Bio 205 Comparative Anatomy

Respiratory functions of respiratory system

Introduction

Vertebrate cells require oxygen to function and need to get rid of waste products like CO2. Vertebrates are too big for oxygen to diffuse into cells from outside our bodies. Hence there has to be a way to transport oxygen from the external environment to the cells. The resiratory system, which exchanges gases between the blood and external environment, and the circulatory system, which carries O2 and waste products around the body, are these transport systems.

Some background

• respiration is the exchange of gas (O2 and CO2) across a membrane
• do not confuse this with cellular respiration, which is the aerobic oxidation of sugar or fat
• ventilation is the movement of fluid across the respiratory membrane. This could be the movement of air in (inspiration) and out (expiration) of the lungs or the movement of water past the gills
• basic mechanism - essentially, for efficient gas exchange, an animal needs a thin but large surface with the oxygen-rich fluid (air or water) on one side and blood on the other. The more energetic the animal, the more oxygen the animal needs and the larger surface area that is required.

Respiratory design in humans and other mammals

• mammals have a external and internal nares with a nasal cavity that is divided from the oral cavity by a secondary palate
• the nasal cavity contains turbinate bones covered with an epithelium. These provide excess surface area, probably to warm inspired air
• the larynx of many mammals is modified into a structure for vocalization. We won't talk about this, unfortunately.
• the trachea divides into primary bronchi, which divide into secondary bronchi, which divide about 14 more times into bronchioles, which divide a few more times and finally end in a little sac called an alveolus. This is called the bronchial tree.
• alveoli are very thin walled, only a single layer of squamous epithelium with a thin basement membrane. Dense capillary networks cover the alveoli. Consequently, there is a thin respiratory membrane between the air in the alveolus and the blood in the capillary. The total surface area of the respiratory membrane in a human is about 70 square meters.
• the alveolar wall contains elastic fibers that are part of the lung skeleton. Other important elastic tissues are in the thoracic wall
• mammals inspire air by increasing the volume of the thoracic, which lowers the thoracic pressure, which expands the alveoli, which sucks air through the mouth into the bronchial tree. Thoracic volume is increased by contraction of the diagphragm, a dome-shaped muscle (convex up) that divides the thorax from the abdomen. Contraction causes the diaphragm to flattten (or the dome to lower), thus increasing thoracic volume.
• the expanding thorax and lung during inspiration stretches the elastic tissue of the thorax and lung. The stored strain energy in this elastic tissue powers expiration.
• This bellows mechanism of sucking in air with an elastically powered expriation is called an aspiration pump.

Why mammals need this design and (most) other vertebrates do not

• mammals are homeothermic and the cells need huge amounts of oxygen. Consequently, we need a huge respiratory membrane while [most] other vertebrates do not. In fact, the lungs of many other vertebrates are just a big balloon or a big ballon with internal walls or septae.
• To get alot of oxygen, we need to move alot of air. There are two ways of moving air: squeezing it, forcing into the respiratory structure, or expanding the respiratory structure, which sucks air in. We could squeeze air from our nasal cavity into our lungs but this would be a pitifully small volume. But the squeeze mechanism works well in vertebrates that don't need huge amounts of air or water.

Respiratory organs

skin

gills

• endoderm derivative in pharyngeal region, heavily vascularized
• in general. one way transport (with some exceptions)

lungs

• diverticulum of pharynx
• usually heavily vascularized
• in general. two way transport (with some exceptions)

Survey of respiratory functions in Vertebrates

Primitive pattern for vertebrates

• lampreys - external gills. two-way transport due to parasitic lifestyle.

Most fishes and amphibians

sharks

• no lung
• parabranchial chamber lateral to each gill and closed off from outside by flap valve.
• dual pump for gill ventillation

1. suction pump - parabranchial flap valves close while pharynx and parabranchial chamber expand sucking in water through mouth and spiracle and across gills into parabranchial chamber
2. force pump - mouth and spiracle close, pharynx compressed and water is pushed out through open valve

• ram ventillation - swimming by the shark moves water through the mouth and through the branchial chamber

Actinopterygian fishes

• modified dual pump for gill ventillation - have operculum and opercular cavity
1. expansion of oropharynx and opercular cavity with close operculum sucks water in through mouth
2. compression of oropharynx pushes water out of oropharynx and opercular cavity.
• primitive osteichthyian fishes have lungs (more derived osteichthyan fishes have lungs that don't function as respiratory organs - these are the gas bladders)
• four stroke pulse pump or buccal pump for lung ventillation

1. elastic recoil of lung pushes used air into expanding buccal cavity
2. buccal compression to expel used air
3. buccal expansion to suck in fresh air
4. buccal compression to push fresh air into lung

Powered by pharyngeal and hypobranchial muscles (i.e. cranial)

Sarcopterygian fishes and amphibians

the four stroke buccal pump is found in some aquatic amphibians but more commonly, sarcopterygians have a two-stroke buccal pump

1. pharynx expans sucking in air from outside. Elastic recoil of lung pushes used air into buccal cavity, which mixes with air from outside, excess air is escapes outside
2. buccal compression pushes some of this mixed air into the lungs but most of it outside

In a study of the larval tiger salamander, Ambystoma tigrinum, 80% of inspired gas is fresh. This results from the unequal amount of air that is sucked into the buccal cavity and air that is pushed into the lung. A large volume of fresh air is sucked in and mixes with a small volume of used air. A small volume of this 80/20 mixture is pushed into the lung but most is expelled.

The lungs are slightly compartmentalized with internal septa that act as the respiratory surfaces

frogs are essentially the same but the two-stroke pump can be divided into four stages

1. buccal expansion sucks in air through the open external nares
2. glottis opens and used air exits lungs (via elastic recoil) into buccopharyngeal cavity and out external nares with little mixing with fresh air (which is at floor of cavity)
3. nares close and buccal cavity is compressed to force fresh air into lungs
4. glottis closes

Some salamanders may do it a little differently also, by combining an expiration pump to compress the lungs and push used air into the buccal cavity. This expiration pump is powered by axial muscles, primarily the transversus abdominus m.

Amniotes

have evolved an aspiration pump, in which thoracic expansion decreases the pressure in the lungs which then sucks in the air from the external environment (via the buccal/nasal cavities). More efficient then a buccal pump because air can be moved from outside into lung in one phase (not two as into either 4 stroke pump of actinops or 4 stage pump of frogs).

Lepidosaurs

aspiration pump

• represent the primitive state?
• bilateral contraction of intercostals rotates causes abducts ribs by craniolateral rotation. This increases thoracic cavity, drops thoracic pressure and sucks in air.
• reverse rotation of ribs compresses thorax and pushes out fresh air

constraints

• locomotion hinders respiration because (1) squeezes lungs and (2) contraction of intercostals on one side to aid lateral undulation interferes with respiratory cycle
• dorosventral undulation facilitates breathing while walking/running
• Solution to veranid paradox - buccal (gular) pumping. So some amniotes have retained buccal pumping but this is only used during active locomotion.

turtles

• cannot use ribs to aspirate lungs because these are immobile
• use movement of the plastron and limbs to compress and expand thoraccic cavity

crocodiles (http://cas.bellarmine.edu/tietjen/images/lung_structure_and_ventilation_i.htm)

• hepatic piston pump
• ventral and dorsal rotation of pubes
• may be primitive to many archosaurs

dinosaurs

birds

• elevation and depression of ribs compresses and expands thorax
• one way system that is highly efficient
• large divsion of septae
• nice pages
  • (http://cas.bellarmine.edu/tietjen/images/lung_structure_and_ventilation_i.htm)
  • http://www.biology.eku.edu/RITCHISO/birdrespiration.html