Respiratory System Overview

Respiratory System

• Oxygen and carbon dioxide travel in and out of cells via passive transport (diffusion)
• Diffusion doesn't occur effectively over large distances (>1mm) thus respiratory and circulatory systems are needed to maintain O2 and CO2 partial pressures
• Key factors for diffusion include large surface area, minimal distance for diffusion, tissue permeability, and fluid circulation (inside/outside body)
• Fish and tetrapods both breathe a respiratory medium (water or air) to extract oxygen and distribute it to body tissues for cellular respiration
• Respiratory organs like gills or lungs have specialized surfaces (e.g., secondary gill lamellae, alveoli) for gas exchange through simple diffusion
• Adaptations that aid diffusion include a large surface area (e.g., secondary gill lamellae, alveoli) and minimal diffusion distance (simple epithelium) and tissue permeability.
• Fluid circulation enables continuous delivery of oxygen-rich blood to tissues.

Organs of Respiration (Gills)

• Oxygen dissolves in water, extracted using gills
• Internal gills are located inside the body, lining the pharyngeal slits, protected by operculum (bony fish) or interbranchial septum (cartilaginous fish).
• External gills hang outside the body (e.g., amphibian larvae)
• Gills have dense capillary beds for gas exchange, located in the capillaries, not in arteries, arterioles, veins, or venules
• Gills are respiratory organs for fish breathing water, lungs are respiratory organs for tetrapods breathing air

Additional Gill Details

• Agnathans have simple gill frills lining the interior of branchial pouches; in parasitic lampreys water flows in and out of pharyngeal slits (tidal ventilation)

• Elasmobranchs have more complex gill support structures

• Agnathans (e.g., lampreys) have no outer covering protecting the gill chamber

• In free-living lampreys water flows one-way through mouth and out gill slits and across gill surfaces for oxygen extraction

• In parasitic lampreys, mouth blocked, water flows in and out through gill slits = tidal ventilation (unlike most fish)

• Branchial muscles pump water in and out in fish gill ventilation

• Elasmobranchs (e.g., sharks) have soft skin forming a "flap valve" extending to gill rays for support inside the interbranchial septum

• Hemibranch only has gill lamellae on one side of a gill arch

• Holobranch has lamellae on both sides of a gill arch

Teleost Fish Gill Structures

• Teleost fish have more complex V-shaped gills with primary and secondary lamellae
• Adductor muscles control the amount of space between gills
• Water movement in teleost fish is via dual-pump mechanism (buccal cavity and opercular cavity); these expand or contract to create low or high pressure, respectively

Countercurrent Gas Exchange

• Countercurrent gas exchange in virtually all fish gills is much more efficient than concurrent exchange in transferring oxygen
• Blood and water flow in opposite directions to maintain a steep concentration gradient of oxygen
• This constant gradient is vital for a very efficient exchange of O2 from water into the blood

Ventilation Mechanisms

• Fish gill ventilation can be through a dual pump mechanism involving buccal and opercular cavities.
• Valves in elasmobranch gills, parabranchial ventilation in some species, are crucial for one-way controlled gill flow
• Some large fish also employ ram ventilation to obtain the flow of water needed for oxygen transfer as they swim, during this type of ventilation, forward movement is crucial to allow water to flow past the gills

Explore the fundamental concepts of the respiratory system, including the mechanisms of gas exchange and the adaptations that facilitate efficient diffusion. This quiz covers the roles of respiratory organs such as gills and lungs in extracting oxygen and distributing it to body tissues. Understand the importance of surface area and fluid circulation in maintaining optimal oxygen levels.