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General Biology “Maintenance of Life” Circulation and Gas Exchange Noppadon Kitana, Ph.D. Department of Biology Chulalongkorn University August 29, 2012 Circulation and Gas Exchange • Respiratory system functions in gas exchange (O2 v.s. CO2). • Circulatory system functions in carrying oxygenated blood to other parts of body. • Blood capillaries carry materials to interstitial fluid surrounding cells. August 29, 2012 Circulation in Animals • Every organism must exchange materials and energy with its environment. • This exchange ultimately occurs at the cellular level. – Cells live in aqueous environments. – Nutrients and oxygen move across the plasma membrane to the cytoplasm. – Metabolic wastes (e.g. CO2) move out of the cell. • Diffusion is the simplest and the most common route of exchange. August 29, 2012 Circulation in Animals • Diffusion alone is not adequate for transporting substances over long distances in animals. • Diffusion time is proportional to the square of the distance, e.g. if diffusion through 0.1 mm distance occurs in 1 second. – 1 mm will occur in 100 seconds – 10 mm will occur in 10,000 seconds (~ 3 hrs) • Circulatory system solves this by ensuring that no substance must diffuse very far to enter or leave a cell. August 29, 2012 Circulation in Invertebrates: Gastrovascular Cavity • The body plan of cnidarians (e.g. hydra) makes a circulatory system unnecessary. • A body wall of only two-cell thick encloses a central gastrovascular cavity that serves for both digestion and for diffusion of substances throughout the body. August 29, 2012 • The fluid inside the cavity is continuous with the water outside through a single opening (mouth). • Thus, both the inner and outer tissue layers are bathed in fluid. August 29, 2012 Circulatory System • Open system • Closed system • Heart • Vessels • Circulating fluid August 29, 2012 Open Circulatory System • Insects, other arthropods, and most mollusks • No distinction between blood and interstitial fluid (collectively called hemolymph). August 29, 2012 • One or more hearts pump the hemolymph into interconnected sinuses surrounding the organs. • This process allows exchange between hemolymph and body cells. • Heart is an elongated dorsal tube. August 29, 2012 • Body movements that squeeze the sinuses help circulate the hemolymph. Closed Circulatory System • Earthworm, squid and octopus. • Blood is confined to vessels and is distinct from the interstitial fluid. August 29, 2012 • One or more hearts pump blood into large vessels that branch into smaller ones surrounding organs. • Materials are exchanged by diffusion between the blood and the interstitial fluid bathing the cells. August 29, 2012 Circulation in Vertebrates: Closed Circulatory System • Collectively called cardiovascular system (CVS) • Heart consists of – atrium (atria) = chamber(s) that receive blood returning to the heart – ventricle (ventricles) = chamber(s) that pump blood out of the heart August 29, 2012 • Blood vessels – Arteries, arterioles = carry blood away from the heart to organs – Capillaries = very thin, porous wall vessels form networks, called capillary beds, that infiltrate each tissue – Veins, venules = return blood to the heart • Distinguished by the direction in which they carry blood, not by the characters of the blood they carry. Heart – Arteries carry blood from the heart toward capillaries. – Veins return blood to the heart from capillaries. August 29, 2012 Capillaries Circulation in Fish • 2 chambers (atrium & ventricle) • Blood is pumped from the ventricle to gills (gill circulation) where it picks up O2 and disposes of CO2. • Gill capillaries converge into a vessel that carries oxygenated blood to capillary beds at other organs (systemic circulation) and back to the heart. August 29, 2012 • Blood must pass through two capillary beds, therefore, oxygen-rich blood leaving the gills flows to the systemic circulation quite slowly. • The heart pumps only oxygenpoor blood. August 29, 2012 Circulation in Amphibians August 29, 2012 • Three chambered heart (2 atria & 1 ventricle) • Ventricle pumps blood into a forked artery that splits the ventricle’s output into: – pulmocutaneous circulation – systemic circulation • Blood is pumped a second time after it loses pressure in the capillary beds of lung or skin (= double circulation). • Double circulation provides a vigorous flow of blood to the brain, muscles, and other organs. • In the ventricle, some oxygenrich blood from the lungs mixes with oxygen-poor blood that has returned from the rest of the body August 29, 2012 Circulation in Reptiles • Reptiles also have double circulation with pulmonary (lung) and systemic circuits. – However, there is even less mixing of oxygenrich and oxygen-poor blood than in amphibians. – Although the reptilian heart is threechambered, the ventricle is partially divided. August 29, 2012 Circulation in Other Vertebrates • Crocodilians, birds, and mammals • Four-chambered heart (2 atria & 2 ventricles) • The left side of the heart receives and pumps only oxygen-rich blood. • The right side handles only oxygen-poor blood. August 29, 2012 • Also have double circulation to restore pressure to the systemic circuit. • Evolution of a powerful fourchambered heart was an essential adaptation in support of the endothermic way of life of birds and mammals. August 29, 2012 Circulation Pattern in Human 1. 2. 3. August 29, 2012 Right ventricle Pulmonary arteries Capillary beds in left and right lungs 4. Pulmonary veins, left atrium 5. Left ventricle 6. Aorta 7. Capillary beds in head and arms 8. Capillary beds in abdominal organs and legs 9. Superior (Anterior) vena cava 10. Inferior (Posterior) vena cava 11. Right atrium Human Heart • • • • • • August 29, 2012 4 chambers atria (thin) ventricle (thick) valve artery vein Cardiac Cycle • A complete sequence of pumping (contraction) and filling (relaxation) • Contraction phase = Systole • Relaxation phase = Diastole Cardiac cycle is regulated by electrical impulses that August 29, throughout 2012 radiate the heart. Control of Heartbeat • Cells of vertebrate heart muscle are self-excitable (contract without any signal from nervous system). • Each cell has its own intrinsic contraction rhythm. • However, these cells are synchronized by the pacemaker, which sets the rate and timing of contraction. – In amphibians, pace maker is located in sinus venosus. – In human, sinoatrial (SA) node in wall of the right atrium is a pacemaker. August 29, 2012 Cardiac Electrical Impulse • (1) The SA node generates electrical impulses that spread rapidly (2) through the wall of the atria, making them contract in unison. • The impulse from the SA node is delayed by about 0.1 sec at the atrioventricular (AV) node, the relay point to the ventricle, allowing the atria to empty completely before the ventricles contract. August 29, 2012 • (3) Specialized muscle fibers called bundle branches and Purkinje fibers conduct the signals to the apex of the heart and (4) throughout the ventricular walls. • This stimulates the ventricles to contract from the apex toward the atria, driving blood into the large arteries. August 29, 2012 • The impulses generated during the heart cycle produce electrical currents that are conducted through body fluids to the skin. • The currents can be detected by electrodes and recorded as an electrocardiogram (ECG or August 29, 2012 EKG). Heart of Animals • Vertebrate heart has pace maker within the heart (=myogenic heart) • Invertebrate heart has pace maker from motor nerve outside the heart (=neurogenic heart) August 29, 2012 Blood Vessels • • • • • Artery Arteriole Capillary Venule Vein Structural differences correlate with the different August 29, 2012 functions of arteries, veins, and capillaries • Arteries have thick middle and outer layers. – The thicker walls provide strength to accommodate blood pumped rapidly and at high pressure by the heart. – Their elasticity helps maintain blood pressure even when the heart relaxes. • Capillaries lack the two outer layers and their very thin walls consist of only endothelium and its basement membrane (enhancing exchange). August 29, 2012 • Veins convey blood back to the heart at low velocity and pressure. – Blood flows as a result of skeletal muscle contractions that squeeze blood in veins. – Within larger veins, flaps of tissues act as oneway valves that allow blood to flow only toward the heart. August 29, 2012 Blood Pressure • Hydrostatic pressure that blood exert against blood vessel wall • Much greater in arteries than in veins • Highest in arteries when the heart contracts during ventricular systole, creating the systolic pressure. • The narrow openings of arterioles impeding the exit of blood from the arteries, the peripheral resistance. August 29, 2012 • When the heart contracts, blood enters the arteries faster than it can leave, and the vessels stretch from the pressure. • The elastic walls of arteries snap back during diastole, but the heart contracts again before enough blood has flowed into the arterioles to completely relieve pressure in the arteries, the diastolic pressure. August 29, 2012 Normal value: 120/80 mm Hg Blood Flow • Blood travels 1,000 time faster in the aorta than in capillaries • Law of continuity: fluid will flow through narrower segments faster than wider segments because the volume of flow per sec must be constant. • The total cross-sectional area of capillaries determines flow rate. August 29, 2012 Capillary Blood Flow • At any given time, only about 5-10% of the body’s capillaries have blood flowing through them. • Capillaries in the brain, heart, kidneys, and liver are usually filled, but in other sites, the blood supply varies over times as blood is diverted August 29, 2012 • 2 mechanisms regulate the distribution of blood in capillary beds. 1) contraction of smooth muscle layer in the wall of an arteriole constricts the vessel, decreasing blood flow to a capillary bed. 2) rings of smooth muscles, precapillary sphincters, control the flow of blood between arterioles and venules. • Some blood flows directly from arterioles to venules through thoroughfare channels which are always open. August 29, 2012 Exchange of Substances between Blood & Interstitial Fluid • Takes place across the thin endothelial walls of the capillaries by diffusion and endocytosis. • Bulk flow due to fluid pressure (blood pressure v.s. osmotic pressure) August 29, 2012 Blood Component August 29, 2012 Cellular Elements • Erythrocytes – 5-6 million cells per microliter – carry oxygen and carbondioxide – mammals and birds has biconcave erythrocyte with no nucleus (increase surface area and hemoglobin) – no mitochondria, make ATP by anaerobic respiration • Leukocytes – 5,000-10,000 cells per microliter – 5 types: monocytes, neutrophils, basophils, eosinophils, lymphocytes – play roles in immunity • Platelets – 250,000-400,000 platelets per microliter August 2012 –29,function in blood clotting August 29, 2012 Gas Exchange in Animals • Gas exchange is the uptake of molecular oxygen (O2) from the environment and the discharge of carbon dioxide (CO2) to the environment. • Although it is often called respiration, this process is distinct from the production of ATP in cellular respiration. • Other terms – respiration, breathing – cellular respiration August 29, 2012 Components of Gas Exchange • Respiratory medium (air or water) • Respiratory surface August 29, 2012 Gas Exchange in Animals • Sponges, cnidarians and flatworms: membrane of every cell in the body is close enough to the outside environment for gases to diffuse in and out. • Other animal has a respiratory organ that is extensively folded or branched, enlarging the surface area for gas exchange: – skin – gill – trachea – lung August 29, 2012 Gas Exchange through Skin • Earthworms & some amphibians • Below the moist skin is a dense net of capillaries. • The respiratory surface must be moist, their possible habitats are limited to water or damp places. August 29, 2012 Gas Exchange by Gills • Invertebrates & fishes • Gills are outfoldings of the body surface that are suspended in water. • The total surface area of gills is often much greater than that of the rest of the body. August 29, 2012 Water as a Respiratory Medium • Pros: no problem in keeping the respiratory surface moist since the gills are surrounded by the aqueous environment. • Cons: O2 concentrations in water are low, especially in warmer and saltier environments. • Thus, gills must be very effective to obtain enough oxygen. • Ventilation increases the flow of the respiratory medium over the respiratory surface, ensures that there is a strong diffusion gradient between the gill surface and the environment. – Crayfish and lobsters have appendages that drive a29,current of water over their gills. August 2012 • Fish gills are ventilated by a current of water that enters the mouth, passes through the pharynx, flows over the gills, and exits the body. – Fishes must expend considerable energy in ventilating their gills. – Gas exchange at the gill surface is enhanced by the opposing flows of water and blood at the gills. August 29, 2012 Countercurrent exchange • As blood moves in a gill capillary, it becomes more and more loaded with O2, but it simultaneously encounters water with even higher concentration of O2 because it just pass over the gills. • All along the gill capillary, there is a diffusion gradient favoring the transfer of O2 from water to blood. August 29, 2012 Air as a Respiratory Medium • Pros: air is a good respiratory medium containing high concentration of oxygen. Diffusion rate is thus higher in air. • Cons: high risk of losing moisture from the respiratory surface due to evaporation. • The terrestrial animal must kept its respiratory surface within its body. August 29, 2012 Gas Exchange by Tracheal System • Insects • Air tubes that branch through the body. • The largest tubes open to the outside. • The finest branches extend to the surface of nearly every cell where gas is exchanged by diffusion. • The circulatory system does not transport oxygen and carbon dioxide. August 29, 2012 Gas Exchange by Lungs • Unlike branching tracheal systems, lungs are restricted to one location. • Respiratory surface of the lung is not in direct contact with all other parts of the body. • Circulatory system transports gases between the lungs and the rest of the body. • Lungs have a dense net of capillaries just under the epithelium that forms the respiratory surface. • Lungs have evolved in spiders, terrestrial snails, and vertebrates. August 29, 2012