Respiration and Gaseous Exchange

A respiratory organ consists of a surface across which gas exchange by diffusion can occur between blood and either water or air
The surface must be

moist enough to allow the cells to live
large enough to permit sufficient gas exchange
thin enough to permit rapid diffusion
In respiration blood entering the respiratory organ must be high in CO2 and low in O2
both gases must move into and out of the body tissues through diffusion
requires a functional connection between the respiratory and circulatory system
the external air/water medium must be frequently replenished
The primary respiratory organs of vertebrates are gills and lungs, although the skin is sometimes used
External cutaneous respiration is the ancestral form of respiration found in most protochordates
During external respiration gas exchange occurs at the level of the skin and oxygen and carbon dioxide are passed into and out of tissues
the process still occurs in small vertebrates as long as they have low activity levels and live in cool flowing water or in damp air - frogs meet about half of their needs for gas exchange through their skin
Because most vertebrates are too large for each cell to interact directly with the environment, many organisms have evolved specialized organ systems to undertake the process of diffusion

Generally, fishes use gills and tetrapods use lungs, although the distinction is not absolute

Through ventilation of the organs of the respiratory system, gaseous exchange can occur

Ventilation of respiratory structures depends on

ram ventilation - forward momentum contributes to flow of water across the gill membranes
dual pump - buccal and opercular action operating in tandem drives water in a nearly continuous unidirectional flow across the gill curtain between them - the suction phase begins with compressed buccal and opercular cavities and closed valves
- as the buccal cavity expands, the internal oral valves open and water moves into the buccal cavity and across the gill curtain
- during the force phase, the oral valve closes and water is forced out through the opercular valve
pulse pump - the dual pump is modified into an inhalation/exhalation phase - the exhalation phase begins with transfer of spent air from the lungs into the buccal cavity
- the exhalation phase concludes with expulsion of air from the buccal cavity to the outside either through the mouth or under the operculum
- the inhalation phase begins with the organism taking fresh air into the mouth
- the inhalation phase concludes with transfer of air from the buccal cavity into the lungs
aspiration pump - air is sucked in, or aspirated, by low pressure created around the lungs - the lungs are located within the pump so that the force required to ventilate them is applied directly
- a moveable diaphragm and rib cage cause pressure changes rather than the action of the buccal cavity
Internal gills develop from the pharynx as evaginations called pharyngeal pouches
visceral grooves opposite to the pharyngeal pouches are separated from the pharyngeal pouches by a thin layer of tissue called the closing plate - the closing plates rupture in the embryo to establish the communication between the gill chamber and the surrounding medium
- tetrapods retain the first closing plate, which becomes the eardrum (tympanic membrane), while the remaining ones disappear
the pouches are also separated by the visceral arches, which combine to form the parabranchial gill chambers
the first visceral arch becomes the spiracle
The general structure of a mature gill is composed of several parts: gill rakers are cartilagenous or bony parts on the pharyngeal margin of the gill and function in preventing food particles from entering the gill chambers
gill rays are found within the interbranchial septa and provide support for the gill
gill filaments are the feather-like projections of the gills across which diffusion of gases occurs
gill filaments also possess gill lamellae, which are small crevices through which water passes for diffusion - lamellae are oriented parallel to the stream of water through the gills to maximize efficiency of diffusion
- the blood flow through the gills opposes the flow of water through the lamellae (countercurrent flow) and maximizes the efficiency of diffusion - this is important because water has about 1/30th the oxygen concentration of air
Three primary types of gill morphology are found in fishes: Holobranch - gill bar with anterior and posterior rows of gill filaments (jawed fishes)
Hemibranch - gill bar with gill filaments found on either the posterior or anterior side (sharks)
Pseudobranch - gill bar with posterior filaments modified to serve a nonrespiratory function such as sensory or salt balance - spiracular pseudobranch in rays and skates with much reduced hemibranch providing unobstructed flow of water for gill irrigation
Gills can also be used in excretion of nitrogenous wastes (in the form of ammonia) and regulation of salts in the body

There are three general variations in gills found in fishes:

Pouched gills - Agnatha - have external and internal pores rather than gill slits
- water is drawn into the gill chambers through the mouth and then passed over the gills
Septal gills - Elasmobranchs - have gill slits rather than pores and gill septa that help support gill filaments
- inspiration occurs through the mouth and expiration occurs through the gills - the exception is when the shark is feeding, when water moves into the pharynx through the spiracle
Opercular gills - bony fishes - have no septa (aseptal) but gill bars anchor gill filaments
- the operculum protects the filaments and expiration occurs through a single gill slit
External gills develop from the skin ectoderm of the branchial area but are not directly related to the visceral skeleton or branchial chambers
are found most often in larval or paedomorphic amphibians
Swim bladders and the origin of lungs

Lungs are found among fishes found in warm or stagnant water, as well as in primitive fishes, and allow for the fish to gulp air and undergo diffusion in an environment with relatively low dissolved oxygen

Such fishes undergo long periods of breath-holding (apnea) alternated with short periods of lung ventilation

Swim bladders
Swim bladders are similar to lungs, but are found in fishes that live in more oxygen-rich environments - thus, the air-filled spaces serve less of a purpose in respiration and function more as a hydrostatic organ

are connected to the pharynx by the pneumatic duct
make up approximately 4 - 11% of the body by volume
counters the increased density and sinking tendency from an ossified skeleton
gas is secreted into the swim bladder from blood by action of the gas glands or may be connected directly to the digestive tract via the pneumatic duct in primitive teleosts
air is added to the swim bladder to maintain its volume as fish dive and removed as the fish surfaces
gas glands may be associated with a countercurrent rete mirabile, which affects partial pressure and flow of oxygen into and out of the bladder
Lungs and their ducts

Tetrapod lungs are paired organs surrounded by pleura and contained in the pleural cavity

they have a higher surface to area volume ratio than the gills
are joined to the ventral side of the gut tube by the trachea
in general, 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 in the trachea splits into bronchi
the bronchi then lead to the lungs, which are themselves highly lobed
branching continues from the bronchi into bronchioles, then alveolar sacs, and end in alveoli - small sac-like structure within the lung 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
In the evolution of lungs from amphibians to mammals, several modifications to the respiratory structures are primarily associated with ventilation of the lungs In amphibians, ventilation of the lungs occurs through external nares and choanae rather than the mouth - air is drawn into the pharynx by muscle contraction that lowers the pharynx floor
In reptiles, muscle action against the ribs helps to change internal air pressure, causing inspiration - the action is assisted by contraction of the diaphragmatic muscle, which is not the same thing as the diaphragm
In birds, the lungs are half the size of the lungs of a similarly-sized mammal - however, the lungs connect to a system of air sacs in the bones and abdominal cavity, which increases the capacity to 2 - 3 times that of a similarly-sized mammal
- the result is to decrease overall body mass, but still maintain respiratory efficiency
- conducting passages continue to subdivide into parabronchi, with one-way airflow through the lungs
The primary mammal modification is the formation of the diaphragm dividing the thoracic and abdominal cavity - movement of air into the lungs is facilitated by contraction of the diaphragm to change the pressure in the chest cavity.
Vocalization in relation to respiration
The larynx is the primary organ that functions in producing sounds supported caudally by the cricoid cartilage, dorsally by the arytenoid cartilage, and by the addition of the thyroid cartilage in mammals
all support cartilages are derived from the visceral arches
The vocal cords themselves are flaps of epithelium supported by cartilage - produce bursts of air that can be modified by the pharynx, lips and tongue to produce speech

In birds, the syrinx is located at the distal end of the trachea and contains tympanic membranes to assist in sound production

consists of one or more tympanic-like membranes lying between cartilaginous rings in its wall
membranes are vibrated by air moving across them and changed into meaningful sounds by changes in tension on the tympani, by the configuration of the trachea and buccopharyngeal cavity, and by tongue movements
can be quite elaborate in some birds (such as cranes) and may be incorporated into the sternal keel  
 
Definitions Alvelolus - a small sac-like structure within the lung where gas exchange occurs (plural alveloi)

Closing plate - thin layer of tissue separating the pharyngeal pouch from the external environment

Gill filament - feather-like projection of the gill across which diffusion of gases occurs

Gill raker - bony part on the pharyngeal margin of the gill which functions in preventing food particles from entering the gill chamber

Gill rays - found within the gill filaments, and provides support for the gill

Hemibranch - gill bar with gill filaments found on either posterior or anterior side (sharks)

Holobranch - gill bar with anterior and posterior rows of gill filaments (jawed fishes)

Operculum - bony gill covering in teleosts that protects the gill filaments

Pseudobranch - gill bar with posterior filaments modified to serve a nonrespiratory function, such as sensory or salt balance. Found in the first gill bar of teleosts

Rete mirabile - a network of small arteries or capillaries associated withthe gas glands

Spiracle - the reduced first gill pouch of some fishes through which watermay enter the pharynx; also, the opening from the gill chamber of frogtadpoles

Surfactant - tension depressant found on the lining of the lungs

Syrinx - the voice box of birds, located at the distal end of the trachea