Unit 6.4 Gas Exchange PDF

Department of Biology Gas Exchange A. Gas Exchange in Animals Gas exchange is the uptake of molecular oxygen (O 2) from the environment and the discharge of carbon dioxide (CO2) to the environment.  While often called respiration, this proc...

Department of Biology Gas Exchange A. Gas Exchange in Animals Gas exchange is the uptake of molecular oxygen (O 2) from the environment and the discharge of carbon dioxide (CO2) to the environment.  While often called respiration, this process is distinct from, but linked to, the production of ATP in cellular respiration. Gas exchange, in concert with the circulatory system, provides the oxygen necessary for aerobic cellular respiration and removes the waste product, carbon dioxide. The source of oxygen, the respiratory medium, is air for terrestrial animals and water for aquatic animals.  The atmosphere is about 21% O2 (by volume).  Dissolved oxygen levels in lakes, oceans, and other bodies of water vary considerably, but are always much less than an equivalent volume of air. The part of an animal where gases are exchanged with the environment is the respiratory surface.  Movements of CO2 and O2 across the respiratory surface occur entirely by diffusion.  The rate of diffusion is proportional to the surface area across which diffusion occurs, and inversely proportional to the square of the distance through which molecules must move.  Therefore, respiratory surfaces tend to be thin and have large areas, maximizing the rate of gas exchange.  In addition, the respiratory surface of terrestrial and aquatic animals must be moist to maintain the cell membranes. ▪ As a result, gases must dissolve in water before diffusing across respiratory surfaces. Direct Diffusion In simple organisms like cnidarians and flatworms, gas exchange through diffusion is sufficient to meet their oxygen needs. Every cell of their bodies is in close contact to the external environment. Also, their cells are kept moist which facilitates easy diffusion of gases. In flatworms, their shape increases the surface area for diffusion which ensures that each cell within their body is close to the outer membrane surface and has Figure 1. Direct diffusion: This flatworm’s process of access to oxygen. respiration works by diffusion across the outer membrane.  Department of Biology Skin and Gills Animals like earthworms and amphibians use their integument as the organ for gas exchange. Below their skin are dense network of capillaries which facilitates exchange between the external environment and the circulatory system. For the gases to dissolve and diffuse across cell membranes, the respiratory surface must be kept moist. Aquatic animals need to obtain oxygen. Oxygen dissolves in water, but at a lower concentration in comparison to the atmosphere, which has roughly 21% oxygen. In the case of fishes and other aquatic organisms use gills to take up dissolved oxygen from water. Gills are thin tissue filaments that are highly branched and folded. When water passes over the gills, the dissolved oxygen in the water rapidly diffuses across the gills into the bloodstream. The folded surfaces of the gills provide a large surface area to ensure that fish obtain sufficient oxygen. Blood with a low concentration of oxygen molecules circulates through the gills. The concentration of oxygen molecules in water is higher than the concentration of oxygen molecules in gills. As a result, oxygen molecules diffuse from water (high concentration) to blood (low concentration). Similarly, carbon dioxide molecules diffuse from the blood (high concentration) to water (low concentration). Figure 2. Oxygen transport and gills: As water flows over the gills, oxygen is transferred to blood via the veins. Tracheal Systems In insects, respiration is independent of its circulatory systems; hence, the blood has no direct role in oxygen transport. The tracheal system in insects consists of a network of small tubes which carries oxygen to the entire body. This system of respiration has tubes made of a polymeric material called chitin. Insects also have spiracles which are openings along the thorax and abdomen. Spiracles connect to the tubular network that allows oxygen to pass into the body, regulating the diffusion of CO2 and water vapor. Air enters and leaves the tracheal system through the spiracles. Some insects can ventilate the tracheal system with body movements.  Department of Biology Figure 3. Insect respiration: Insects perform respiration via a tracheal system, in which openings called spiracles allow oxygen to pass into the body. Amphibians and Bird Respiratory System Early in life, amphibians utilize their gills for breathing. They develop primitive lungs later then in life and are also able to breathe through their skin through diffusion. In order for this to happen, their skin must remain moist. Avian Respiration Birds have relatively small lungs and nine air sacs that have important roles in respiration. Bird lungs do not have inflatable capacity because they lack diaphragm and a pleural cavity. Rather than occurring in the alveoli, gas Figure 4. Bird respiration: The process exchange in birds occurs between air capillaries and blood of inhalation and exhalation in birds. capillaries. Three distinct sets of organs perform respiration – the anterior air sacs, the lungs, and he posterior air sacs.  Department of Biology Mammalian Systems of Respiration In mammals, respiration occurs through inhalation and exhalation. Air passes through the nasal cavity and is warmed to body temperature and humidified. From the nasal cavity, air passes through the pharynx and the larynx to the trachea. The function of the trachea is to funnel the inhaled air to the lungs and the exhaled air out of the body. The human trachea, a cylinder about 10-12cm long, 2cm in diameter found in front of the esophagus, extends from the larynx into the chest cavity. It is made of incomplete rings of hyaline cartilage and smooth muscle that divides into the two primary bronchi at the midthorax. The terminal bronchioles then subdivide into respiratory bronchioles which subdivide into alveolar ducts. Numerous alveoli (sing. alveolus) and alveolar sacs surround the alveolar ducts. Alveoli are made of thin-walled, parenchymal cells that are in direct contact with capillaries of the circulatory system. This ensures that oxygen will diffuse from alveoli into the blood Figure 5. Route of inhalation: Air enters the and that carbon dioxide produced by cells as a respiratory system through the nasal cavity waste product will diffuse from the blood into and pharynx. It then passes through the alveoli to be exhaled. The anatomical trachea and into the bronchi, which bring air arrangement of capillaries and alveoli into the lungs. emphasizes the relationship of the respiratory and circulatory systems. As there are so many alveoli (around 300 million per lung) within each alveolar sac and so many sacs at the end of each alveolar duct, the lungs have a sponge-like consistency. This organization produces a very large surface area that is available for gas exchange. Videos ✓ https://www.youtube.com/watch?v=LXGG-HgtJoI ✓ https://www.youtube.com/watch?v=8NUxvJS-_0k References 1. Biology, 7th edition by Campbell and Reece Department of Biology, Caraga State University Lecture Notes by: Chris Romero, 2005 2. Online Sources ✓ https://courses.lumenlearning.com/boundless-biology/chapter/systems-of-gas-exchange/ LESSON COMPILED BY: Rovelyn Pallega Gallego Faculty, Department of Biology Caraga State University