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Gas Exchange Part I Respiration – taking up O2 giving up CO2 Photosynthesis – taking up CO2, giving up O2. Respiratory medium (air or water) O2 CO2 Respiratory surface Organismal level Circulatory system Cellular level Energy-rich fuel molecules from food Cellular respiration ATP What is diffusion? epswww.unm.edu/.../eps462/graphics/diffusion.gif Depends on partial pressure, surface area A gas always diffuses from an area of high partial pressure to low partial pressure. What is equilibrium? Partial pressure of gases: pressure exerted by a particular gas in a mixture of gases. We need to know: Pressure that is exerted by mixture Fraction of mixture represented by the particular gas Atmosphere is 21% by volume O2. At sea level atmospheric pressure is 760mm Hg. PO2 is 760mm Hg X 0.21 = 160mm Hg What happens in water? Amount of gas dissolved in water is proportional to partial pressure in air solubility in water. At equilibrium partial pressure of a gas in air (PO2 of 160mm Hg) = partial pressure of that gas in solution (PO2 of 160mm Hg) Concentration of a gas depends on the solubility of the gas. Solubility decreases with increase of temperature and dissolved solids. Concentration of O2 [O2] is about 40 times more in air than water. Comparison of the two respiratory media: Air Water density less more viscosity less more [O2] higher lower Aquatic animals have had to evolve very effective and efficient gas exchange strategies. Respiratory surfaces are plasma membranes which must be moist. Gas exchange takes place by diffusion. Rate of diffusion is directly proportional to the surface area across which it occurs inversely proportional to the square of the distance the molecules have to travel. To speed up the rate of diffusion, respiratory surfaces have to be LARGE and THIN. In unicellular and simple animals diffusion occurs between all cells and environment. If body surface is enough then skin can be a respiratory organ. Earthworm – surface is moist, supplied richly by capillaries Dorsal vessel (main heart) Auxiliary hearts A closed circulatory system. Ventral vessels If body surface area is insufficient – need for specialized respiratory organs Larger animals have respiratory organs consisting of respiratory surfaces and other structures. Size of respiratory surface depends on Size of organism Metabolic demands To accommodate large respiratory surfaces inside the body – Folded Branced Examples: gills, trachea, lungs Gills: outfoldings of the body that are suspended in water; surface area much larger than the rest of the body. There are a large variety of gills Parapodia Gill Marine Worm Marine worm LE 42-20d Gills Crayfish Crayfish LE 42-20a Gills Coelom Tube foot Sea Star Sea star Oxygen-poor blood Oxygen-rich blood Gill arch Gill arch Water flow Lamella Blood vessel Operculum Gill filaments Water flow O2 over lamellae showing % O2 Blood flow through capillaries in lamellae showing % O2 Countercurrent exchange Ventilation: movement of respiratory medium over respiratory surface. Promoted by moving the gills moving water over the gills swimming Countercurrent exchange: exchange of substance between two fluids (blood and water) flowing in opposite directions and thereby maximizing gas exchange efficiency (about 80%) Gills are unsuitable for land: water supports the filaments and keep them separate gills would dry up Tracheal systems: Most common respiratory structure. Consists of: Large tubes (trachea – supported by chitin rings) branch into… Smaller tubes, tracheoles (fluid at terminal end); bring enough O2 to the tissues and removes enough CO2 from the tissues. Air sacs: supply air to organs with higher O2 needs. Tracheae Air sacs Spiracle Body cell Air sac Tracheole Trachea Air Body wall Tracheoles Mitochondria Myofibrils 2.5 µm O2 demand can go up during flight by up to 200X. The demand is met by: Contraction and relaxation of the flight muscles pumps air through the tracheal system Flight muscles rich in mitochondria. Withdrawal of fluid from tracheole into body increases surface area. Lungs: localized respiratory organs; inflodings of the body surface separated consisting of numerous small pockets. Circulatory system transports O2 to the body from the lungs and CO2 from the body to the lungs Most reptiles, all birds and mammals use lungs for gas exchange Amphibians and some reptiles (turtles) supplement lungs with parts of their skin. Some aquatic animals (lungfishes) use lungs for gas exchange Gills Trachea Lungs habitat of organisms water land land involves circulatory system yes no yes location in body hangs outside in localized areas through out the body localized organs inside the body For animals with gills or lungs – endotherms have greater surface area than ectotherms. Gas Exchange Part II Pathway of air to the gas exchange surface in mammals: Nasal cavity Pharynx Nasal cavity Pharynx Larynx Glottis (covered by epiglottis during swallowing) Larynx Esophagus Left lung Trachea Trachea Right lung Bronchi Bronchus Bronchiole Diaphragm Bronchioles Heart Alveoli Mucus traps dust, beating cilia move the mucus to esophagus Millions of alveoli in lungs, total area about 100 m2. Alveoli are surrounded by capillaries. Branch from pulmonary vein (oxygen-rich blood) Branch from pulmonary artery (oxygen-poor blood) Terminal bronchiole Surface is coated by moist fluid that helps in gas exchange. Alveoli Surfactants keep alveoli from collapsing. SEM Colorized SEM Breathing: process to ventilate lungs. Amphibian breathing: positive airflow. Mammalian breathing: negative pressure breathing. Mammalian breathing During inhalation - expand thoracic cavity, causes lower air pressure in thoracic chamber, air rushes in; opposite process for exhalation. Rib muscles, diaphragm, double layered membrane between lungs and thoracic cavity participate. During exercise muscles of neck, back and chest are also involved. LE 42-24 Rib cage expands as rib muscles contract Air inhaled Rib cage gets smaller as rib muscles relax Air exhaled Lung Diaphragm INHALATION Diaphragm contracts (moves down) EXHALATION Diaphragm relaxes (moves up) Tidal volume: volume of air inhaled and exhaled at each breath (~ 500ml) Vital capacity: maximum volume of air that a person can exhale after maximum inhalation, OR maximum volume of air that a person can inhale after maximum exhalation. 3.4L in college age women, 4.8L in college age men. decreases with age. Residual volume: Air that remains after forced exhalation. Avian breathing: Ventilation is more efficient and more complex. Maximum PO2 is higher than that of mammals. Birds are better adapted to higher altitudes than humans. Airflow over gas exchange surface is in one direction only No mixing of fresh and used air. 8 – 9 pairs of air sacs that act as bellows. Parabronchi in the lungs, no alveoli 2 sets of inhalation and exhalation are needed to completely pass air through the system. LE 42-25 Air Air Anterior air sacs Trachea Posterior air sacs Lungs Lungs Air tubes (parabronchi) in lung INHALATION Air sacs fill EXHALATION Air sacs empty; lungs fill 1 mm Breathing is controlled (involuntarily) to ensure Gas exchange coordinates with circulation Metabolic needs are met Cerebrospinal fluid Pons Breathing control centers Medulla oblongata Breathing is controlled by two regions at the base of the brain – pons and medulla oblongata During respiration cells produce CO2. CO2 concentration in blood goes up. CO2 diffuses from blood to cerebrospinal fluid (CSF). In CSF CO2 + H2O H2CO3 HCO3- + H+ Increased metabolic activity (exercise) – [CO2] increases Results in increase in [H+] Results in decrease in pH. pH in CSF is an indicator of blood [CO2]. Decrease in pH is an indicator of increased [CO2] Decreased pH in cerebropspinal fluid results in control centers of the brain increasing the rate and depth of breathing. When CO2 is exhaled, pH increases and breathing is returned to normal. Cerebrospinal fluid Pons Breathing control centers Medulla oblongata Carotid arteries Aorta Diaphragm Rib muscles CO2 concentration is primarily used to control breathing O2 concentration influences breathing only when it is very low. Aorta and carotid arteries have O2 sensors which signal the brain ti increases breathing Increased breathing is always coupled with increased cardiac output. Coordination of circulation and gas exchange. LE 42-5 Heart is a dual pump. Circulatory system is divided into pulmonary circuit and systemic circuit. Blood with higher PCO2 and lower PO2 comes from the heart to the lungs. Capillaries of head and forelimbs Anterior vena cava Pulmonary artery Pulmonary artery Capillaries of right lung Pulmonary vein Right atrium Right ventricle Posterior vena cava Aorta Capillaries of left lung Pulmonary vein Left atrium Left ventricle Aorta Capillaries of abdominal organs and hind limbs Air in the alveoli has higher PO2 and lower PCO2 than blood in the capillaries. O2 in the alveoli dissolves in the fluid coating the alveolar epithelium and diffuses into the blood. CO2 dissolves from blood to the air in the alveoli. Blood leaving the lungs and going to the heart has higher PO2 and lower PCO2 than the blood entering the lungs. From the heart the blood goes into the systemic circulation. In the tissues cellular respiration removes the O2 from the cells and adds CO2. PO2 is higher and PCO2 is lower in the blood in the tissue capillaries than in the tissues. O2 diffuses out of the blood and enters the cells and CO2 diffuses out of the cells and enters the blood. This blood is returned to heart and sent to lungs. Exhaled air Inhaled air 160 0.2 O2 CO2 Alveolar spaces 120 27 O2 CO2 104 40 Alveolar epithelial cells Blood entering alveolar capillaries 40 45 O2 CO2 O2 CO2 CO2 O2 Alveolar capillaries of lung Pulmonary arteries Systemic veins Blood leaving alveolar capillaries 104 40 O2 CO2 Pulmonary veins Heart Systemic arteries Tissue capillaries Blood entering tissue capillaries Blood leaving tissue capillaries 40 45 O2 CO2 CO2 O2 Tissue cells < 40 > 45 O2 CO2 100 40 O2 CO2 Diffusion of O2 in the blood alone is inadequate for meeting metabolic needs. O2 transport is done to a large degree by respiratory pigments. During exercise cardiac output is 12.5L of blood per minute with respiratory pigment 555L without the pigment Respiratory pigments: protein bound to metal, have distinctive color Hemoglobin: protein and iron (vertebrates) Hemocyanin: protein and copper (some arthropods and molluscs) Hemoglobin (Hb): 4 polypeptide subunits each with an iron atom cofactor. Found in red blood cells. Heme group Iron atom O2 loaded in lungs O2 unloaded in tissues Polypeptide chain Functions of hemoglobin: carries O2, carries C O2, acts as a buffer in blood Binds to oxygen reversibly. Subunits show cooperativity in binding and release. O2 saturation of hemoglobin (%) 100 O2 unloaded from hemoglobin during normal metabolism 80 60 O2 reserve that can be unloaded from hemoglobin to tissues with high metabolism 40 20 0 0 20 40 60 Tissues during Tissues exercise at rest PO2 (mm Hg) PO2 and hemoglobin dissociation at 37°C and pH 7.4 80 100 Lungs Cellular respiration increases CO2 production. CO2 production lowers pH Lower pH decreases Hb affinity for oxygen. (Bhor shift) O2 saturation of hemoglobin (%) 100 pH 7.4 80 Bohr shift: additional O2 released from hemoglobin at lower pH (higher CO2 concentration) 60 pH 7.2 40 20 0 0 20 40 60 80 PO (mm Hg) 2 pH and hemoglobin dissociation 100 When cellular respiration is higher Hb releases more O2. CO2 transport. In solution (7%) Bound to Hb (23%) Bicarbonate (HCO3-) 70% Tissue cell CO2 transport from tissues CO2 produced CO2 transport from tissues to alveolar space Interstitial fluid CO2 Blood plasma within capillary CO2 Capillary wall CO2 H2 O Red H2CO3 Hb blood cell Carbonic acid HCO3– + Bicarbonate Hemoglobin picks up CO2 and H+ H+ HCO3– To lungs CO2 transport to lungs HCO3– HCO3– + H2CO3 H+ Hb Hemoglobin releases CO2 and H+ H2 O CO2 CO2 CO2 CO2 Alveolar space in lung Tissue cell CO2 produced From tissue and interstitial fluid to plasma Large part (~90%) diffuses into the red blood cells Some picked up by Hb CO2 and water in red blood cells react forming carbonic acid Carbonic acid dissociates into bicarbonate and hydrogen ions. Hb binds most of the H+; this helps maintain pH, preventing Bhor sift. Interstitial fluid CO2 Blood plasma within capillary CO2 CO2 transport from tissues Capillary wall CO2 H2O Red H2CO3 Hb blood Carbonic acid cell HCO3– + Bicarbonate H+ HCO3– To lungs Hemoglobin picks up CO2 and H+ To lungs CO2 transport to lungs HCO3– HCO3- diffuses into the plasma. At the lungs HCO3diffuses back into the red blood cells. Combines with H+ to form CO2 and water CO2 is unloaded from Hb. Diffuses from plasma into interstitial fluid. CO2 diffuses into alveolar space, exhaled out. HCO3– + H2CO3 H+ Hb Hemoglobin releases CO2 and H+ H2O CO2 CO2 CO2 CO2 Alveolar space in lung Animals like cheetah, pronghorned antelope have been selected enhancement normal physiological mechanisms at every stage of O2 metabolism. Diving mammals have myoglobin that have higher affinity for O2 than human myoglobin.