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Circulatory System Function • Move circulatory fluid (blood) around body Circulation Chapter 3 Types of Circulation • Sponges – intracellular spaces – allows water to flow through • Nematodes, Platyhelminths, etc – gut cavity, coelomic fluid • Arthropods, annelids, chordates, etc – distinct circulatory system – pumps and channel system Types of Channel Systems – – – – – – – Gas Transport Nutrient Transport Excretory Product Transport Cell Signal Transport Hydraulic Force Heat Conductance Immunity Types of Pumps (Hearts) • Peristaltic – waves of muscular contraction along tubes drives blood flow • Chamber – muscular pump divided into chambers which contract • Pressure – contraction of muscles external to the circulatory system drives flow Invertebrate Circulation: Annelids • Closed circulatory systems – blood carried in tubes (blood vessels) • arteries, capillaries and veins – vertebrates, cephalopods, echinoderms, annelids • Open circulatory systems – blood (hemolymph) passes from heart through short arteries into open sinuses surrounding the tissues • Closed circulatory system • Dense capillary network at integument (respiration) • Peristaltic dorsal blood vessel drives blood flow – most mollusks and arthropods 1 Invertebrate Circulation: Bivalves and Gastropods • Open circulatory system – (hemolymph) circulated in an open space (hemocoel) divided into lacunae • Two- or three-chambered heart • Hydraulic force used to control movement of the foot in bivalves Invertebrate Circulation: Insects • Open circulatory system • Minimal gas transport • Large dorsal vessel w/peristaltic heart in posterior segment – hemolymph runs anteriorly to head, then ends in hemocoel – flow directed through hemocoel by longitudinal membranes – flows back to posterior dorsal vessel • Auxillary pumps supply wings limbs, and antennae Invertebrate Circulation: Crustaceans • Some small or sessile spp. lack heart or blood vessels • Larger spp possess open system similar to insects • Extensive circulation in gills – heart receives oxygenated hemolymph from the gills then pumps it to the rest of the body Invertebrate Circulation: Cephalopods • Closed circulatory system – pair of branchial hearts (drive blood to gills) – single chambered systemic heart (ventricle) – similar to system in higher vertebrates • separate pulmonary and systemic circuits Invertebrate Circulation: Arachnids • Similar to insect design • Hemolymph contains higher [hemocyanin] – O2 transport • More extensive arterial systems in arachnids with books lungs • Specific arteries supply hydraulic pressure to legs for locomotion – legs of spiders lack extensor muscles Vertebrate Circulation: General Patterns • Single passage through heart during circuit (e.g., fish) – Single circuit • Double passage through heart during circuit (e.g., mammals) – Separate pulmonary and systemic circuits 2 Vertebrate Circulation: Cyclostomes • Partially open system – large blood sinuses • Multiple “hearts” – branchial (regular) heart • two chambered – – – – cardinal heart portal heart caudal hearts gills (drive arterial blood) Vertebrate Circulation: Dipnoi (Lungfish) • Three-chambered heart – Two-chambered atrium – Partially divided ventricle & bulbus cordis (conus arteriosis) • Separates oxygenated (left) and deoxygenated (right) blood • Can shunt blood to lungs or gill lamellae Vertebrate Circulation: Non-Archosaur Reptiles • Three chambered heart – Two chambered atrium – partly divided ventricle • Ventricle contains three sub-chambers – divided upon contraction – “five-chambered” heart – allows heart to redirect blood flow btw pulmonary and systemic circuits – “cardiac shunting” Vertebrate Circulation: Teleosts and Elasmobranchs • Two-chambered heart – atrium + ventricle • Atrial contraction (systole) pushes blood into ventricle – valves prevent flow into sinus venosus • Ventricular systole forces blood into bulbus arteriosus • Backflow upon relaxation (diastole) prevented by valves – elastic recoil of bulbus arteriosus drives blood through blood vessels Vertebrate Circulation: Amphibians • Three chambered heart – Two chambered atrium – Undivided ventricle – Spiral valve - separates blood flow in conus arteriosus • Right side (pulmonary) – Receives blood from tissues and skin – Pumps to skin and lungs • Left side (systemic) – Receives blood from lungs – Pumps to tissues Vertebrate Circulation: Crocodilians • Four-chambered heart – Left aortic arch and pulmonary artery arise from right ventricle – L and R arches connected by foramen of Panizza • Allows cardiac shunting – blood directed to lungs during air breathing – blood directed to tissues during diving 3 Vertebrate Circulation: Mammals and Birds • Four-chambered heart • Complete separation into right and left halves • Blood pressure can differ between pulmonary and systemic circuits – systemic BP = 95 mmHg – pulmonary BP = 14 mmHg Mammalian/Avian Cardiac Cycle • Systole (contraction) – Muscular walls of the ventricles contract – Elevation of blood pressure in the ventricles – Closure of atrioventricular valves – Blood pushes through arterial valves – Blood flows into arteries Cardiac Output • amount of blood pumped by the heart per min. Qh = ƒh * Vh • ƒh = heart rate – frequency of contraction • Vh = stroke volume Vertebrate Circulation: Mammals and Birds • Atria – Thin walled, support ventricular filling • Ventricles – Primary pumps for driving blood through circulation • One-way valves – Atrioventricular valves – Arterial (semilunar) valves – ensure unidirectional flow • veins → atria → ventricles → arteries Mammalian/Avian Cardiac Cycle • Diastole (relaxation) – Muscular walls of the ventricles relax – Blood pressure in the ventricles falls below arterial pressure – Closure of arterial valves – Pressure falls below atrial pressure – Blood pushes through atrioventricular valves – Ventricular volume increases Cardiac Output • Adjusted to meet metabolic demands of an organism – ↑ activity, ↑ cardiac output • Modify cardiac output by changing either heart rate or stroke volume – volume of blood pumped by heart per contraction 4 Heart Excitation: Myogenic (Vertebrates) • Heart excitation and contraction can occur in absence of external stimulation • Presence of internal “pacemakers” (modified muscle cells) form conduction system – – – – Sinoatrial node Atrioventricular node Atrioventricular bundle Purkinje fibers Regulation of Cardiac Output (Mammals) • Heart rate (modify pacemaker activity): – The autonomic nervous system: • Parasympathetic nervous system (vagus nerve) – acetylcholine slows HR • Sympathetic nervous system (accelerans nerve) – norepinephrine increases HR – Hormones • Epinephrine (released from adrenal glands) Heart Excitation: Neurogenic (Arthropods) • Signals received from neurons directly responsible for muscle contraction – Posterior cells act as pacemakers – Anterior cells stimulate muscle contraction Regulation of Cardiac Output (Mammals) • Stroke volume (modify force of contraction): – neural/hormonal • epinephrine and norepinephrine – increases force of muscle contraction – autoregulation • Frank-Starling Law – increased venous return increases stretch on the heart – increased stretch leads to stronger contractions – increased HR Oxygen Delivery During Exercise • ↑ activity, ↑O2 requirements and CO2 production • Three mechanisms of obtaining more O2 – ↑ O2 extraction from the blood • only 25% of O2 removed from blood at rest • increase to 80-90% during exercise – ↑ Heart Rate – ↑ Stroke Volume Animal Size and Cardiac Output • Smaller animals have relatively higher metabolic rates (b ~ 0.75) • Smaller animals have relatively higher cardiac outputs (b ~ 0.75) • Higher cardiac output due to higher heart rates, not larger stroke volumes 5 Blood Vessels • Arteries - large, elastic tubes, multiple layers of muscles • Arterioles - smaller diameter, less elastic, fewer muscle layers • Capillaries - thin diameter, thin walls, low diffusion resistance • Venules - larger diameter, thin walled, no muscle • Veins - large diameter, elastic walls, little muscle, may possess valves Blood Flow • Blood flows from an area of high total fluid energy to low total fluid energy • Bernoulli’s Theorem E = pv + mgh + 1/2mu2 – – – – E = total fluid energy pv = potential energy of pressure generated by the heart mgh = gravitational potential energy 1/2mu2 = kinetic energy Blood Vessels • Structural Patterns – ↓ diameter, ↑ number, ↑ cross-sectional area • Functional Patterns – Blood volume: largest in veins, smallest in capillaries – Blood pressure: ↓ with ↑ distance passed – Blood flow velocity: ↓ with ↓ diameter and ↑ crosssectional area Overview of Blood Flow • Reasonable assumptions that will help simplify things… – Kinetic Energy varies little from one location to another within the system being analyzed – Flow is horizontal (gravitational potential energy is constant) Blood Flow: Poiseuille’s Law • For the laminar flow of a fluid through a straight, rigid tube: Q = (∆pr4π) / (8Lη) – Q = blood flow (volume per unit time) – ∆p = difference in pressure between both ends – r = radius of the tube – L = length of the tube – η= viscosity Blood Flow: Poiseuille’s Law • Q α ∆p – as pressure gradient increases, flow increases • Q α r4 – increased radius, large increase in flow – decreased radius, large decrease in flow • Q α 1/L – flow decreases with increased tube length • Q α 1/ η – increased viscosity decreases flow 6 Gravity Effects on Blood Pressure • As height ↓’s, gravitational potential energy ↓’s, pressure ↑’s • Venous return – blood pressure in lower body greater than upper body due to gravity • pressure in veins exceeds arterial pressure • blood pools in leg veins • returned by venous pressure pumps Capillaries • Enormous number of capillaries – overall large cross-sectional area • Extremely thin diameter – slow blood flow – high SA/V ratio • Thin walls (simple squamous endothelium) Gravity Effects on Blood Pressure • Head perfusion – arterial blood pressure must be high enough for blood to reach head – giraffes - long vertical neck • high arterial BP • venous values prevent backflow when head brought to ground level Ultrafiltration • Small molecules can diffuse into and out of capillaries • Additional amounts of fluid driven out by hydraulic pressure inside the capillaries = ultrafiltration. – Small particles driven out with water – large molecules (e.g. plasma proteins) remain in blood – low diffusion distance Ultrafiltration • Loss of water with retention of proteins increases the colloid osmotic pressure of the blood – generates tendency for water to flow back into the blood as pressure in the capillaries decreases Lymphatic System • Generally water loss by ultrafiltration exceeds water uptake by colloid osmotic movement of water – lost fluid enters lymphatic system – returned to the blood 7