C. RESPIRATORY SYSTEM
1. Differentiate external and internal respiration.
External respiration – gas exchange between environment and body occurs by diffusion; specialized organs are used to accomplish gaseous exchange; ex. skin surface, gills, lungs, etc.
Internal respiration – gas exchange between blood and cells occurs by diffusion;
2. Define ventilation and diffusion.
Ventilation – mechanism of bringing gas in contact with respiratory exchange surface; ex. moving water through gills in fish, breathing to move gas to lungs in mot tetrapods
Diffusion – the spontaneous movement of gases, without the use of any energy or effort by the body, between the gas in the alveoli and the blood in the pulmonary capillaries
3. Name an animal that uses cutaneous respiration.
Frogs use cutaneous respiration, or “skin respiration.”
4. Name the germ layer from which external gills develop. Elaborate on the function of external gills and name a species that has these gills.
External gills develop from skin ectoderm. External gills function in respiration using a diffusion barrier (as in lungs and internal gills). They are found in some urodeles and dipnoans.
5. Discuss the development of internal gills, including descriptions of pharyngeal pouch, visceral groove, visceral arch and aortic arch.
Pharyngeal pouches form in the pharynx; visceral grooves form on the outside of the pouches; closing plates are partitions between the inside pharyngeal pouches and the outside visceral grooves, but these plates rupture in embryos to form slits (in tetrapods, the first closing plate does not rupture and forms the eardrum); visceral arches separate adjacent pouches and support the gill bars (which contains arteries or aortic arches); in agnathans, the first pharyngeal pouch becomes a typical gill chamber, but in other fish is it lost, reduced and modified into the spiracle; in tetrapods; this first pharyngeal pouch becomes the cavity of the middle ear.
6. Define spiracle.
A spiracle is a water intake hole. In species where they are present, other spiracles provide oxygenated blood directly to the eye and brain through a separate blood vessel.
7. Compare the development of internal gills in primitive fish, jawed fish and tetrapods.
Fate of 1st pharyngeal pouch: in Agnatha becomes gill chamber, in Other fishes lost or modified to become cavity of spiracle, in Tetrapods becomes cavity of middle ear; closing plates rupture in gill-bearing vertebrates, in tetrapods the 1st is retained as the ear drum (tympanum), but the other leave no derivatives; 1st visceral arch becomes jaws of gnathostomes; 2nd visceral arch usually supports jaws in fishes, contributes to middle ear in tetrapods; More posterior arches support gill bars of fishes and become various derivatives in tetrapods (hyoid apparatus)
8. Describe the following gill structures:
Gill bar – one of the bony or cartilaginous arches on each side of the pharynx that support the gills of fishes and aquatic amphibians; part of the visceral skeleton; contains blood vessels, nerves, branchial muscles, respiratory epithelium
Gill septae – interbranchial septum that supports gills and connects gill bar to body surface
Gill rakers – trap debris carried in the water over the gills
Gill filaments – complex structures, primary lamellae which have a large surface area. Off each are numerous smaller secondary lamellae. Tiny blood capillaries flow through the secondary lamellae of each gill filament. The direction of blood flow is opposite to that of water flow. This ensures that as the blood flows along each secondary lamella, the water flowing beside it always has a higher oxygen concentration than that in the blood. In this way oxygen is taken up along the entire length of the secondary lamellae.
9. Contrast holobranch, hemibranch, and pseudobranch.
Holobranch – a single gill bar with anterior and posterior rows of gill filaments
Hemibranch – a gill bar with filaments on one surface only
Pseudobranch – filaments on posterior surface of mandibular arch, serve a nonrespiratory function
10. Trace the blood flow through the gill.
When a fish swims, water moves over its gills from anterior to posterior. Blood flow in the gill capillary bed is oriented from posterior to anterior. The blood picks up O2 from the external water (and loses CO2) as it flows through the gill capillaries.
11. Describe how countercurrent flow allows maximal O2/CO2 exchange.
The blood in the capillaries flows in the opposite direction from the water in the adjacent channels. Dissolved gases diffuse faster between fluids with a large difference in gas concentration (a high concentration gradient) than between fluids with only a small difference. In the fish gill, low-oxygen blood enters the capillaries, encountering water at the end of its travel through the gills, which is thus relatively low in oxygen. As blood travels in the direction opposite to the water, it encounters “fresher” water with ever-higher oxygen concentrations. Thus, along the capillary, a steep diffusion gradient favors transfer of oxygen into the blood.
12. Address any other functions of gills besides gas exchange.
The primary function of gills is respiration; other functions include osmoregulation and the excretion of salts (in marine fishes) and nitrogenous wastes.
13. Name the three classifications of gills. Describe their structures, ventilation, and name a species that possesses each type.
Pouched gills – In pouched gills, each pouch has an internal and external pore, and there may be 5-15 pouches in cyclostomes, depending on the group. In some types of hagfish the external pores open via a single duct to the outside; in other agnathans there are individual external openings. In hagfishes, water enters the single median naris and passes through the pharynx and out through the gill pouches. In lampreys, a tidal-flow mechanism is used whereby water enters and leaves through the external gill slits. In cyclostomes, their mouths are sometimes occupied (parasitic lifestyle) so they intake water through their branchial pores then pass it out back through the same pores.
Septal gills – These are also called elasmobranchs, hence the name given to sharks. The openings are vertical slits rather than pores as in living agnathans. The first gill chamber of sharks is reduced to a spiracle. Each anterior and posterior surface of a gill slit has a gill surface called a demibranch or hemibranch). The two hemibranchs of a single gill arch are called a holobranch. The interbranchial septum separates the two hemibranchs in sharks; the septum is supported by cartilaginous gill rays. The gill surface of the spiracle is called a pseudobranch; sharks draw water into their pharynx through the mouth and spiracles; they force water through their gill slits; (they can suck up water through their gill slits and spit it out their spiracles and mouth to clean the gills or reject food.)
Opercular gills – These are protected by the bony operculum (part of the dermal opercular series of the skull) which functions in a pump-and-valve-system for respiration. The septa are very poorly developed or absent in bony fish. A spiracle is present in Latimeria, Polypterus, and Chondrostei; the pseudobranch may be present even if the spiracle is absent. In tetrapods the spiracle becomes the middle ear cavity and Eustachian tube. Some lungfish lose the more anterior holobranchs. Ventilation is very similar to septal gills
14. Discuss the development/evolution of the swim bladder.
Gas bladders (swim bladders) are evolutionarily closely related to lungs. It is believed that the first lungs, simple sacs that allowed to gulp air under oxygen-poor conditions, evolved into the lungs of today’s terrestrial vertebrates and into the gas bladders of today’s fish. In embryonal development, both lung and gas bladder originate as an outpocketing from the gut; in the case of gas bladders, this connection to the gut continues to exist as the pneumatic duct in more “primitive” teleosts, and is lost in the higher orders.
15. Describe the structure and function of the swim bladder. Define gas gland, rete mirabile, pneumatic duct, and weberian ossicles.
This organ serves many functions. It is a gas-filled organ that is used for regulating buoyancy and equalizing internal and external pressures. Gases, including nitrogen, carbon dioxide, oxygen and hydrogen, are produced by the gas gland, a modification of the inner lining of the bladder, which works with the rete mirabile to force gases into the bladder. Another specialized structure, called the rete mirabile (meaning “wonderful net”) pumps gases into the bladder. The rete is made up of tightly packed capillaries, arranged so that those carrying incoming blood are right next to those carrying outgoing blood. This allows a very efficient exchange of blood gases. In order for the bladder to hold and release these gases, there is a channel that connects the bladder to the esophagus called the pneumatic duct. This connecting duct is present in most larval fishes while their swim bladder completes its development, and only some – the physostomes – retain this duct through maturity. In some fishes such as the Grunts (Family Pomadasyidae) the swim bladder is believed to function as a resonator to amplify the grunting sounds the fish make by grinding their pharyngeal teeth. In some catfish, carp, and minnows, weberian ossicles function in sound and pressure reception by transmitting sound waves to the inner ear in the skull.
16. Contrast physostomous and physoclistous.
Physostomous – fish with a connection (pneumatic duct) between the gas bladder and the pharynx, e.g. many freshwater fishes
Physoclistous – fish with a closed pneumatic duct (i.e. without any connection between swim bladder and pharynx)
17. Compare fish and tetrapod respiration.
Fish extract oxygen from the water and transfer it to their bloodstream. This is done by gills, lungs, specialized chambers, or skin, any of which must be richly supplied with blood vessels in order to act as a respiratory organ. Extracting oxygen from water is more difficult and requires a greater expenditure of energy than does extracting oxygen from air. Water is a thousand times more dense (heavier per unit volume) than air, and at 20 deg C (68 deg F) it has 50 times more viscosity (resistance to flow) than air and contains only 3% as much oxygen as an equal volume of air. Fishes, therefore, have necessarily evolved very efficient systems for extracting oxygen from water; some fishes are able to extract as much as 80% of the oxygen contained in the water passing over the gills, whereas humans can extract only about 25% of the oxygen from the air taken into the lungs.
Tetrapod lungs are paired organs surrounded by pleura and contained in the pleural cavity. They have a higher surface to area volume than the gills, and are joined to the ventral side of the gut tube by the trachea. In tetrapods, any increase in overall body size leads to an increased amount of compartmentalization of the lungs. During respiration, air enters through the mouth, or into the external nares to the choanae, and then passes into the pharynx. From there, air travels through the glottis to the trachea, and then in the trachea splits in to bronchi. The bronchi then lead to the lungs, which are themselves highly lobed. In the lungs, there is further branching into a respiratory tree. Branching continues form the bronchi into bronchioles, then alveolar sacs, and end in alveoli. Alveoli are smaller saclike structures within the lungs where gas exchange occurs. As in gills, the diffusion of oxygen and carbon dioxide is facilitated by counter-current flow in the alveoli. The lining of the lungs is lubricated by surfactant, a tension depressant. Surfactants are generally lipoproteins and reduce the resistance to lung expansion as well as the energy needed to fill the lungs.
18. Characterize the respiratory tree of anurans and urodeles. Name their laryngeal cartilages.
Anurans and urodeles have large, short lungs and a short trachea that divides into two short bronchi. The larynx is positioned at the end of the trachea, supported by a pair of cartilaginous arcs. These are divided into an anterior pair of arytenoid cartilages and posterior cricoid cartilages that form a ring.
19. Discuss ventilation in amphibians, reptiles, and mammals.
Amphibians ventilate by using a pulse pump to force movement of water or air
Reptile ventilation includes inspiration – suction in amniotes to produce negative pressure in chest cavity – and expiration – passive due to recoil.
Mammal ventilation utilizes a diaphragm. As it contracts, it moves caudally which creates negative pressure in the thoracic cavity causing air to move into airways during inspiration. As it relaxes, it moves cranially resulting in air passively moving out of airways.
20. Discuss the respiratory anatomy in reptiles and mammals.
Air enters the body through the nose, is warmed, filtered, and passed through the nasal cavity. Air passes the pharynx (which has the epiglottis that prevents food from entering the trachea). The upper part of the trachea contains the larynx. The vocal cords are two bands of tissue that extend across the opening of the larynx. After passing the larynx, the air moves into the bronchi that carry air in and out of the lungs. Bronchi are reinforced to prevent their collapse and are lined with ciliated epithelium and mucus-producing cells. Bronchi branch into smaller and smaller tubes known as bronchioles. Bronchioles terminate in grape-like sac clusters known as alveoli. Alveoli are surrounded by a network of thin-walled capillaries.
21. Name the laryngeal cartilages unique to mammals.
In addition to arytenoids and cricoids, mammals posses thyroid cartilage (“Adam’s apple”) and an epiglottis which prevents food from going into the larynx.
22. Characterize the unique respiratory tree of birds. What are air sacs?
In birds, the trachea splits into 2 primary bronchi and forms the syrinx, the vocal organ, at that point. Also unique to a bird’s respiratory system are large air sacs join lungs and serve in ventilation. Air sacs are devoid of respiratory epithelium and serve to ventilate the system by providing continuous air flow to the lungs. Birds possess 2 abdominal air sacs, 2 posterior thoracic air sacs, 2 anterior thoracic air sacs, 2 cervical air sacs, and 1 interclavicular air sac.
23. Define syrinx.
The syrinx is a bird’s vocal apparatus, located in the interclavicular air sac region. There are no vocal cords in the larynx region of a bird.