Gas Exchange Getting oxygen to each cell of the body Campbell et al. “Biology: concepts and connections” Chapter 22 Overview: getting O2 from the atmosphere to the mitochondria 1. Breathing: mechanically draws air into lungs – O2 diffuses into blood vessels – CO2 diffuses from blood vessels into lungs and is exhaled 2. The blood transports gases to the body – O2 attaches to hemoglobin 3. Gas exchange between blood and body cells – O2 serves as final electron acceptor in in respiration – ATP is produced in the mitochondria – CO2 + H2O produced as a by- product Gas exchange in animals occurs across moist surfaces • O2 must be in solution to be used in biological systems • Rate of diffusion depends on: – Surface area – Thickness of surface (1/x2) – Relative concentrations Gas exchange structures in animals • Some smaller animals use their entire body surface for gas exchange… – Small size and/or shape (flatten) large surface area to volume ratio – Must remain moist Gas exchange structures in animals Water contains ~ 20 times less available O2 than air. Countercurrent Many animals living in aquatic Exchange environments use gills for gas exchange Countercurrent Exchange in Gills Gas exchange structures in animals • Insects use a “tracheal system” of air sacs and tracheoles allowing direct exchange of gas between the air and cells – Circulatory system not involved The tips of the tracheoles – All cells are near an contain fluid / gas exchange air tube (tracheole) occurs across the epithelial layers The development of lungs enabled vertebrates to colonize land • Some ancient forms likely had gills and lungs and lived in shallow water • Longer snout and muscular neck allowed them to lift their heads above water surface to gulp air • Terrestrial tetrapods diverged into three major groups: – Amphibians (small lungs / diffusion across skin) – Reptiles / Birds Various types of lungs – Mammals • Size and complexity of lungs relate to metabolic needs. Gas exchange structures in animals Most terrestrial vertebrates have lungs Each human lung contains about 150 million alveoli Breathing forces air into and out of our lungs (negative pressure breathing) Birds have more efficient lungs Inhalation – all the air sacs fill - read air sacs fill with fresh air from atmosphere - front air sacs fill with depleted air which has pass through the lungs Exhalation expels air from air sacs - air from rear air sacs passes through the lungs into front air sacs - air from front air sacs is expelled to the atmosphere -- Two cycles required for air to pass through whole system -- Countercurrent exchange system increases efficiency Flow through lung & countercurrent exchange greater efficiency Don’t forget to breath… • Breathing centers in the brain send signals to muscles in the rib cage and diaphragm • CO2 levels in the blood are monitored • Sensors in the aorta & carotid arteries monitor O2 levels in blood • Breathing and heart beat are coordinated Gases (O2 and CO2) are transported by the blood • Gases will diffuse down their concentration gradients according to their partial pressure • Direction of flow is from areas with high partial pressures to those with lower partial pressures Carbon Dioxide is constantly being produced and oxygen is being consumed in the body’s cells Hemoglobin carries oxygen in the blood • Oxygen is not highly soluble in water • Hemoglobin can reversibly carry oxygen in the blood • Hemoglobin also acts as a buffer and assists in transporting carbon dioxide • CO2 + H2O H2CO3 H+ + HCO3- Carbonic Acid Hydrogen ion + Bicarbonate ion Hemoglobin loading and unloading… Hemoglobin carries out multiple functions: -- Oxygen carrier -- Carbon Dioxide carrier -- Controls blood pH Bohr shift a change in the conformation of hemoglobin which lowers its affinity for oxygen. Developing fetus exchanges gases with the mother’s bloodstream • Baby’s lungs are filled with amniotic fluid and are non- functional • Network of capillaries in the placenta serve as gas exchange system • Fetal hemoglobin has higher affinity for oxygen than adult hemoglobin • (note: smoking reduces oxygen supply to placenta by ~25%) • At birth gas exchange via the placenta stops and CO2 levels in blood increase (pH drops) • Lower pH signals the brain to start using the lungs • WOW!! - Lung Cancer Smoking kills!! - Emphysema - Heart Disease - Many other cancers Cigarette smoke contains toxin-laden particles end Rubber cast of human lungs… (courtesy of the Anatomical Institute, Bern) • During the first inspiration, the air travels through the nostrils, also called nares, of a bird, which are located at the junction between the top of the upper beak and the head. The fleshy tissue that surrounds them, in some Bird Breathing birds, is called the cere. As in mammals, air moves through the nostrils into the nasal cavity. From there it passes through the larynx and into the trachea. Air moves through the trachea to the syrinx, which is located at the point just before the trachea divides in two. It passes through the syrinx and then the air stream is divided in two as the trachea divides. The air does not go directly to the lung, but instead travels to the caudal (posterior) air sacs. A small amount of air will pass through the caudal air sacs to the lung. • During the first expiration, the air is moved from the posterior air sacs through the ventrobronchi and dorsobronchi into the lungs. The bronchi continue to divide into smaller diameter air capillaries. Blood capillaries flow through the air capillaries and this is where the oxygen and carbon dioxide are exchanged. • When the bird inspires the second time, the air moves to the cranial air sacs. • On the second expiration, the air moves out of the cranial air sacs, through the syrinx into the trachea, through the larynx, and finally through the nasal cavity and out of the nostrils.