Cardiovascular System PDF

Cardiovascular System What is the cardiovascular system? Open circulation: Closed circulation: ▪ Blood or hemolymph bathes ▪ Blood contained within tissue vessels - (e.g. most in vertebrates) - (e.g. most vertebrates) ▪ Low flow, low pr...

Cardiovascular System What is the cardiovascular system? Open circulation: Closed circulation: ▪ Blood or hemolymph bathes ▪ Blood contained within tissue vessels - (e.g. most in vertebrates) - (e.g. most vertebrates) ▪ Low flow, low pressure pump ▪ High flow, high pressure associated with a mostly pump requires a mostly ‘spongy’ myocardium ‘compact’ myocardium - (e.g. fishes, amphibians, - (e.g. birds and most reptiles) mammals) Cardiovascular system is the heart , vessels and blood ▪ Heart pumps blood around the body ( systemic circulation, leaves oxygenated, returns deoxygenated) and through the lungs ( pulmonary circulation, leaves deoxygenated, returns oxygenated) ▪ Transport of blood occurs along vessels (enclosed tubes) of varying size and characteristic (arteries, arterioles, capillaries, venules , veins) ▪ Blood composed of erythrocytes (red blood cells), leukocytes (white blood cells) and the thrombocytes (platelets) contained within plasma ▪ Blood transports respiratory gases (O 2 and CO 2 ), nutrients (fuel for metabolism), waste materials (breakdown products of metabolism), electrolytes, hormones, drugs → to every single cell of the body! General function of cardiovascular system Cardiovascular system is responsible for 1. Transport of respiratory gases - O2 and CO2 2. Nutrients, waste, electrolytes, hormones - Metabolic fuel (e.g. carbohydrates and lipids), metabolic waste (e.g. urea), and electrolytes(Na+, K+, Mg++, Ca++, Cl-) 3. Maintenance of body fluid balance - Hydration status and electrolyte balance 4. Protection from infection and blood loss 5. Thermoregulation The heart Location of the heart: 1. Located in the middle of the thoracic cavity, between the two lungs 2. Housed within the mediastinum space, a division of the thoracic cavity containing the heart, large (major) blood vessels, portions of the esophagus and trachea, and other smaller structures… 3. Generally, in a standing animal, the heart is located between (and just above) the two elbows Left side of animal Right side of animal Size, shape and position of the heart 1. Heart has the shape of a prolate spheroid (elongate sphere) 2. Counterintuitively, the cranial end (at the top) is the base of the heart , the caudal end (at the bottom) is the apex of the heart 3. The heart does not sit straight along the median plane (see very last slide) of the animal, the base is shifted to the right and faces more dorsally, the apex is shifted to the left and sits more ventrally Coverings of the heart: 1. Heart is enclosed with the pericardium , which has three - Fibrous pericardium - ‘Parietal layer’ of the serous pericardium - ‘Visceral layer’ of the serous pericardium (i.e. 2. The pericardial sac is comprised of the fibrous pericardium and the ‘parietal layer’ of the serous pericardium , and fits loosely around the heart so that it can beat inside, but it is not elastic and will not stretch if the heart becomes abnormally large 3. The serous pericardium (both the parietal & visceral layers) are smooth membranes that slide past each other when the heart beats 4. Pericardial fluid between parietal & visceral layers acts as a lubricant ▪ Wall of the heart 1. Responsible for pumping action 2. Wall of the heart has three layers: - Myocardium middle and thickest layer, comprises cardiac muscle, cardiomyocytes (cardiac cells) are anatomically joined by intercalated disks allowing all cells to functionally ‘beat as one’ (syncytium), cardiomyocytes are auto rhythmic (spontaneously beats) and are resistant to fatigue (high capillary and mitochondrial investments) - Epicardium outer layer (i.e. visceral layer of the serous pericardium)  - Endocardium inner layer, thin and flat (squamous) epithelium lining chambers of the heart, continuous with lining of valves and vessels Chambers of the heart: 1. Four chambers two atria receive blood into heart and two ventricles pump blood out of heart 2. The two atria sit on top of the two ventricles 3. The left atrium and right atrium (LA and RA) - Thin walled chambers, separated by the inter atrial septum - Receives blood from large veins that carry blood to heart - When atria have filled with blood, they contract, push blood through one way valves into the ventricles - Atria are seen from the outside by their auricles (= ear flap), which are pouch like structures that form part of the atria 4. The left ventricle and right ventricle (LV and RV) - Thick walled chambers, separated by the inter ventricular septum - Inter ventricular septum is evident from the outside by he interventricular groove (large ‘coronary’ blood vessels and fat) - When ventricles have filled with blood (from atria), they contract, push blood through one way valves into the major arteries - LV pumps into aorta → systemic circulation (around body) RV pumps into pulmonary artery → pulmonary circulation (to lungs) 5. Thus, LV has thicker walls than the RV due to the different pressures required to pump against the different resistances! Valves of the heart: 1. Four one way valves control direction of blood flow through heart two of the valves are located between atria and ventricles (‘left and right atrioventricular valves’) and two of the valves are located between the ventricles and the major arteries they pump into (‘aortic and pulmonary semilunar valves’) 2. Left atrioventricular valve (bicuspid/mitral valve) has two cusps (flaps) and opens when pressure in LA exceeds that in LV 3. Right atrioventricular valve (tricuspid valve) has three cusps (flaps) and opens when pressure in RA exceeds that in RV 4. Papillary muscles control tension along chordae tendineae that attach to the atrioventricular cusps , preventing the atrioventricular valves from opening backward when they snap shut. 5. Aortic semilunar valve has three cusps (crescent moon shape), and opens when pressure in the LV exceeds that in aorta  6. Pulmonary semilunar valve also has three cusps, and opens when pressure in the RV exceeds that in pulmonary artery Skeleton of the heart: 1. Ringed structures located between the atria and ventricles 2. Composed of dense fibrous connective tissue 3. Provides four (4) functions - Separates the atria and ventricles - Provides anchorage for the heart valves - Provides point of attachment for myocardium (cardiac muscle) - Separates electrical insulation (delays the electrical impulse) between the atria and the ventricles Blood flow through chambers of heart Blood flow through the heart: 1. Blood flow is ‘one way’ (unidirectional) through chambers of the heart 2. The function of the heart is to: - Receive de-oxygenated blood from the systemic circulation (flows into RA) and pump it to the pulmonary circulation (flows from the RV) - Receive oxygenated blood back from the pulmonary circulation (flows into LA) and pump it to the systemic circulation (flows from the LV) 3. The heart and the systemic circulation and pulmonary circulation can be represented as a ‘figure 8’ (see next slide) 4. Let’s follow the path of the blood from an arbitrary starting point - De-oxygenated blood flows from the vena cava into the RA - Blood flows from RA, through right atrioventricular valve (tricuspid valve), and into RV (passively at first then ‘topped up’ by RA) - When RV is full, it begins to contract, forcing right atrioventricular valve ‘shut’ and pulmonary semilunar valve ‘open’ allowing blood to enter the pulmonary artery and head toward the lungs - Oxygenated blood returns from lungs, via the pulmonary veins, and empties into LA - Blood flows from LA, through the left atrioventricular valve (bicuspid/mitral valve), and into LV (passively at first then ‘topped up’ by LA)  - When LV is full, it begins to contract, forcing left atrioventricular valve ‘shut’ and aortic semilunar valve ‘open’ allowing blood to enter the aorta and head toward the body 5. The heart pumps blood in a rhythmic fashion despite the left and right sides pumping to different parts (left side → body, right side → lungs) - De-oxygenated blood enters the RA and oxygenated blood enters the LA at the same time - The RA and LA contract and relax at the same time - The right atrioventricular valve (tricuspid valve) and left atrioventricular valve (bicuspid/mitral valve) open and close at the same time - The RV and LV contract and relax at the same time - The pulmonary semilunar valve and aortic semilunar valve open and close at the same time 6. Heart valves ‘open’ and ‘shut’ in response to differences in pressure - When ventricles are full and begin to contract , blood pressure in then exceeds pressure in atria, forcing atrioventricular valves ‘shut’ - As ventricles continue to contract , pressure in them rises above the pressure in the major outflow arteries (aorta and pulmonary artery), forcing aortic and pulmonary semilunar valves ‘open’ - As ventricles begin to relax , pressure in them decreases below that of the major arteries, forcing aortic and pulmonary semilunar valves ‘shut’ - As ventricles continue to relax , pressure in them decreases below the pressure in the atria, forcing atrioventricular valves ‘open” 7. To keep blood flowing continuously through the chambers of the heart, the ventricles empty while the atria fill (and vice versa) 8. The period when the chambers are contracting is referred to as ‘systole’ (e.g. atrial or ventricular systole), and the period 9. when the chambers are relaxing is refereed to as ‘diastole’ (e.g. atrial or ventricular diastole) 10.The valves closing ‘shut’ produce sounds that can be heard with a stethoscope (i.e. the heartbeat) Normal heart sounds: 1. The valves closing ‘shut’ produce sounds heard with a stethoscope 2. Two distinct sounds ‘lub dub’ 3. First sound (‘ lub ’) is the atrioventricular valves snapping ‘shut’ after atrial systole (because pressure in the ventricles becomes greater than pressure in the atria) 4. Second sound (‘dub’) is the aortic & pulmonary semilunar valves snapping ‘shut’ after ventricular systole (because pressure in the ventricles decreases below that of pressure in the major outflow arteries) Abnormal heart sounds: 1. ‘Extra heart sounds’ the two atrioventricular valves or the two aortic pulmonary semilunar valves may not be closing simultaneously 2. ‘Valvular insufficiency’ valve/s are not closing completely , produces a ‘murmur’ (a ‘whooshing’ sound rather than ‘lub dub’) due to turbulence as blood flows in the wrong direction (e.g. mitral/bicuspid valvular insufficiency allows some blood flow from LV back to LA) 3. ‘Valvular stenosis’ valve/s are not opening completely , again produces a ‘murmur’ due to turbulence (e.g. mitral/bicuspid valvular stenosis prevents some of the blood flowing from LA to LV) Cardiac output & blood pressure Cardiac output, stroke volume, heart rate: 1. Cardiac output is the volume of blood pumped per unit time and is dependent on stroke volume and heart rate (see equation below) 2. Cardiac output of the LV (to systemic circulation) and of the RV (to pulmonary circulation) are equal! They have to be! 3. Cardiac output = stroke volume ×heart rate (mL min-1 ) = (mL) ×(min-1) 4. Stroke volume is the volume of blood pumped per beat of a ventricle (e.g. in units of ‘mL’) 5. Heart rate is the number of beats by a ventricle per unit time (e.g. in units of ‘ b.p.m ’ = ‘min-1) 6. Let’s try the following: - If stroke volume is 15 mL - And if heart rate 100 b.p.m. - Then, cardiac output is … 1500 mL min-1 7. Larger animals (e.g. horses and cattle) have larger stroke volumes but somewhat slower heart rates … smaller animals  (e.g. dogs and cats) have smaller stroke volumes but somewhat faster heart rates Preload and afterload: 1. Preload is the volume of blood the ventricle receives, measured just before ventricular contraction (a measure of how much the ventricle is ‘stretched’ prior to contraction) 2. Reduced preload can occur if the atria fail to contract properly (remember, most of ventricular filling occurs passively, but the atria provide an important ‘top up’) 3. Increased preload often occurs during exercise because there is an increased return of blood to the heart (ventricles) 4. Frank Starling law: preload ‘stretches’ the fibers, increasing the force of contraction, which in turn increases the stroke volume  5. Afterload is the resistance of the arterial tree into which the ventricle is ejecting 6. Reduced afterload can occur if there is decreased resistance to flow along the arterial tree 7. Increased afterload can occur if there is increased resistance to flow along the arterial tree (e.g. caused by vessel constriction or blockage) Blood Pressure 1. Blood pressure is the amount of force exerted by blood over an area 2. Traditionally, it has units of mmHg (‘millimeters of mercury’), the amount of pressure to raise a column of mercury a given height 3. These days, it is often expressed in units of kPa, which is 1000 N (of force) distributed over an area of one square meter (1 m2) 4. Blood pressure is also dependent on factors such as cardiac output (i.e. heart rate and stroke volume) and resistance to flow (e.g. diameter and elasticity of the arteries and arterioles) 5. Blood pressure varies depending on the location (e.g. atria, ventricles, arteries, capillaries and veins) and across the cardiac cycle  6. Systemic arterial blood pressure is the most common measure (often referred to simply as ‘blood pressure’) and it has three parts: - Systolic arterial pressure pressure produced by the ejection of blood into the systemic arteries during LV contraction (systole), e.g. 120 mmHg - Diastolic arterial pressure pressure remaining in the systemic arteries during LV relaxation (diastole), e.g. 80 mmHg - Mean arterial pressure (MAP) average blood pressure in the systemic arteries across the cardiac cycle, e.g. 100 mmHg 7. Blood pressure measured with a ‘sphygmomanometer’ (pressure cuff) Regulation of blood pressure MAP is regulated by adjusting CO and TPR 1. MAP drives blood flow, and thus is regulated within a narrow range (about 90 mmHg or thereabouts) 2. How? Well, if MAP increases or decreases, baroreceptors register this change, and either CO is adjusted (when large response needed --> vary heart rate and/or stroke volume) and/or TPR is adjusted (when subtle response needed --> vary 'tone ’ of the arterioles)  3. This quick adjustment in CO and/or TPR brings MAP back to normotensive levels before you know it! Unless, you jump out of a hot bath too quickly… 4. Baroreceptors are stretch sensitive mechanoreceptors located in the walls of the major blood vessels that respond to MAP 5. Most important are the aortic body baroreceptors that monitor MAP to the body, and the carotid artery baroreceptors that monitor MAP to the brain Aerobic energy for cardiac tissue Blood supply to the heart (coronary vessels): 1. Cardiac muscle requires a constant supply of blood, transported along vessels of the heart (i.e. coronary vessels), to supply nutrients (e.g. O2 and metabolic fuel) and remove wastes (e.g. CO2) 2. LV receives ca. ⅔ & RV receives ca. ⅓ of heart’s total blood supply 3. Two coronary arteries branch off the base of the aorta, heart is first organ supplied with blood, arteries divide and encircle the whole heart 4. Coronary capillaries service cardiomyocytes (cardiac cells) ensuring adequate supply of oxygen, even during maximum cardiac work  5. Coronary veins connect to form the coronary sinus , which drains directly into the RA Mitochondrial investment of the heart: 1. Cardiomyocytes (cardiac cells) require a constant supply of ATP so that they can perform work (i.e. cycle or contraction and relaxation) 2. Cardiomyocyte mitochondria use the O 2 (and fuel) supplied by the capillaries to generate ATP for work 3. Cardiac capillary investment (i.e. capillary density) appears well matched to cardiac mitochondrial investment (i.e. mitochondrial density) Nerve supply conduction pathways Nerve supply and pacemaker of the heart: 1. Cardiomyocytes (cardiac cells) are ‘autorhythmic’ they can create their own spontaneous contractions and relaxations 2. However, external motor input (autonomic control) is also present nerve fibers enter heart and terminate at the pacemaker (sinoatrial node in wall of RA) 3. Pacemaker is a specialized collection of cardiomyocytes (sinoatrial node in wall of RA), which sets the heart rate, by propagating its electrical impulse through the entire myocardium to all the cardiomyocytes 4. Pacemaker keeps the heart beating at a steady rate 5. Cardiomyocytes of the atria contract first, then cardiomyocytes of the ventricles contract second 6. Cardiomyocyte contraction is followed immediately by cardiomyocyte relaxation 7. The cardiac cycle is one complete cycle of atrial and ventricular contraction relaxation (producing one heart beat) Conduction system of the heart: 1. The key structures of the conduction system of the heart are sinoatrial node (SA node), atrioventricular node (AV node), bundle of His, and the Purkinje fiber system 2. Impulse generated by the pacemaker (sinoatrial node in wall of RA) passes from cell to cell across the atria (= contraction of atria), then moves from the atrioventricular node (in atrioventricular septum) where there is a short delay, through the bundle of His to the apex of heart, and then via  the Purkinje fiber system back to the base of heart spreading cell to cell across the ventricles (= contraction of ventricles) 3. 3. When impulse reaches atrioventricular node, there is a short delay, allowing atria to finish contraction before ventricles begin contraction 4. Impulse is transmitted rapidly along the bundle of His (in ventricular septum) to the apex of the heart, before Purkinje fiber system picks up the impulse, makes a U turn and delivers impulse through the myocardium of the ventricles where it is spread cell to cell 5. Because the impulse is delivered rapidly along the bundle of His, it causes ventricular contraction from the apex toward the base of the heart (squeezing blood into aorta and pulmonary artery) Depolarization (contraction) and repolarization (relaxation) of the cells: 1. In the relaxed state, cardiomyocyte is ‘polarized’ or ‘ re polarized’, with Na+ and Ca++ displaced outside the cell , and K+ ions displaced inside the cell , producing a resting membrane potential of around -90 mV (i.e. inside of cell more negative than outside of cell) 2. For contraction to occur, cardiomyocyte must be ‘ de polarized’, with Na+ and Ca++ rushing in to the cell through membrane channels (this reverses polarity of the membrane) 3. Then, K+ moves out of the cell (i.e. in the other direction) also through membrane channels (and this restores polarity of the membrane) 4. But now Na+, Ca++ & K+ are on the wrong side of the cell membrane 5. For relaxation to occur, Na ++, Ca ++ & K ions need to be pumped back, so that the cell is ‘ re polarized’ (and ready to be de polarized again) Blood vessels of the circulation Arteries, veins, capillaries: 1. Blood vessels of the systemic and pulmonary circulations make continuous loops (figure 8) providing passage for blood to & from the heart 2. Blood vessels are composed of up to three layers - Endothelium: inner layer of all vessels (also lines heart and valves), simple squamous epithelium, smooth surface reduces friction - Smooth muscle & elastic tissue layer (absent in capillaries): middle layer, smooth muscle provides control over vessel diameter (autonomic control), elastic fibers allow vessel to stretch and recoil - Connective tissue layer (absent in capillaries): outer layer, connective tissue is strong and flexible, collagen fibers anchor to provide stability 3. Blood vessels are broadly grouped into 5 categories - Arteries: carry blood away from heart and become progressively smaller, large elastic arteries closest to heart (e.g. aorta) have greater ability to stretch and recoil (more elastic fibers), smaller muscular arteries farther from heart have greater ability to modulate pressure and re direct blood flow (more smooth muscle fibers) - Arterioles: smallest branches of arterial tree, high resistance, in effect they are small muscular arteries that modulate pressure and re direct blood flow (work in partnership with precapillary sphincters) - Capillaries: microscopic blood vessels, one endothelial cell thick wall, occur in groups of ‘capillary beds’, all cells within close proximity to a capillary, site of exchange of gases, nutrients, wastes, electrolytes  - Venules : smallest veins, thin walls allow some fluid exchange between plasma and interstitial fluid - Veins: carry blood back to heart and become progressively larger, often major veins accompany corresponding major arteries (e.g. femoral vein runs alongside femoral artery), largest vein is closest to heart (i.e. vena cava), many veins work against gravity to get blood back to heart, valves ensure one way blood flow of blood Fetal circulation Blood flow in the fetus: 1. Fetal circulation directs fetal blood to the ‘placenta’, embedded in wall of ‘uterus’, for gas exchange with maternal blood viz. postnatal blood to lungs for gas exchange with air) 2. In the fetus, the lungs are filled with fluid and are non functional! 3. Fetal circulation has bypasses that direct blood away from lungs - Foramen ovale: shunt from RA to LA - Ductus arteriosus: shunt from pulmonary artery to aorta 4. Some mixing of oxygenated & de oxygenated blood occurs in fetus 5. The umbilical cord contains two umbilical arteries (partially de-oxygenated fetal blood) and one umbilical vein (oxygenated fetal blood) 6. At birth (parturition), the shunts that bypass the lungs close, the lungs fill with air, become functional for gas exchange, and the neonate takes its first breath! 7. At birth, closure of the shunts means that the heart transforms from pumping blood in ‘parallel’ (as a fetus) to pumping blood in ‘series’ (as a neonate to adult) Cardiovascular monitoring Auscultation: 1. Because the heart is not visible by direct observation, a number of direct and indirect tests have been developed to assess cardiac function 2. Auscultation of thorax, using a stethoscope, allows one to listen to the internal sounds of the heart 3. Possible to detect irregularities (e.g. extra heart sounds, valvular insufficiency, valvular stenosis) Pulse and pulse points: 1. The pulse rate is the frequency of stretching and recoiling of an artery as blood passes through it with each heart beat 2. It occurs because every time the LV contracts (systole) it ejects a bolus of blood into the aorta, when the LV relaxes (diastole) blood flow into the aorta stops 3. Produces a pulse wave of stretching recoiling through the arterial tree 4. Pulse is felt only on superficial arteries that lie close to the surface 5. Pulse is used to evaluate the strength and regularity of the heartbeat - Pulse is best felt in different arteries (locations) depending on the species - Pulse can only be taken over an artery (veins do not have a pulse) - Tips of index and middle fingers used to detect pulse (your thumb has a pulse!)  - Common pulse points: femoral artery (cats, dogs, sheep, goats) coccygeal artery (cattle, swine) facial artery (cattle) mandibular artery (horse) Electrocardiography (ECG): 1. Evaluate the electrical activity of the heart 2. Electrical impulse that originates at SA node and spreads through the cardiac conduction system can be detected on the body surface 3. ECG machine records the heart’s electrical activity via leads placed on the body surface 4. ECG output (electrocardiogram) of cardiac cycle has 3 components: - P wave: Time taken for depolarization of atria (contraction) Corresponds to mechanical activity of atrial contraction - QRS complex: Time taken for depolarization of ventricles (contraction) Corresponds to mechanical activity of ventricular contraction Q wave = Depolarization of interventricular septum R wave = Depolarization of main ventricular mass  S wave = Depolarization of ventricular mass near base of the heart - T wave: Time taken for repolarization of ventricles (relaxation) Corresponds to time taken for ventricles to refill with blood and get ready for next contraction P wave: Time taken for depolarization of atria (contraction) QRS complex: Time taken for depolarization of ventricles (contraction) T wave: Time taken for repolarization of ventricles (relaxation) Echocardiography (ultrasound): 1. Evaluate the size, shape, and movement of structures of the heart 2. Ultrasound waves are used to image the heart 3. Two dimensional echocardiography produces a simple cross sectional view of the heart (e.g. the ‘four chamber view’) and allows measurement of chamber size, wall thickness, valve function 4. Doppler echocardiography images blood flow and allows measurement of valvular insufficiency (valve/s not closing completely) and valvular stenosis (valve/s not opening completely) Venipuncture: 1. Superficial blood vessels that lie just beneath the skin can be used to collect blood samples, administer medications, place arterial or venous catheters 2. Jugular vein is the most common venipuncture site in nearly all species, runs along muscular grooves on the ventral aspect of each side of the neck 3. Other important venipuncture sites include the cephalic vein in the forelimb, and the femoral vein and saphenous vein in the hindlimb, of cats and dogs Introduction to ‘Scaling’ To understand the structure and function of the cardiovascular system we must first understand the effect of body size ––‘scaling’ or ‘ 1. Body size is the most important factor in all of biology! It affects: - Most physiological functions (including those of the c/v system) - Most anatomical features (including those of the c/v system) - Most ecological characteristics - Many behavioural traits 2. Etruscan shrew (2 g), human (60 kg), African bush elephant (6 000 kg), blue whale (100 000 kg) 3. So, an elephant is 100 times larger than a human, but its metabolic rate is only ca. 30 times greater! It’s allometric. Nobody knows why Scaling of the cardiovascular system Geometry of the heart scales isometrically (i.e. in direct proportion to body mass): ▪ e.g. heart wall thickness ∝ 𝑀0.33 ▪ e.g. heart wall area ∝ 𝑀0.67 ▪ e.g. heart volume (or heart mass) ∝ 𝑀1.0 Scaling of key cardiovascular variables as a function of body mass ( M ) among mammals: 1. Heart mass ∝ M 1.0 2. Stroke volume ∝ M 1.0 3. Mean arterial blood pressure ∝ M 0.0 (MAP is ~100 mmHg independent of body mass) 4. Heart rate ∝ M 0.25 at rest, and ∝ M 0.15 at VO2 max 5. Cardiac output ∝ M 0.75 at rest, and ∝ M 0.85 at VO2 max Solve for unknown scaling exponent: Solve for unknown scaling variable given body mass: 1. Heart mass = 6.0M 0.97 , where heart mass is in g and body mass is in kg (Spector 1956). 2. What is the heart mass of an average 60 kg human? = 6.0 ×( 0.97 ) = 318 g 3. What is heart mass expressed as a percentage of body mass in an average 60 kg human? = 318 / (60 ×1000) = 0.005 = 0.5%