Lecture Outline. The Heart. Blood Vessels. Blood. Overview of the Cardiovascular System. Overview of the Cardiovascular System

Transcription

1 Lecture Outline Overview of the Cardiovascular System Anatomy of the Heart and blood vessels Electrical Activity of the Heart The Cardiac Cycle, Cardiac Output and Its Control Blood flow and blood pressure Anatomy of the respiratory system Respiratory function - breathing, gas transport, gas diffusion Diagrams: Germann and Stanfield, Principles of Human Physiology Dr. Áine Kelly aikelly@tcd.ie ie Overview of the Cardiovascular System The Heart Blood Vessels Blood 1

2 Overview of Cardiovascular System Functions Transport of substances Respiratory: oxygen & carbon dioxide Nutritive: absorbed products of digestion Excretory: metabolic wastes delivered to liver and kidneys Regulation & protection: Hormones, immune cells, clotting proteins Regulation Hormones Thermoregulation (skin blood vessel) Protection Blood clotting (protects against haemorrhage) Pathogens (immune system) The Heart 2

3 Blood Vessels Heart Arteries Arterioles Capillaries Venules Veins Arteries relatively large, branching vessels that conduct blood away from the heart Arterioles small branching vessels with high resistance Capillaries site of exchange between blood and tissue Venules small converging vessels Veins relatively large converging vessels that conduct blood to the heart Closed system Blood On average, 5L of blood (approx 8% body weight) Arterial Blood Blood leaving the heart Bright Red High oxygen content (oxyhaemoglobin) Cellular portion of blood (45% blood volume) Erythrocytes (red blood cells): oxygen transport Leukocytes (white blood cells): immune function Platelets: Blood clotting Venous blood Blood returning to the heart Darker colour Lower oxygen content (deoxyhaemoglobin) Plasma (55% blood volume) Water Dissolved solutes eg. ions Plasma proteins Other components eg. metabolites, hormones, enzymes, antibodies. 3

4 Path of blood flow through cardiovascular system Cardiovascular system is a closed system Flow through systemic and pulmonary circuits are in series Left ventricle aorta systemic circuit vena cavae right atrium right ventricle pulmonary artery pulmonary circuit pulmonary veins left atrium left ventricle Flow within systemic (and pulmonary) circuit is in parallel Parallel flow allows independent regulation of blood flow to organs (see next slide) Parallel Flow in the Cardiovascular System 4

5 Oxygenation of Blood Exchange between blood and tissue takes place in capillaries Pulmonary capillaries Blood entering lungs is deoxygenated Oxygen diffuses from tissue to blood Blood leaving lungs is oxygenated Systemic capillaries Blood entering tissues is oxygenated Oxygen diffuses from blood to tissue Blood leaving tissues is deoxygenated Anatomy of the Heart Size of fist; weighs approximately grams Location of the Heart Located in thoracic cavity Diaphragm separates abdominal cavity from thoracic cavity 5

6 Cardiac Muscle (myocardium) Gap junctions - Contracts as a unit Desmosomes - Resist stress Atria & Ventricles are separate units separated by fibrous skeleton 99% contractile cells 1% autorhythmic cells Internal anatomy of the heart Walls of ventricles thicker than walls of atria Left ventricle wall thicker than right ventricle wall 6

7 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 Ventricles to arteries Valves prevent backward flow of blood All valves open passively based on pressure gradient 7

8 Heart Valves Atrioventricular valves = AV valves Right: tricuspid valve; Left: bicuspid (mitral) Papillary muscles and chordae tendinae keep AV valves from everting Semilunar valves Aortic Valve Pulmonary Valve Electrical Activity of the Heart Autorhythmic cells generate their own rhythm Conduction System Autorhythmic cells that provide pathway to spread excitation through the heart 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 8

9 Anatomy of the Conduction System Figure 14.9 Spread of Excitation Between Cells Atria contract first followed by ventricles (fibrous skeleton) Coordination due to presence of gap junctions and conduction pathways 9

10 Spread of Excitation The Electrocardiogram Records the Electrical Activity of the Heart (external measure) Non-invasive technique Used to test 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 10

11 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 11

12 Ventricular Fibrillation Loss of coordination of electrical activity Atrial fibrillation - weakness Ventricular fibrillation - death within minutes Damage to heart muscle Cardiac Cycle Events associated with the flow of blood through the heart during a single complete heartbeat Mechanical Events Systole - Ventricle contraction Diastole - Ventricle relaxation 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 12

13 Cardiac Cycle Heart Sounds occur due to turbulent flow when valves close First sound (lubb): AV valves close Second sound (dubb): Semilunar valves close Continuous Blood Flow During Cardiac Cycle Aorta (and large arteries) are elastic: Pressure reservoir Store energy during systole as walls expand Release energy during diastole as walls recoil inward Maintains blood flow through entire cardiac cycle 13

14 Ventricular Volume and Stroke Volume EDV (end diastolic volume): volume of blood in ventricle at end of diastole ESV (end systolic volume) volume of blood in ventricle at end of systole SV (stroke volume) volume of blood ejected from heart each cycle SV = EDV - ESV 130 ml 70 ml = 60 ml Cardiac Output Volume of blood pumped by each ventricle per minute Cardiac Output = CO = SV x HR Average CO = 5 litres/min at rest (70ml/beat x 70beat/min) Can increase 5-fold during exercise 14

15 Regulation of Cardiac Output Regulate heart rate and stroke volume Extrinsic and Intrinsic regulation Extrinsic - neural and hormonal Intrinsic - autoregulation Extrinsic: Neural and hormonal input to the Heart Neural: Nerves can increase or decrease (a) heart rate and (b) contracility 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) 15

16 Intrinsic Control - Frank-Starling s Law Increase venous return Increase strength of contraction Increase stroke volume Length-Tension Curve (Starling Curve) for Cardiac Muscle Increase EDV stretches muscle fibres Closer to optimum length Greater strength of contraction Increased SV Length = EDV Tension = SV 16

17 Blood Flow and Blood Pressure Physical Laws governing blood flow and blood pressure Pressure gradients in the Cardiovascular System Resistance in the Cardiovascular System Flow Rule Circulatory system = closed system Pressure = force exerted by blood Flow occurs from high pressure to low pressure Heart creates pressure gradient for bulk flow of blood A gradient must exist throughout circulatory system to maintain blood flow Flow = ΔP/R = pressure gradient/resistance 17

18 Pressure Gradient Across Systemic Circuit Pressure gradient = pressure in aorta minus pressure in vena cava just before it empties into right atrium P in aorta = mean arterial pressure (MAP) = 90 mmhg P in vena cava = central venous pressure (CVP) = 0 mmhg Pressure gradient = MAP CVP = 90 0 = 90 mmhg Pressure Gradient Across Pulmonary Circuit Pressure gradient = pressure in pulmonary arteries minus pressure in pulmonary veins Pulmonary arterial pressure = 15 mmhg Pulmonary venous pressure = 0 mmhg Pressure gradient = 15 0 = 15 mmhg 18

19 Pressures and Pressure drops in the Pulmonary and Systemic Circuits Resistance in the Cardiovascular System systemic circuit is high pressure, high resistance pulmonary circuit is low pressure, low resistance 19

20 Poiseuille s Law R = Length of vessel (normally doesn t change) Viscosity of fluid = η (normally doesn t change) Radius of vessel 8 η L Flow = ΔP/R = ΔP r 4 r 4 8 η L Factors Affecting Resistance to Flow In arterioles (and small arteries) - can regulate radius RADIUS IS THE MOST IMPORTANT FACTOR Regulate Blood Flow by Regulating Radius Regulation of radius of arterioles (and small arteries) Vasoconstriction Vasodilation decrease radius increase resistance Decrease blood flow increase radius decrease resistance increase blood flow 20

21 Overview of the Vasculature Arteries carry blood away from heart Microcirculation Arterioles Capillaries site of exchange Venules Veins return blood to heart Blood Vessel anatomy 21

22 Arteries Rapid transport pathway: large diameter - little resistance Under high pressure: walls contain elastic and fibrous tissue Arteries: a Pressure Reservoir Storage site for pressure Thick elastic arterial walls Expand as blood enters arteries during systole Recoil during diastole Arterial Blood Pressure Pressure in the aorta varies during cardiac cycle Systolic blood pressure = maximum pressure Diastolic blood pressure = minimum pressure Calculating Mean Arterial Pressure (MAP) Measured BP = SP/DP = 110/70 Pulse Pressure = SP DP = = 40 mmhg MAP = DP +(PP)/ 3 = /3 = 83.3 mmhg 22

23 MAP 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 Short and Long-Term Regulation of MAP Short-term regulation seconds to minutes Involves heart and blood vessels Primarily neuronal control Long-term regulation minutes to days Regulate blood volume Involves kidneys Primarily hormonal control 23

24 Arteries and 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 and cause clots - heart attacks or strokes can result Often occurs with age Smoking, diabetes and obesity are other risk factors Angioplasty or stent implantation can be used as treatments Arterioles Resistance vessels Part of microcirculation Connect arteries to capillaries Contain smooth muscle: regulate radius (& thus resistance) Arterioles provide greatest resistance to blood flow Largest pressure drop in vasculature (90 mmhg to 40 mmhg) 24

25 Changes in Arteriole Radius 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 Control of Blood Flow Distribution to Organs Regulation of blood flow to organs based on need (eg to skeletal muscles during exercise) Regulated by varying radius (and therefore resistance) Organ blood flow = MAP / organ resistance 25

26 Independent regulation of blood flow during exercise 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) billion in the body Total surface area = 600 m 2 Most cells within 1 mm of a capillary 1 mm long 26

27 Exchange Across Capillary Walls Venules Smaller than arterioles Connect capillaries to veins Little smooth muscle in walls Some exchange of material between blood and interstitial fluid 27

28 Veins Large diameter, but thin walls Valves allow unidirectional blood flow Function as blood reservoir 60% total blood volume in systemic veins at rest Skeletal Muscle Pump One-way valves in peripheral veins Skeletal muscle contracts Squeezes on veins increasing pressure Blood moves toward heart Blood cannot move backwards due to valves Skeletal muscle relaxes: Blood flows into veins between muscles 28

29 Summary of cardiovascular physiology Respiratory Physiology Pulmonary ventilation (breathing) Gas exchange between lungs and blood Transportation of gases in blood Gas exchange between blood and body tissues 29

30 Overview of Respiration Exchange of oxygen and carbon dioxide between atmosphere and body tissues Respiration:Transport in Blood 30

31 Respiration:Exchange between blood and tissues Cellular Respiration 31

32 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. 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 32

33 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 Resin Cast of Blood Vessels to the Lungs 33

34 Scanning Electron Micrograph of capillaries around alveoli 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 34

35 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 = P atm (760mmHg at sea level) Intra-alveolar pressure = P alv Pressure of air in alveoli During inspiration = negative (less than atmospheric) During expiration = positive (more than atmospheric) Difference between P alv and P atm 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 35

36 Muscles of Respiration Inspiratory muscles increase volume of thoracic cavity Diaphragm External intercostals Expiratory muscles decrease volume of thoracic cavity Internal intercostals Abdominal muscles Expiration is generally passive (no muscle contraction required) Respiratory Muscles 36

37 Inspiration and Expiration Figure 17.11b Expiration Expiration normally a passive process When inspiratory muscles stop contracting, recoil of lungs and chest wall to original positions decreases volume of thoracic cavity Active expiration requires expiratory muscles Contraction of expiratory muscles creates greater and faster decrease in volume of thoracic cavity 37

38 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 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 (V T ) - the volume of a normal breath (approx. 500ml) 38

39 Minute Ventilation Total volume of air entering and leaving respiratory system each minute Minute ventilation = V T x RR Normal respiration rate = 12 breaths/min Normal V T = 500 ml Normal minute ventilation = 500 ml x 12 breaths/min = 6000 ml/min The Pulmonary Circulation Low-pressure, low-resistance Ventilation-perfusion matching: blood flow through the pulmonary circulation is matched to ventilation 39

40 Movement of Oxygen and Carbon Dioxide Diffusion of gases across respiratory membranes 40

41 Gas Composition of Air Composition of Air: 79% Nitrogen; 21% Oxygen; trace amounts of carbon dioxide, 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 Partial Pressures of Oxygen and Carbon Dioxide 41

42 Oxygen Transport in the Blood Oxygen not very soluble in plasma Only 1.5% arterial blood oxygen dissolved in plasma Other 98.5% transported by haemoglobin - a protein present in red blood cells Each haemoglobin protein can bind 4 oxygen molecules Hb = deoxyhaemoglobin Hb + O 2 Hb. O 2 Hb. O 2 = oxyhaemoglobin Haemoglobin has greater affinity for carbon monoxide (CO) than for oxygen: Prevents oxygen from binding to hemoglobin. This is why CO is poisonous Carbon Dioxide Transport Mechanisms Some is transported dissolved in plasma Some is transporeted bound to haemoglobin Most is converted to bicarbonate ions by red blood cells, then transported into plasma 42

43 Summary 43