Chapter 41 Respiration

I. The Nature of Respiratory Systems
A. Factors That Affect Gas Exchange
1 Fick’s Law
a. Respiratory systems rely on the diffusion of gases down pressure gradients.
1) Partial pressures for each gas in the atmosphere can be calculated; for example,
oxygen’s is 160mm Hg.
2) Gases will diffuse down a pressure gradient across a membrane if it is permeable and moist.
b. According to Fick’s Law, the amount of diffusion depends on the surface area of the membrane
and the differences in partial pressure.
2 Surface-to-Volume Ratio
a. As an animal grows, its surface area increases at a lesser rate than its volume, making
diffusion of gases into the interior a problem.
b. Therefore, animals either must have a body design that keeps internal cells close to the
surface (flatworms) or must have a system to move the gases inward.
3 Ventilation
a. Animals have adaptations to move the air, or water, over the respiratory surfaces.
b. Bony fish move the covers over the gills; sponges move the flagella on their collar cells;
humans move the muscles of the thorax to expand and contract the chest cavity and
move air in and out of the lungs.
4 Transport Pigments
a. Hemoglobin is the main transport pigment.
b. It binds four molecules of oxygen in the lungs (high concentration) and releases them in the
tissues where oxygen is low.
5 Aquatic Environments
a. Water holds much less oxygen than does air and is more dense and viscous.
b. Water of high salt content and high temperature holds less oxygen; lack of sunlight and
stagnation also reduce oxygen content.
6 Land Environments
a. Air has more oxygen, but membranes are subject to drying.
b. Air-breathing animals can have their oxygen reduced by excess water in their environments.
B. Integumentary Exchange
1 In small animals with low metabolic rates, the epidermis at the body surface is used for
integumentary exchange.
2 For water dwellers, the surroundings keep the respiratory surface moist.
3 For terrestrial animals, like the earthworm, mucus helps keep the surface moist to allow the
oxygen to diffuse inward through the thin epidermis.
C. Specialized Respiratory Surfaces of Tracheas, Gills, and Lungs
1 These adaptations are needed because the integument may be too thick, too hardened, or too
sparsely supplied with blood vessels; and, of course, most animals are just too large to
rely on body surface for gas exchange.
2 Tracheas
a. These special air-conducting tubes are found in arthropods such as insects and spiders.
b. By repeated branching, fine tubules provide gaseous exchange at the cellular level—no
participation by the circulatory system is needed.
3 Gills
a. A gill has a moist, thin, vascularized epidermis.
b. External gills project from the body surface of a few amphibians and some insects.
c. The internal gills of adult fishes are positioned where water can enter the mouth and then
flow over them as it exits just behind the head.
1) Water flows over the gills and blood circulates through them in OPPOSITE DIRECTIONS.
2) This mechanism, called countercurrent flow, is highly efficient in extracting oxygen from
water, whose oxygen content is lower than air.
4 Lungs
a. Lungs contain internal respiratory surfaces shaped as a cavity or sac.
b. Simple lungs evolved about 450 million years ago to assist respiration in oxygen-poor
habitats; some evolved into swim bladders, others into complex respiratory organs.
c. Lungs provide a membrane for gaseous exchange with blood.
1) Air moves by bulk flow into and out of the lungs.
2) Gases diffuse across the inner respiratory surfaces of the lungs.
3) Pulmonary circulation enhances the diffusion of dissolved gases into and out of lung
capillaries.
4) In body tissues, oxygen diffuses from blood Æ interstitial fluid Æ cells; carbon dioxide
travels the route in reverse.

II. Human Respiratory System
A. Air-Conducting Portion
1 Through nasal cavities, air enters or leaves the respiratory system; hair and cilia filter dust and
particles. Blood vessels warm, and mucus moistens, the air.
2 Air moves via this route: pharynx Æ larynx (route blocked by epiglottis during swallowing) Æ
vocal cords (space between is glottis) Æ trachea Æ bronchi Æ bronchioles Æ alveoli.
B. Gas Exchange Portion
1 Human lungs are a pair of organs in the rib cage above the diaphragm.
2 Each lung lies in a thin-walled pleural sac, which leaves a very thin intrapleural space between
the membranes.
3 Inside the lungs, respiratory bronchioles bear outpouchings of their walls called alveoli, which
are usually clustered as alveolar sacs.
4 Alveoli provide a tremendous surface area for gaseous exchange with the blood located in the
dense capillary network surrounding each alveolar sac.

III. Air Pressure Changes in the Lungs
A. Ventilation
1 To inhale, the diaphragm contracts and flattens, muscles lift the rib cage upward and outward,
the chest cavity volume increases, internal pressure decreases, air rushes in.
2 To exhale, the actions listed above are reversed; the elastic lung tissue recoils passively.
B. Lung Volumes
1 About 500 ml of air enters and leaves with each breath (tidal volume).
2 The maximum volume that can be moved in and out is called the vital capacity.

IV. Gas Exchange and Transport
A. Gas Exchange in Alveoli
1 Each alveolus is only a single layer of epithelial cells through which gases can readily diffuse into
interstitial fluid and blood capillaries.
2 The partial pressure gradients are sufficient to move oxygen in and carbon dioxide out of the
blood, passively.
B. Gas Transport Between Lungs and Tissues
1 Blood cannot carry sufficient oxygen and carbon dioxide in dissolved form as the body requires;
hemoglobin helps enhance its capacity.
2 Oxygen Transport
a. Oxygen diffuses down a pressure gradient into the blood plasma Æ red blood cells Æ binds to
hemoglobin (4 molecules/hemoglobin to form oxyhemoglobin).
b. Hemoglobin gives up its oxygen in tissues where partial pressure of oxygen is low, blood is
warmer, partial pressure of carbon dioxide is higher, and pH is lower; all four conditions
occur in tissues with high metabolism.
3 Carbon Dioxide Transport
a. Because carbon dioxide is higher in the body tissues, it diffuses into the blood.
b. Seven percent is dissolved in plasma, 23 percent binds with hemoglobin to form
carbaminohemoglobin, and 70 percent is in bicarbonate form.
c. Bicarbonate and carbonic acid formation is enhanced by the enzyme carbonic anhydrase,
which is located in the red blood cells.
C. Controls Over Respiration
1 Gas exchange in the alveoli is most efficient when air flow equals the rate of blood flow.
2 Neural Controls
a. The nervous system controls oxygen and carbon dioxide levels for the entire body by
adjusting contraction rates of the diaphragm and chest wall muscles.
b. The brain monitors input from carbon dioxide sensors in the bloodstream and from receptors
sensitive to decreases in oxygen partial pressure (carotid bodies and aortic bodies).
3 Local controls within the lungs correct imbalances in air and blood flow by constricting/dilating
both bronchioles and arterioles.

V. Respiration in Unusual Environments
A. Decompression Sickness
1 If a diver ascends too rapidly, the change in pressures will force the nitrogen to leave the tissues
of the body and pass into the blood, often as bubbles.
2 The bubbles can cause pain in the joints (“bends” or decompression sickness) or can obstruct
blood flow to vital organs such as the brain.
B. Hypoxia
1 Hypoxia occurs when tissues do not receive enough oxygen.
2 At high altitudes, the partial pressure of oxygen is lower than at sea level; hyperventilation may
occur.
3 Carbon monoxide combines with hemoglobin 200 times faster than does oxygen; CO poisoning can
result.