RESPIRATORY SYSTEM = responsible for bringing oxygen to blood in lungs or gills and expelling CO2 from blood.
- Contains large surface area by folding and refolding upon itself; also exhibits:
1) steep diffusion gradient for O2, CO2
2) short diffusion distance
3) high degree of vascularization
4) coordinated air and blood movement
- Diffusion of gases occurs 2 layers of thin squamous epithelium w/ basal lamina; gases must be dissolved in fluid in respiratory tract to diffuse (causes water loss problems in Tetrapods)
Respiratory Structures:
1) Lungs - Terrestrial
2) Gills - Aquatic, in some fish buccal cavity and anterior stomach serve as accessory respiratory structures
3) Cutaneous - amphibians (esp. lungless salamanders)
- Terrestrial Situation = bidirectional flow (expired and inspired air over same surfaces), except in birds (unidirectional crosscurrent system)
- Aquatic Situation = water flow over gills is unidirectional, countercurrent exchange = blood and water pass in opposite directions
Terrestrial Respiratory System consists of paired lungs and branching system of airways bringing air to respiratory surface for gas exchange.
- Sequence (anterior to posterior, for mammal):
NASAL CAVITIES
- Paired, divided by nasal septum
- Open to outside via external (anterior) nares, open interiorly to nasopharynx via internal nares (choanae)
- Walls of nasal cavities of hyaline cartilage and bone to keep them open
- Consist of 3 regions, anterior to posterior:
1) Vestibule = lined by str. squamous with filtering hairs, sweat and sebaceous glands present
2a) Olfactory Region = contains olfactory mucosa, dorsally situated
2b) Respiratory Region = contains respiratory mucosa, ventrally situated
Olfactory Epithelium = PSC containing:
1) Olfactory Receptor Cells = modified bipolar neurons with dendrite that extends to free surface with olfactory cilia and axon extending into Lamina Propria; large spherical nucleus found in middle of epithelium
2) Sustentacular Cells = columnar cells with microvilli, oval nucleus in uppermost region of epithelium
3) Basal Cells = basal location, triangular
4) No goblet cells present
5) Nuclear-free Border is 1/5 height of epithelium
6) L. propria with serous Bowman's Glands to rinse away stimulus molecules and dissolve incoming stimulus molecules
Respiratory Epithelium = ciliated PSC with goblet cells; nuclear-free border about 1/3 of epithelial height; located ventrally in nasal cavity and lines larger respiratory tubes
- In nasal cavity L. propria composed of cavernous (erectile) tissue - swelling gives plugged nose; L. propria also contains mucoserous glands
Nasal Conchae (Turbinates) = shelves of bone projecting medially from lateral walls of nasal cavity
- Covered by a mucous membrane
- Act to increase surface area of nasal cavity; function as countercurrent heat exchanger (exhaled air temperature is less than body temperature) and therefore decrease respiratory water loss (since cold air holds less water than warm air)
RESPIRATORY TUBES Larynx = connects pharynx to trachea, tube supported by cartilage interconnected by dense CT and skeletal muscle; functions to guard respiratory tract from possible food entry and produces voice
- Vestibular Folds of mucous membrane extend from lateral walls (false vocal cords). They lie above and protect the Vocal Folds (vocal cords) beneath.
- Vocal Ligaments = elastic fibers in vocal cords with skeletal muscle at core. Contraction of muscle changes tension and length of cords and opening between them, thereby creating changes in voice.
- Epiglottis = extension of anterior wall of larynx, supported by elastic cartilage plate; covers opening to larynx to keep food out
Trachea = shares a large number of characteristics with lesser ducts; contains mucosa, submucosa, fibrocartilaginous coat (tunic) of adventitia and cartilage; mucosa and submucosa separated by an elastic membrane
SEE HANDOUT FOR CROSS-SECTIONAL STRUCTURE OF RESPIRATORY TUBES
Alveolus = terminal division of respiratory system where gas exchange occurs; open into alveolar sac, atrium, alveolar duct or respiratory bronchiole.
- Alveoli are tightly packed, partitions (interalveolar septa) separate individual alveoli. These septa are composed of basement membrane + delicate elastic and reticular fibers and numerous capillaries
- Alveoli lined primarily by extremely flat simple squamous epithelial cells (Type 1 Pneumocytes)
- Alveolar - Capillary Barrier = simple squamous pneumocyte + fused basement membrane + simple squamous endothelial cell lining capillary. Varies from 0.2 - 2.5 µm in thickness.
- Type 2 Pneumocytes are scattered along alveolar walls. These are rounded cells incorporated into simple squamous epithelium. Secrete surfactant = phospholipid film that counteracts surface tension forces on alveoli preventing their collapse on expiration.
- Fibroblasts and Macrophages may be present in interalveolar septa (Septa Cell).
