Chapter
11
Respiratory Systems
I.
Overview
1. Respiration can be either aerobic or anaerobic. Some organisms can switch between aerobic and anaerobic respiration.
2.
Cellular respiration is
the process by which mitochondria consume oxygen, produce carbon dioxide, and
supply the ATP.
3.
External respiration is the
process by which animals exchange gases with the environment to supply oxygen to
the mitochondria and to remove the resulting carbon dioxide.
4.
Larger animals,
however, rely on a
combination of bulk flow and diffusion for gas exchange. Bulk flow in the
circulatory system is a process called gas transport.
5.
Ventilation is an active
movement of the respiratory medium (air or water) across the respiratory
surface.
II.
Respiratory Strategies
1.
The physics of respiratory
systems
(1)
The ideal gas law n/V = P/RT
is the relationship between pressure, volume, and gas concentration. The
concentration of gas (number of moles per unit volume, n/V) is proportional to
the pressure at a constant temperature.
(2)
Henry's law [G] = PgasSgas
states that the amount of gas that will dissolve in a liquid (solvent) is determined by
the partial pressure of the gas (Pgas) and the solubility of the gas
in the liquid (Sgas).
(3)
Graham's law states that when
gases are dissolved in liquids, the relative rate of diffusion of a given gas is
proportional to its solubility in the liquid, and inversely proportional to the
square root of its molecular weight, diffusion rate = solubility/√MW.
(4)
Diffusion rate = D x A x Pgas
x Sgas/X x √MW, whereas D is the diffusion coefficient, A is the
cross-sectional area, and X is the diffusion distance.
2.
Types of respiratory systems
(1)
Three major respiratory
strategies to facilitate bulk flow of gases from external environment to every
cell in the body:
1)
circulating the external
medium through the body
2)
diffusion of gases across most
of the body surface (cutaneous respiration)
3)
diffusion across a specialized
respiratory surface
(2)
Circulating the external
medium through the body: this strategy is found in the sponges, cnidarians, and
many terrestrial arthropods.
(3)
Diffusion of gases across most
of the body surface (cutaneous respiration): this strategy is found in most
aquatic invertebrates, terrestrial annelid worms, and some vertebrates
(amphibians).
(4)
Diffusion across a specialized
respiratory surface: this strategy is classified as either gills or lungs.
III.
Ventilation and Gas Exchange
1.
The oxygen content of air is
almost 30 times that of water at 20oC. Thus, water-breathing animals
must ventilate their respiratory surface nearly 30 times more to move the same
amount of oxygen across the respiratory surface than do air-breathing animals.
2.
Ventilation and gas exchange
in water
(1)
In sponges (=
poriferans; Porifera), the beating of
flagellated choanocytes move water through ostia and into the spongocoel.
(2)
In cnidarians
(Cnidaria), muscle
contractions move water through the mouth into the gastrovascular cavity.
(3)
Most mollusks
(Mollusca) ventilate their
gills using cilia.
(4)
Crustacean gells are located
on the appendages.
(5)
Echinoderms
(Echinodermata) have diverse
respiratory structures. Most sea stars and sea urchins use their tube feet with
external gills for gas exchange. Brittle stars and sea cucumbers have
respiratory tree (so called lungs).
(6)
Jawless fishes (hagfish and
lamprey) have multiple pairs of gill sacs located at the anterior end of the
body. Elasmobranchs
(cartilaginous fishes) use a buccal pump for ventilation, while teleost
(bony) fishes use
a buccal-opercular pump for ventilation.
3.
Ventilation and gas exchange
in air
(1)
Arthropods
(Arthropoda) use a variety of
mechanisms for gas exchange. Some air-breathing chelicerates (spiders and
scorpions) have 4 book lungs. Some myriapods (centipeds and millipeds) have
tracheal systems, while some aquatic insects breathe through siphons or carry
air bubbles.
(2)
Both water- and air-breathing fish (lungfish)
ventilate their lungs
(swim bladders)
and gills using a buccal force pump similar to other fishes. Animals
similar to lungfish
(and
coelacanths
belonging to lobe-finned
fishes) are thought to be the common ancestor of the tetrapods
(amphibians, reptiles, birds, and mammals).
(3)
Amphibians use cutaneous
respiration, external gills, lungs (buccal force pump), or combination of these
three methods of gas exchange.
(4)
Reptiles ventilate their lungs
using a suction pump. During inspiration (inhalation), the volume of the chest
cavity increases, decreasing the pressure, and causing air to enter the lungs.
During expiration (exhalation), the volume of the chest cavity decreases,
increasing the pressure, and causing air to exit the lungs.
(5)
Birds ventilate their lungs
associated with many air sacs. Air enters the nares (singular: naris) and mouth, passing down the
trachea (plural: tracheae). The trachea divides into two primary bronchi
(singular: bronchus) that branch into secondary bronchi (dorsobronchi) as they
enter the lungs, then into parabronchi (smallest pathways of a bird lung),
lead into secondary bronchi (ventrobronchi),
and back into primary bronchi.
(6)
The alveoli (singular:
alveolus) are the site of
gas exchange in mammals.
1)
The mammalian respiratory
system is located within the chest cavity (thorax), and is divided into an upper
respiratory tract (mouth, nasal cavity, pharynx, larynx, and trachea) and a
lower respiratory tract (bronchi and alveoli).
2)
Air enters the nares and
mouth, passing through the pharynx and larynx, and entering the trachea. The
trachea divides into two primary bronchi which branch into secondary and
tertiary bronchi, and then bronchioles. The bronchioles terminate in
thin-walled, blind-ended sacs called alveoli that are the site of gas exchange.
(7)
The tidal volume (VT)
is the total volume of air moved in or out of the lungs in one ventilatory
cycle.
IV.
Gas Transport to the Tissues
1.
Oxygen transfer
(1)
The blood of many animals
contains specialized metalloproteins which have metal ions to bind oxygen
reversibly.
(2)
Hemoglobin (Hb) is the oxygen
carrier in vertebrate
erythrocytes, and Hb increases the oxygen carrying capacity by
50-fold.
(3)
There are 3 main types of
respiratory pigments:
1)
hemoglobins
2)
hemocyanins
3)
hemerythrins
(4)
Hemoglobins
1)
Hemoglobins are the most
common type of respiratory pigments in vertebrates, nematodes, some annelids,
some crustaceans, and some insects.
2)
A hemoglobin contains 4 globin
molecules which are members of the globin family. Hemoglobin is a tetrameric
protein (2 α-chains
and 2
b-chains)
with a molecular mass of 68 kDa.
3)
Each globin contains a heme
molecule, which has a porphyrin ring containing ferrous iron at the center.
4)
The iron molecules in
hemoglobin give vertebrate blood its reddish color (oxyhemoglobin when
deoxyhemoglobin is a dark maroon-red color).
5)
Myoglobin is a type of
hemoglobins found in muscles. Myoglobin is monomeric, whereas the blood
hemoglobin of vertebrates is tetrameric.
6)
The hemoglobins of annelids
(earthworms) contain
extracellularly nearly 150 globin molecules plus linker proteins that do
not contain heme.
7)
A few marine annelids have
chlorocruorins, also called as the green hemoglobins, that are composed of a
globin molecule complexed to an iron porphyrin (slightly differs from that in
heme of hemoglobin).
(5)
Hemocyanins
1)
Hemocyanins are found in
arthropods and mollusks.
2)
Hemocyanins contain copper
which is complexed directly to protein without a heme.
3)
Hemocyanins are very large
multimeric proteins containing up to 48 subunits per molecule.
4)
Hemocyanins are usually
dissolved in hemolymph at high concentrations, rather than being located within
blood cells.
(6)
Hemerythrins
1)
Hemerythrins are found in
species from 4 invertebrate phyla (sipunculids, priapulids, brachiopods, and
annelids).
2)
Hemerythrins do not contain
heme. Instead, iron is bound directly to protein.
3)
Hemerythrins are generally
trimeric or octameric molecules in which each subunit contains 2 iron ions.
4)
Most hemerythrins are found
inside circulating cells (pink blood cells) and in muscle cells.
(7)
The shapes of oxygen
equilibrium curves differ.
1)
Myoglobin is a monomeric
respiratory pigment containing a single heme molecule with one oxygen-binding
site. Thus, myoglobin exhibits a hyperbolic oxygen equilibrium curve.
2)
Hemoglobin is a tetrameric
respiratory pigment containing four heme molecules with four oxygen-binding
sites. The cooperative binding (cooperativity) is a phenomenon demonstrated by
multimeric proteins in which binding of a ligand to one subunit increases the
binding of other subunits. Thus, hemoglobin exhibits a sigmoidal oxygen
equilibrium curve.
(8)
The hemoglobin-oxygen affinity
is reduced by:
1)
decrease in pH
2)
increase in PCO2
3)
increase in temperature
4)
organic phosphate modulators
(e.g., 2,3-diphosphoglycerate (2,3-DPG), ATP, and GTP)
(9)
The Bohr effect: a decrease in
pH or increase in PCO2 reduces the oxygen affinity of a respiratory
pigment.
(10)
The Root effect: a decrease in
pH or increase in PCO2 causes not only a Bohr effect, but also a
reduction in the oxygen carrying capacity of the respiratory pigment in some
crustaceans, cephalopods, and many teleost fishes.
2.
Carbon dioxide transport
(1)
Carbon dioxide is much more
soluble in body fluids than is oxygen.
(2)
The majority of the CO2
is transported as bicarbonate (HCO3-) in blood.
(3)
Carbon dioxide reacts
spontaneously in water to form carbonic acid (H2CO3),
which can further dissociate into HCO3- and H+.
Carbonic anhydrase (CA) catalyzes the formation of bicarbonate in the red blood
cells.
(4)
Carbon dioxide binds to
hemoglobin and forms carbaminohemoglobin.
(5)
In mammals, approximately 70%
of the blood CO2 content is in the form of HCO3-
and
23% is in the form of carbaminohemoglobin in erythrocytes, and 7% is present as dissolved CO2
in plasma.
(6)
The Haldane effect: the
deoxygenated blood can carry more CO2 than can oxygenated blood.
Thus, deoxygenated hemoglobin at the tissues promotes CO2 uptake by
the blood, whereas oxygenated hemoglobin at the respiratory surface promotes CO2
uploading.
(7)
The chloride shift is the
exchange of chloride and bicarbonate (Cl-/ HCO3-)
across the erythrocyte membrane.
(8)
The respiratory system can
regulate blood pH. The normal pH of blood is approximately 7.4 in humans; a pH
above 7.7 or below 6.8 can be fatal. A Davenport diagram is a pH-bicarbonate
plot ahowing the blood buffer line in the relationships between pH, HCO3-,
and PCO2.
V.
Regulation of Vertebrate
Respiratory Systems
1.
Regulation of ventilation
(1) Breathing is automatically controlled by involuntary mechanisms (autonomic nervous system). Sensors in the medulla (medulla oblongata) monitor the pH of the cerebrospinal fluid as an indicator of CO2 level in the blood.
(2)
In vertebrates, the central
pattern generators (the central nervous system) are located within the medulla
of the brain to initiate ventilatory movements.
(3)
In mammals, these central
pattern generators are located in the caudal medulla.
(4)
The pre-Botzinger complex is
the primary respiratory rhythm generator of mammals located in the caudal
medulla.
(5)
The parafacial respiratory
group (pre-I complex), another neuronal complex, is coupled to the pre-Botzinger
complex. Neurons in the parafacial respiratory group fire before those in the
pre-Botzinger complex, and play a role in specifying timing of the rhythm.
2.
Environmental hypoxia
(1)
Hypoxia represents a condition
with low environmental oxygen.
(2)
Hypoxemia represents a
condition with low blood oxygen levels.