Chpts 45 Lecture Notes Four Steps of Gas Exchange 1 Ventilation occurs when air or water moves through a specialized gas exchange organ such as lungs or gills 2 Gas exchange takes place as CO2 and O2 diffuse between air or water and the blood at the ventilatory surface 3 Through circulation the dissolved O2 and CO2 are transported throughout the body 4 Gas exchange between blood and cells occurs in tissues where cellular respiration occurs and O 2 and CO2 diffuse between blood and cells Oxygen and carbon dioxide diffuse into water from the atmosphere but the amount of gas that dissolves depends on several factors 1 The solubility of the gas in water 2 The temperature of the water 3 The presence of other solutes 4 The partial pressure of the gas in contact with the water Fick s Law of Diffusion states that the rate of diffusion of a gas depends upon five parameters 1 Solubility of the gas 2 Temperature 3 Surface available for diffusion 4 Differences in partial pressures of the gas across the gas exchange surface 5 Thickness of the barrier in diffusion Specifically Fick s law states that all gases including O2 and CO2 diffuse in the largest amounts when three conditions are met 1 The surface area for gas exchange is large 2 The respiratory surface is extremely thin 3 The partial pressure gradient of the gas across the surface is large The Fish Gill Is a Countercurrent System The flow of blood through the capillaries is in the opposite direction to the flow of water over the gill surface This sets up a countercurrent exchange system in each lamella This exchange system creates a large partial pressure of oxygen and the carbon dioxide in water over blood The result is efficient exchange of gases over the gills How Do Insect Tracheae Work Insects have air filled tubes called trachea Trachea open to the outside through pores called spiracles Air moves into the trachea and then by diffusion into the cells How Do Vertebrate Lungs Work In terrestrial animals air enters the body through the mouth and nose The trachea carries inhaled air to narrow tubes called bronchi The bronchi branch off into even narrower tubes called bronchioles The organ for gas exchange is the lung which encloses the bronchioles and portions of the bronchi Vertebrates actively ventilate their lungs by pumping air via muscular contractions two mechanisms for pumping air 1 Positive pressure ventilation used by frogs 2 Negative pressure ventilation used by humans and other mammals Ventilation of the Human Lung The air inside the human lung is under negative pressure The pumping action in humans is achieved by a muscle called the diaphragm Ventilation in Birds Birds are able to extract enough oxygen for extremely long flights and flights at high elevations There are four general steps to ventilation in birds How Are Oxygen and Carbon Dioxide Transported in Blood The blood has many functions Transport oxygen and carbon dioxide Transport nutrients to cells from the digestive system Conveys hormones to target tissues and organs Delivers cells of the immune system Distributes heat The blood has cellular components Platelets are cell fragments that minimize blood loss 1 White blood cells WBCs are part of the immune system Red blood cells RBCs transport oxygen from the lungs to body tissues and participate in transporting carbon dioxide from tissues to lungs In humans red blood cells make up 99 9 of the formed elements Red blood cells contain an oxygen carrying molecule called hemoglobin Hemoglobin consists of four polypeptide chains each of which binds to a nonprotein group called a heme Each heme contains an iron ion Fe2 that can bind to an oxygen molecule Each hemoglobin molecule can thus bind up to four oxygen molecules In blood 98 5 of the oxygen is bound to hemoglobin the other 1 5 is dissolved in plasma The Bohr Shift Hemoglobin is sensitive to changes in pH and temperature Decreases in pH and increases in temperature alter hemoglobin s conformation such that it is more likely to release O2 at all values of PO2 Note A conformational change means a change in the shape of the molecule The Bohr shift makes hemoglobin more likely to release oxygen during exercise in which PCO2 is high pH is low and tissues are under oxygen stress CO2 Transport and the Buffering of Blood pH CO2 that is produced by cellular respiration enters the blood and RBCs where Carbonic anhydrase catalyzes the formation of carbonic acid from carbon dioxide in water Most CO2 is transported in blood specifically in plasma in the form of the bicarbonate ion HCO 3Circulatory systems In the open system the hemolymph is pumped throughout the body in open vessels 1 The hemolymph comes in direct contact with body tissues 2 The open system is characteristic of invertebrates 3 The hemolymph is pumped by an organ called the heart In the closed system blood flows in a continuous circuit of closed vessels 1 Pressure provided by the pumping action of the heart 2 Closed systems are characteristic of vertebrates Blood Vessels Arteries are tough thick walled vessels that take blood away from the heart under high pressure Capillaries are the smallest vessels Their walls are just one cell thick allowing gases and other molecules to exchange with tissues in networks called capillary beds Veins are vessels that return blood to the heart s under low pressure The area between cells is called the interstitial space the fluid that leaks into it is interstitial fluid In animals with closed circulatory systems the heart contains at least two chambers The atrium receives blood returning from circulation The ventricle generates force to propel the blood through the system Systemic and pulmonary circulations both have arterial and venous sides veins carry blood to heart Arteries Away from heart both veins and arteries can carry deoxygenated blood or oxygenated blood this depends on where they are coming from E g a vessel carrying blood FROM lungs TO heart is a VEIN because it is going towards the heart not away from it This vessel carries OXYGENATED BLOOD because it was just in the lungs picking up O2 Cardiac Cycle Signals from the sinoatrial SA node ensure that the atria contract simultaneously then relax Signals from the atrioventricular AV node ensure that the ventricles contract while the atria are relaxed The contraction phase is the systole The relaxation phase is the diastole Electrical Activation of the Heart Key events in the heart s electrical activation include The SA