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Chapter 44
GAS EXCHANGE - RESPIRATORY SYSTEM
The exchange of gases between an organism and its environment is called respiration.
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Organismic respiration brings oxygen from the environment to the cells.
Aerobic respiration occurs within the cell in the mitochondria.
Ventilation is the movement of air or water over the respiratory surfaces.
In order for oxygen and carbon dioxide to diffuse across a cell membrane, they must dissolve in water.
Water is more viscous and dense than air and the aquatic animal must spend a lot of its energy moving
water over the gills.
Aquatic animals spend 20% of its energy while terrestrial animals spend 1 -2 % of its total energy.
Air contains much larger concentration of oxygen than water.
Terrestrial animals have to compensate for water loss during breathing.
Respiratory surfaces must be maintained moist and air has to pass through a long series of tubes to reach
these surfaces.
TYPES OF RESPIRATORY SURFACES
1. Body surface. Used by small animals with low metabolic rate.
2. Tracheal tubes that deliver oxygen to all parts of the body.
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It consists of a network of tracheal tubes that open on the body surface through up to 20 tiny
openings called spiracles.
3. Gills containing capillaries.
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Echinoderms have dermal gills.
Chordates usually have internal gills.
In bony fish the gills are protected by a bony plate, the operculum
Counter current system is an efficient method of obtaining oxygen.
4. Lungs formed by in-growth of the body surface or from the wall of the body cavity.
Spiders have "book lungs".
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Located in an inpocketing of the abdominal wall.
Open to the outside by a spiracle.
A series of plates rich in hemolymph separated by air spaces.
Osteichthyes have a swim bladder.
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It is used to control buoyancy.
Lungfishes use it breath air at certain times in their life cycle.
Amphibians and reptiles have simple lungs.
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The lungs of toads and frogs are simple sacs with ridges that increase the respiratory surface.
Some amphibians do not have lungs and exchange gases through the skin.
Reptiles have sacs with folding of the wall to increase the respiratory surface.
Birds have the most efficient respiratory system of any living vertebrate.
Their lungs have air sac extensions that reach into many parts of the bird's body.
Hemoglobin increases the capacity to transport oxygen by about 75 times.
There are several types of hemoglobin.
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All contain iron as part of a heme group.
Heme group is bound to a protein called globin.
Protein portion varies in size and AA in different species.
It is present in some invertebrates like annelids, nematodes, mollusks and arthropods.
Hemocyanins are copper containing proteins found in arthropods and some mollusks.
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Lack heme group.
Dissolved in the blood rather than contained in cell.
Blue when combined with oxygen.
It is found many species of mollusks and arthropods.
HUMAN RESPIRATORY SYSTEM
The human respiratory system is typical of air-breathing vertebrates.
Nostrils are the opening of the nose.
Nasal cavities moisten, warm and filter the air.
Pharynx or throat is used also by the digestive system.
Larynx also called "voice box" contains the vocal cords and is supported by a cartilage.
Epiglottis is a small flap of tissue that closes the larynx during swallowing.
Trachea or windpipe is supported by rings of cartilage.
Bronchi are branches of the trachea that lead to each lung.
Bronchioles and alveoli make most the lungs.
Each lung is covered with a pleural membrane, which also lines the thoracic cavity.
Pleural cavity is the space in between the pleural membranes and it's filled with a fluid.
The alveoli are tiny air sacs at the end of the bronchioles and are lined with a very thin epithelium.
Capillaries surround the alveoli.
Gas exchange occurs in the alveoli of the lungs.
The lungs as such consist mostly of air tubes and elastic tissue with a very large internal surface.
Passage of air:
Nostrils → nasal cavities → pharynx → larynx → trachea → bronchi→ bronchioles → alveoli.
BREATHING
Breathing is the mechanical processes of moving air from he environment into the lungs (inspiration)
and expelling the air from the lungs (expiration).
During inspiration, the volume of the thoracic cavity is increased by contraction of the diaphragm.
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Contraction moves the diaphragm downward.
With the increase in volume in the thoracic cavity, the pressure drops and air is forced in by the
atmospheric pressure.
Expiration occurs when the diaphragm and rib muscles relax.
The normal amount of air inhaled at rest is called tidal volume.
The vital capacity is the maximum amount of air a person can exhale after filling the lungs to the
maximum extent.
O2 and CO2 are exchanged between alveoli and blood by diffusion.
The difference in partial pressure of oxygen between the inhaled air and the blood allows the oxygen to
diffuse.
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Pox= 160 mm in air and Pox= 40 mm in venous blood.
Therefore oxygen diffuses into the blood.
Pox= 100 mm in arterial blood; Pox= 0- 40 mm in tissues.
Therefore oxygen diffuses into the tissues.
Fick's law: the greater the partial pressure difference and the larger the surface area, the
faster the gas will diffuse.
About 97% of the oxygen is transported as oxyhemoglobin and 3% dissolves in the plasma.
The maximum amount of oxygen that can be transported by hemoglobin is called the oxygen carrying
capacity.
The actual amount of oxygen bound to hemoglobin is the oxygen content.
The ration of oxygen content to oxygen capacity is the percent oxygen saturation.
Oxyhemoglobin dissociates faster in an acid medium.
Lactic acid released during muscular activity lower the pH and therefore decreases the affinity of
hemoglobin to oxygen and oxygen is released more easily in the muscles.
Bohr effect: Changing the blood pH affects the % of O2 saturation of blood.
CARBON DIOXIDE TRANSPORT.
Carbon dioxide is transported mainly as bicarbonate ions.
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About 70% of the CO2 dissolves in the plasma and forms HCO3- and H+ lowering the pH.
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About 7% 10% dissolves in the plasma.
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About 20% enters the red blood cells and combines with hemoglobin.
This reaction occurs in the RBC catalyzed by carbonic anhydrase.
CO2 + H2O → H2CO3 → H+ → HCO3Most of the H+ released from carbonic acid combine with hemoglobin.
Many of the bicarbonate ions leave the RBCs and diffuse into the plasma.
Chloride ions diffuse into the RBC to replace the bicarbonate ions. This is known as the chloride shift.
As CO2 diffuses out of the alveolar capillaries, the resulting lower CO2 concentration reverses the
previous reaction.
BREATHING
Breathing is regulated by respiratory centers in the pons and medulla.
Chemoreceptors sensitive to increases in CO2 and H+ and to low O2 concentrations regulate the
respiratory centers.
Neurons originating in the medulla send messages to the diaphragm and intercostal muscles causing them
to contract and inspiration occurs.
After several seconds the neurons become inactive, the muscles relax, and expiration occurs.
Hyperventilation reduces the concentration of CO2 in the blood.
Certain amount of CO2 is needed to maintain normal blood pressure.
CO2 stimulates vasoconstrictors in the brain in order to maintain muscle tone in the walls of blood
vessels.
As altitude increases, barometric pressure decreases and less oxygen enters the blood and leads to
hypoxia.
A rapid drop of barometric pressure that produces gas bubbles in the blood causes decompression
sickness or bends.
Dissolved gases and some liquids bubble out of solution when the barometric pressure drops below the
total pressure of the gases in solution.
Diving mammals have high concentration of myoglobin, an oxygen binding pigment found in muscles.
Myoglobin stores oxygen in diving mammals up to ten times more than in terrestrial mammals.
Diving reflex reduces the heart rate, blood is redistributed and other physiological changes occur that
allow the diving mammal to conserve oxygen.
Polluted air results in bronchial constriction, increase mucus secretion and damage to ciliated cells.
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Chronic bronchitis causes the production of too much mucus and damaged ciliated cells
cannot expel it. The body resorts to coughing as the way of clearing respiratory passages.
Emphysema is caused the loss of elasticity in the alveoli and air cannot be expelled
effectively. Also, gas exchanged is impaired. The right ventricle compensates by pumping
harder and becoming enlarged. Heart failure is common among emphysema patients.
Lung cancer.