Respiratory System - USD Biology

Respiratory System - USD Biology

Respiratory System Forms as an outpocketing from the pharynx region of the digestive tract Embryonically, all vertebrates possess a series of paired outgrowths of the gut tube projecting laterally to meet the ectoderm of the body wall. These outgrowths are termed pharyngeal pouches, and the corresponding ectodermal invaginations of the body wall are termed pharyngeal grooves.

Together, a pouch plus a groove make up a pharyngeal gill slit. Mesoderm is restricted to columns between the pouches = pharyngeal (branchial) arches. Pharynx In most primitive vertebrates, the number of gill slits is higher (6-14) than in more advanced vertebrates (3).

Evolutionary trend = reduction in number of gill slits in more advanced vertebrates. Primitively, the pharynx was used in filter-feeding (e.g., Amphioxus) Structurallly, the pharynx forms from only a short tube connecting the mouth with the esophagus; but the pharyngeal derivatives form the respiratory system (gills and lungs) and certain glands.

Pharynx Pharynx is shared between digestive and respiratory tracts. Fate of pharyngeal arches in jawed fishes and tetrapods: The first branchial arch (mandibular) forms the jaws The second arch (hyoid) functions in jaw support, support of respiratory tubes, sound transmission,

and respiration (gills) The remainder of the arches form gills (in fish) or derivatives (in tetrapods) Fig 7.5 Fates of branchial arches in gill-breathing vertebrates Gills Develop from branchial arches plus ectodermal and endodermal lining of the arches.

Mesoderm contributes to the skeletal and muscular components (gill arches). The epithelial lining of the arches becomes folded as filaments; lamellae form as folds in the lining of the filaments. Gas exchange occurs between water and capillaries in the gill lamellae in a countercurrent arrangement. This is the most efficient arrangement for gas transfer.

Fig 11.19 Gill ventilation and gas exchange in teleost fish ANATOMY of GILLS IN ELASMOBRANCHS (primitive) AND TELEOSTS (advanced) (Chondrichthyes vs. Osteichthyes) Elasmobranchs Teleosts

Gill septum extends to surface Gill septum reduced to base of filaments Gill slits open individually to surface exit Gill slits open to common chamber (opercular chamber) with single opening to exterior

Spiracle present = slit between mandibular (1) Spiracle absent and hyoid (2) arches Fig 11.18 Gill ventilation and gas exchange in a shark External Gills Outgrowths of epithelium covering gill arches

3-5 in extant vertebrates Occur in larval lungfish, amphibians, and some adult aquatic amphibians Gas exchange is facilitated by muscles located at the gill base acting to wave the gills through the water, thereby creating currents to move new oxygen-rich water over the gill surface. Fig 11.4 Gill

coverings in vertebrate animals Lungs and Swim Bladders Embryonic Origin Lungs = ventral outpocketing from floor of pharynx Swim Bladder = dorsal outgrowth from pharynx

Phylogenetic Origin Lungs are characteristically associated with Tetrapods, but they are more primitive. Lungs are present in: Dipnoans (Lungfishes) Polypterus (bichir) = most primitive living Actinopterygian Evidence for lung-like structures in a Devonian placoderm suggests that lungs are a very ancient structure.

Lung Evolution It is likely that some ancient fishes used a vascularized mouth and pharynx for accessory air breathing (to supplement gill-breathing, as in some modern fishes and amphibians). Any outgrowth of the pharynx would increase surface area for respiration and would presumably be advantageous. A continuation of this trend would then result,

eventually, in the formation of lungs. Swim Bladder Consists of elongate sac arising as a dorsal outgrowth from the pharynx Present in most Actinopterygians Major Function = hydrostatic organ (accessory respiration function in Holosteans) Polypterus (bichir) has no swim bladder, but has paired

ventral lungs Thus, it appears that the swim bladder appeared early in the history of the Actinopterygians by the lungs becoming singular and assuming a dorsal position The only intermediate condition is in Erythrinus (a Teleost) which shows a lateral attachment of the swim bladder to the gut

Swim Bladder Conditions Physostomous = swim bladder retains an open connection to the digestive tube (= pneumatic duct) Most Primitive = Chondrosteans, Holosteans; Pneumatic duct is short and broad, connects to back of pharynx Primitive Teleosts have a narrow elongate pneumatic duct opening to the digestive tract at the esophagus or more posteriorly (Salmonids, etc.)

Physoclistous = connection with digestive tube is lost entirely Swim bladder fills with gas by a complicated countercurrent gas exchange system Occurs in advanced teleosts Fig 11.22 Swim bladders

Mammal Lungs Air passages branch to form tree-like structures Passages terminate in thin-walled sacs (alveoli) where gas exchange occurs Flow is bidirectional (inspiration and expiration occur over the same passageways). Lungs are relatively large, reside in pleural cavity separated from coelomic cavity by

diaphragm Mammals = only vertebrates with a diaphragm Mammal Lungs Breathing is accomplished by a negative pressure system. Outward movement of ribs + downward movement of diaphragm expands lung cavity creates a partial vacuum air rushes in to

equilibrate pressure Fish Lungs Lungs are simple sacs with few internal folds (Polypterus - low surface area) or a minor degree of subdivision (Lungfish) Lungfish lungs are dorsal, but the duct arrangement suggests this is a secondary condition because it opens to the ventral

pharynx Bidirectional flow as in mammals Amphibian/Reptile Lungs Amphibians similar to lungfish condition some supplementary respiratory surface provided by ridges and septa in epithelial lining Reptiles show a modest internal subdivision Bidirectional air flow.

Amphibians (and fish) = lungs filled by buccal pump (positive pressure) swallowing air Reptiles = lungs filled by suction pump system similar to mammals, but not as efficient since no diaphragm is present Bird Lungs Complicated air sac system (no gas exchange in air sacs) allows unidirectional (1-way) flow over lungs

Gas exchange occurs in air capillaries that open to a tube (parabronchus) through which air flows in only one direction Blood flow in avian lung is via a crosscurrent mechanism relative to air flow This functions similar to a countercurrent system, and is more efficient for oxygenating blood than the bidirectional flow system of other Tetrapods This explains their success at high altitudes relative to

mammals Fig 11.21 Fish lungs Fig 11.27 Reptile lungs Fig 11.34 Mammalian lung Figs 11.35 & 36

Bird respiratory system Figs 11.37 & 38 Bird respiratory system

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