Gas Exchange and Respiratory System

• The primary function of the respiratory system is to deliver oxygen to the cells of the body’s tissues and remove carbon dioxide, a cell waste product.
• Gas exchange is necessary to support the production of ATP in cellular respiration and usually involves the respiratory system and the circulatory system of an animal.
• The source of O2, called the respiratory medium, is air for terrestrial animals and water for most aquatic animals.
• The atmospheric air is Earth’s main reservoir of O2 and is about 21% O2 (by volume).
• The part of an animal’s body where gas is exchanged with the surrounding environment is called the respiratory surface.
• Animals do not move O2 and CO2 across membranes by active transport, so the movement of these gases between the respiratory surface and the environment occurs entirely by diffusion.
• All aerobic organisms require oxygen to carry out their metabolic functions.
• The environment in which the animal lives greatly determines how an animal respires.
• The complexity of the respiratory system is correlated with the size of the organism.
• As animal size increases, diffusion distances increase and the ratio of surface area to volume drops.
• Air enters the lungs through the two primary (main) bronchi, which divide into secondary bronchi, then into tertiary bronchi, and finally into bronchioles.
• Bronchi are made of cartilage and smooth muscle, and are innervated by nerves of both the parasympathetic and sympathetic nervous systems.
• In humans, bronchioles with a diameter smaller than 0.5 mm are the respiratory bronchioles, which lack cartilage and rely on inhaled air to support their shape.
• The passageways decrease in diameter as they split, increasing the relative amount of smooth muscle.
• The terminal bronchioles subdivide into microscopic branches called respiratory bronchioles.
• The respiratory bronchioles subdivide into several alveolar ducts, which are surrounded by numerous alveoli and alveolar sacs.
• Gas exchange occurs only in alveoli, which are made of thin-walled parenchymal cells that look like tiny bubbles within the sacs.
• Alveoli are in direct contact with capillaries of the circulatory system, ensuring that oxygen will diffuse from alveoli into the blood and be distributed to the cells of the body.
• The anatomical arrangement of capillaries and alveoli emphasizes the structural and functional relationship of the respiratory and circulatory systems.
• The lungs have a sponge-like consistency due to the organization of alveoli within each alveolar sac.
• The surface area of alveoli in the lungs is approximately 75 m2, allowing gases to easily diffuse across the cells.
• The respiratory system contains several protective mechanisms to avoid problems or tissue damage, including hairs and mucus in the nasal cavity that trap small particles, viruses, bacteria, dust, and dirt to prevent their entry.
• In unicellular organisms, diffusion across the cell membrane is sufficient for supplying oxygen to the cell.
• Diffusion is a slow, passive transport process.
• In order for diffusion to be a feasible means of providing oxygen to the cell, the rate of oxygen uptake must match the rate of diffusion across the membrane.
• Larger organisms had to evolve specialized respiratory tissues, such as gills, lungs, and respiratory passages accompanied by complex circulatory systems, to transport oxygen throughout their entire body.
• For small multicellular organisms, diffusion across the outer membrane is sufficient to meet their oxygen needs.
• Gas exchange by direct diffusion across surface membranes is efficient for organisms less than 1 mm in diameter.
• In simple organisms, such as cnidarians and flatworms, every cell in the body is close to the external environment.
• Their cells are kept moist and gases diffuse quickly via direct diffusion.
• Flatworms are small, literally flat worms, which ‘breathe’ through diffusion across the outer membrane.
• Simple squamous epithelium is found where filtration or the exchange of substances by rapid diffusion is a priority.
• Nasal and Nose Cavity: In mammals, pulmonary ventilation occurs via inhalation (breathing).
• A system of branching ducts conveys air to the lungs.
• During inhalation, air enters the body through the nasal cavity located just inside the nose.
• As air passes through the nasal cavity, the air is warmed to body temperature and humidified.
• The respiratory tract is coated with mucus to seal the tissues from direct contact with air.
• Mucus is high in water.
• As air crosses these surfaces of the mucous membranes, it picks up water.
• These processes help equilibrate the air to the body conditions, reducing any damage that cold, dry air can cause.