Lecture 17: Respiratory System

Question 1

Give the function of the respiratory system

Answer 1

• Facilitate the exchange of respiratory gases (oxygen and carbon dioxide) between the external and internal environments of the organism.
• Internally, the respiratory gases are transported by the circulatory system.

Question 2

What are the respiratory system’s requirements?

Answer 2

• There must be sufficient surface area across which gas diffusion can occur rapidly and efficiently.
• This surface area must be well vascularized.
• The surface area must be kept moist (gases must be dissolved in water in order to diffuse across the membrane).
• some sort of protection for the delicate gas exchange surfaces is generally required.

Question 3

What is the respiratory system like in amphioxus?

Answer 3

• Uses its gill slits for filter feeding

* Gas exchange mostly occurs through the integument.

Question 4

What is the respiratory system like comparative to amphioxus in other vertebrates?

Answer 4

• Most vertebrates:
• Skin is too thick or is covered by structures that prevent diffusion, such as dermal or epidermal scales, feathers, or hairs.
• In larger animals, the surface area is not great enough to allow for adequate gas exchange.
• Most vertebrates, including fishes, develop additional gas exchange surfaces.
• In fishes these are usually modified gill slits.
• Some fishes also developed lungs.
• Some tetrapods use cutaneous surfaces and/or gills.
• Most tetrapods use lungs.

Question 5

Restate the purpose of the pharynx

Answer 5

• In higher vertebrates the pharynx is a short segment of the gut tract.
• In fishes it is well-developed.
• The pharynx begins just caudal to the oral cavity and is the anterior end of the gut tube, leading directly into the esophagus.
• The main characteristic of the pharynx:
• Presence of pharyngeal pouches, arches, and slits

Question 6

Describe respiration in the lamprey

Answer 6

• In higher vertebrates the pharynx is a short segment of the gut tract.
• In fishes it is well-developed.
• The pharynx begins just caudal to the oral cavity and is the anterior end of the gut tube, leading directly into the esophagus.
• Longitudinal partition that divides the pharynx into a dorsal esophagus and a ventral respiratory tube. Lateral and parallel to the respiratory tube on both sides are a series of spherical pouches, the respiratory pouches.
• The pouches open to the respiratory tube via internal gill slits and to the exterior via external gill slits.
• The respiratory pouches are lined with gill lamellae, thin epithelial shelf-like processes that are highly vascularized and that serve as the actual gas exchange surfaces.
• This divided pharynx in the lamprey is a specialization for the sucking method of feeding used by the lamprey.

Question 7

Give the generalized structure of a fish gill

Answer 7

• Gill slits are elongated and open into the pharynx via internal gill slits and to the exterior via external gill slits.
• Phylogenetically and ontogenetically the pharynx is a filter-feeding system redesigned for gas exchange with the environment.
• The gill slits are no longer needed for filter feeding
• Nevertheless, all vertebrate embryos develop a pharynx with gill slits.
• In fishes and a few amphibians these gill slits persist in the adult as functional structures.
• Each gill arch is associated with a skeletal, muscular, vascular, and nerve component.
• The gill lamellae are usually found on both sides of a gill septum.
• Such a gill, consisting of gill lamellae on both sides of the supporting septum, is called a holobranch.
• If the septum is covered only on one side by gill lamellae, it is called a hemibranch.
• Usually there is some kind of mechanical support for the actual gas exchange surfaces of the gills.
• This may be in the form of:
• Fleshy septum extending out from the gill arch
• Pairs of stiffer gill rays.
• The highly vascularized gill lamellae, where gas exchange actually occurs, are located on the septum or rays.
• See Slides 12-16

Question 8

Describe the gill slits in fish (part II)

Answer 8

• The first gill slit in chondrichthyes is usually reduced:
• The hyoid element from the second arch cuts across the gill slit in order to brace the upper jaw.
• Called a spiracle
• Bottom-dwelling sharks and skates:
• Spiracle is large
• Can be used for water intake when the mouth of the fish is blocked by food or the searching for food along the ocean floor.
• Fishes living in open waters, tend to considerably reduce or lose their spiracles.
• Not counting spiracles, the highest number of gill slit pairs in modern fishes is generally five
• Note that the lampreys and hagfishes are exceptions to this general rule.
• Sharks utilize a suction pump mechanism.
• During inspiration the mouth is closed and the volume of the pharynx is expanded, drawing water into the pharynx
• It is then forced across the gill surfaces by the closing of the mouth and the contraction of the pharynx.
• Some fast-swimming sharks may also use a positive pressure ventilation called ram ventilation.
• In teleosts an alternating negative-positive pressure is also used.
• Water is drawn into the pharynx by opening the mouth and lowering the pharyngeal floor.
• The mouth is then closed and the operculum is raised, lowering the pressure in the opercular chamber.
• This draws water from the pharynx across the gill surfaces and out.

Question 9

Describe swim bladders and lungs

Answer 9

• Most actinopterygian fishes also possess a swim bladder in addition to gills.
• The swim bladder develops as a dorsal outgrowth from the pharynx or anterior gut region.
• The gas volume in the swim bladder can be decreased or increased at will.
• It functions as a hydrostatic organ, allowing fish to control their depth in the water with little muscle effort.
• The swim bladder develops as an evagination from the foregut region and is initially connected to the gut tube by a duct (pneumatic duct).
• In most fishes, swim bladders lose their duct.
• Gas volume in these disconnected swim bladders is regulated internally by a gas-producing organ called the red body.

Question 10

Describe the red body

Answer 10

• Red body consists of:
• Gas gland
• An extensive system of capillaries called the rete mirabile
• • The rete mirabile functions to keep the gas (mostly oxygen) from being reabsorbed back into the blood from the swim bladder.
• The red body is located at the anterior end of the swim bladder.
• A gas-absorbing tissue is found at the posterior end of the swim bladder.

Question 11

Describe the fate of the pneumatic duct

Answer 11

• Because the swim bladder is dorsally located, we would expect the pneumatic duct, when present, to come off the roof of the pharynx rather than the floor.
• This is essentially the pattern seen in the sturgeon, the gar, the bowfin, and in some teleosts that still retain the duct.
• In the more primitive teleost, Erythrinus, the pneumatic duct comes off the pharynx from a lateroventral position, even though the swim bladder itself is dorsally located.
• In the primitive actinopterygian fish, Polypterus, the pneumatic duct comes off the ventral floor of the pharynx, and the paired sacs attached to it are more ventrally located than dorsally.
• This ventral position of the air sacs and the pneumatic duct is identical to the position of the lungs and trachea in tetrapods.
• For this reason, and also because of evidence from the vascular structures supplying the swim bladder in primitive fishes, it has been strongly suggested that lungs in fishes preceded the swim bladder.
• See Slide 24
• Two of the three extant species of lung fishes living today do exactly this.
• The third species lives in permanent bodies of water.
• Thus, lungs evolved before swim bladders as an accessory gas exchange mechanism.
• Later, when fishes moved into the oceans, which do not periodically dry up, their lungs were no longer needed and may have evolved into swim bladders.
• Hydrostatic organs are more useful than lungs to a fish that lives in an open permanent body of water.

Question 12

Describe the amphibian respiratory system

Answer 12

• Adult anurans replace their larval gills with lungs but also utilize the skin and floor of the buccal cavity as gas exchange surfaces.
• Amphibian larvae have external gills.
• Anuran larval external gills are replaced by internal gills in older anuran larvae but are retained throughout larval life in the salamanders.
• In some salamanders (i.e., Necturus) the external gills are retained throughout adult life as well.
• This is an example of neoteny.
• Some salamanders (the plethodontids) lack gills and lungs in the adult.
• Even in those amphibians that utilize lungs, the simplicity of these structures usually necessitates the use of other means of gas exchange as well.
• In Necturus the lungs are simple, long narrow sac-like structures with very little internal subdivision.
• In anurans the lungs are still small but in most show some level of internal subdivision.
• During ventilation, frogs use an alternating negative-positive pressure system.
• Air is drawn into the buccal cavity by opening the valved nostrils and lowering the floor of the buccal cavity.
• The nostrils are then closed and the floor of the buccal cavity is raised, creating positive pressure and forcing air into the lungs through the glottis.
• This pumping sequence is repeated several times in order to fill the lungs with air.
• The frog may utilize the air in the lungs for several minutes without again emptying and refilling the lungs.
• See Slide 30

Question 13

Describe the mammalian lung

Answer 13

• Mammals, like reptiles and birds, utilize a pair of lungs for respiratory gas exchange.
• Lungs (Figure 10-3) are divided into two or more lobes, each served by a secondary bronchus.
• Each lobe is divided into lobules, each served by a tertiary bronchus.
• The lobules are subdivided into smaller and smaller units.
• The smallest units where gas exchange actually occurs are called alveoli.
• Alveoli are microscopic sac-like structures that are intimately associated with blood capillaries.
• Each alveolar wall is a single cell-layer thick. Respiratory gases must diffuse across the single cell-layer thickness of the capillary wall and the alveolar wall.

Question 14

Describe the trachea, glottis, epiglottis, and larynx

Answer 14

• Respiratory gases pass to and from the lungs via the trachea.
• The trachea opens into the floor of the pharynx, which in mammals is quite short.
• At the opening of the trachea is the larynx, where the vocal cords and related cartilages and muscles are located.
• The opening into the trachea at the top of the larynx is the glottis.
• Guarding the glottis against the entrance of solid particles such as food is a flaplike structure called the epiglottis.
• During swallowing movements, the epiglottis deflects particles that might otherwise enter and possibly block the trachea.

Question 15

Describe the bronchi and alveoli

Answer 15

• The trachea bifurcates into a pair of primary bronchi, each of which supplies a lung.
• Each primary bronchus then divides into two or more secondary bronchi, each of which supplies a lobe of the lung.
• Secondary bronchi divided into tertiary bronchi, each supplying a lobule.
• There are approximately 16-17 additional levels of branching, terminating in alveoli.
• In reptiles the lungs lie alongside the heart and are separated from it by the pericardial membranes.
• There is a fibrous connective tissue layer, the transverse septum, which separates the lungs from the abdominal contents and creates a pleural cavity on either side of the heart.

Question 16

Describe the pleural cavities, and diaphragm

Answer 16

• In mammals the pleural cavities are well demarcated from the abdominal cavity by a muscular sheet, the diaphragm.
• The diaphragm is an inverted bowl-shaped sheet consisting of striated muscle tissue and is, therefore, under voluntary control.
• However, the breathing centers in the brainstem normally control breathing rate at a subconscious level. The movement of the diaphragm in mammals is performed by a series of striated muscles which converge from all sides toward its center.
• These muscles originate from the sternum, ribs, and lumbar vertebrae.
• Innervation is provided by the phrenic nerve which arises in the cervical region.
• It is suggested that the diaphragm probably originated from the anterior part of the rectus musculature and migrated posteriorly in premammalian history to accommodate the expanding lungs.

Question 17

Describe the diaphragm’s relationship with the space of the pleural cavity

Answer 17

• In order for the diaphragm to function the pulmonary cavities must be completely closed off from the outside and from other body cavities.
• The pulmonary cavities are lined with a serous membrane, the parietal pleura.
• This membrane is continuous with the visceral pleura that covers the lungs themselves.
• The parietal and visceral pleura secrete a thin serous fluid that helps to lubricate the membranes and allows them to slide over one another during breathing movements without generating friction.
• The potential space between the two pleural membranes is the pleural cavity.

Question 18

Describe the mechanism the diaphragm uses to alter the volume of the pleural cavity

Answer 18

• The diaphragm functions by flattening out when it contracts.
• This lowers the floor of the diaphragm in the pleural cavities and increases the volume of the cavities.
• The increase in volume of these closed cavities decreases air pressure within the cavities, causing air to be drawn into the lungs from the exterior.
• Relaxation of the diaphragm along with the elastic recoil of the lungs causes the air to move back to the exterior.

Question 19

What other ways can the pleural cavity’s volume be changed?

Answer 19

• The volume, and therefore the air pressure, in the thoracic cage can also be altered by movements of the rib cage, such that the rib cage volume itself changes.
• Change in rib cage volume depends on the ability of the ribs to move both at their attachment to the thoracic vertebrae as well as their attachment at the sternum.
• Outward and upward movement of the rib cage results in inspiration and is accomplished by muscles that attach to the rib cage.
• Normally these are the external intercostal muscles.

Question 20

What other muscles are important in controlling pleural volume and what problems or conditions can halt or hinder respiratory activity?

Answer 20

• But other muscles such as the scalenes and the serratus anterior (ventralis) are also important.
• Expiration is primarily due to the elastic recoil of the lungs, but forced expiration may result again from muscular activity (i.e., the internal intercostals).
• Note that any puncture of the rib cage will cause loss of the ability to form a vacuum and will hinder respiratory activity.
• Likewise, any interference with the ability of the ribs to move at the vertebral column or sternum will also create problems with breathing movements.

Question 21

See Slides 40-46

Answer 21

I dunno, just felt like this needed it’s own card space.