - little resistance Under high pressure: walls contain elastic and fibrous tissue

Arteries are pressure reservoirs Thick elastic arterial walls expand as blood enters arteries during systole & recoil during diastole

Arteries & disease Atherosclerosis - ‘hardening of the arteries’ A plaque composed of cholesterol, calcium and other substances builds up in an artery Plaques reduce blood flow; they can rupture & cause clots – heart attacks or strokes can result Risk factors: age; smoking; diabetes; obesity Treatments: Angioplasty; stent implantation

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Blood pressure: Mean Arterial Pressure MAP is the driving force for blood flow F = ΔP/R Regulating MAP is critical for normal function MAP < normal Hypotension Inadequate blood flow to tissues

MAP > normal Hypertension Stress on heart and walls of blood vessels

Arterioles Resistance vessels in microcirculation Connect arteries to capillaries Contain smooth muscle: regulate radius (& thus resistance; below) Arterioles provide greatest resistance to blood flow Largest pressure drop in vasculature (90 mmHg to 40 mmHg)

Radius dependent on contraction state of smooth muscle in arteriole wall Vasoconstriction: increased contraction (decreased radius) Vasodilation: decreased contraction (increased radius) Functional importance Controlling blood flow to individual capillary beds Regulating mean arterial pressure

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Control of Blood Flow Distribution to Organs

Cardiac output increases during exercise Distribution of blood does not increase proportionally Dilation to skeletal muscle and heart increases blood flow Constriction to GI tract and kidneys decreases blood flow Dilation to skin increases heat loss to environment (thermoregulatory response mediated by the brain in response to increased body temp during exercise)

Capillaries Site of exchange between blood and tissue 5-10 µm diameter - small diffusion distance Walls : 1 cell layer (small diffusion barrier) 10-40 billion in the body. Total surface area = 600 m2 Most cells within 1 mm of a capillary 1 mm long

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Venules Smaller than arterioles Connect capillaries to veins Thin walls Little smooth muscle in walls Some exchange of material between blood and interstitial fluid

Veins Large diameter, but thin walls, which contain muscle and elastic tissue Valves allow unidirectional blood flow Volume reservoirs: at rest, systemic veins contain 60% of total blood volume Return of blood to heart from veins is called venous return

Summary of cardiovascular physiology

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Respiratory Physiology Pulmonary ventilation (breathing) Gas exchange between lungs and blood

Transport of gases in blood

Gas exchange between blood and tissues

Anatomy of the Respiratory System Conducting airways (Nasal passages, pharynx, trachea, bronchii, bronchioles) Inspired air is warmed and humidified in these tubes. Moistening of air is essential to prevent drying out of alveolar linings. Photomicrograph of Tracheal Epithelium

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Defence mechanisms Respiratory system is largest area of the body in direct contact with the environment. Large particles filtered out in hairs in nasal passages Respiratory airways lined with mucus to trap foreign objects Cilia move mucus upwards towards throat to be swallowed Coughs and sneezes Alveolar macrophages scavenge within the alveoli

Function of the Alveoli Exchange of gases between air and blood by diffusion

300 million alveoli/lung (tennis court size) Rich blood supplycapillaries form sheet over alveoli Alveolar pores

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Resin cast of pulmonary blood vessels

Scanning electron micrograph of capillaries around alveoli

Pulmonary Circulation is low-pressure, low-resistance Ventilation-perfusion matching: blood flow through the pulmonary circulation is matched to ventilation

Structures of the Thoracic Cavity Chest wall – air tight, protects lungs Skeleton: rib cage;sternum; thoracic vertebrae Muscles: internal/external intercostals; diaphragm Lungs are surrounded by pleural sac

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Role of Pressure in Pulmonary Ventilation Air moves in and out of lungs by bulk flow Pressure gradient drives flow (air moves from high to low pressure) Atmospheric pressure = Patm (760mmHg at sea level) Intra-alveolar pressure = Palv Pressure of air in alveoli during inspiration is negative (< atmospheric) Pressure of air in alveoli during expiration is positive (> atmospheric) Difference between Palv and Patm drives ventilation

Mechanics of Breathing Movement of air in and out of lungs due to pressure gradients Mechanics of breathing describes mechanisms for creating pressure gradients Boyle’s Law (pressure and volume are inversely related) The lungs follow the movement of the rib cage

Respiratory Muscles

Expiratory muscles (internal intercostal & abdominal muscles) decrease volume of thoracic cavity Expiration is generally passive (recoil; no muscle contraction required)

Inspiratory muscles (diaphragm & external intercostals) increase volume of thoracic cavity

INSPIRATION

EXPIRATION

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Spirometry A pulmonary function test (method of measuring lung volumes) Can be used diagnostically Volume-time curves Flow volume loops Dependent upon patient effort

Used to measure several lung volumes, including tidal volume (VT) - the volume of a normal breath (approx. 500ml)

Minute Ventilation Total volume of air entering and leaving respiratory system each minute Minute ventilation = VT x RR Normal respiration rate = 12 breaths/min Normal VT = 500 mL Normal minute ventilation = 500 mL x 12 breaths/min = 6000 mL/min NB: Ventilation-perfusion matching: blood flow through the pulmonary circulation is matched to ventilation

Airway Resistance Like blood vessels, the resistance of the airways affects air flow Airway radius affects airway resistance Airway Resistance and disease Asthma – caused by contractions of smooth muscle of bronchioles Chronic obstructive pulmonary diseases – COPD

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Movement of Oxygen and Carbon Dioxide

Gas Composition of Air Composition of Air: 79% Nitrogen; 21% O2; trace amounts of CO2, helium, argon, etc.; water vapour (varies with humidity)

Diffusion of Gases Gases diffuse down pressure gradients High pressure  low pressure In gas mixtures, gases diffuse down partial pressure gradients High partial pressure  low partial pressure A particular gas diffuses down its own partial pressure gradient presence of other gases irrelevant

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Gas Transport in the Blood Oxygen

Carbon dioxide

Not very soluble in plasma (1.5%)

Some transported dissolved in plasma

Most (98.5%) transported by haemoglobin - a protein present in red blood cells

Some transported bound to haemoglobin

Each haemoglobin protein can bind 4 oxygen molecules

Most converted to bicarbonate ions by red blood cells, then transported into plasma

Hb + O2  Hb.O2 Haemoglobin has greater affinity for carbon monoxide (CO) than for oxygen: Prevents oxygen from binding to haemoglobin. CO is poisonous

Summary

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