Energy that we consume in our food is temporarily stored in the bonds of ATP (adenosine triphosphate) before being used by the cell. Cells use ATP for movement and to drive chemical reactions. Normally, after food is consumed, it is digested and broken down into simple compounds including glucose. The energy in glucose is used to synthesize ATP by a process called cellular respiration. Oxygen is needed for cellular respiration because it picks up hydrogen atoms and that were part of the glucose molecule. In this process, it forms water. The remaining carbon and oxygen atoms from glucose form carbon dioxide. The equation below summarizes the process of cellular respiration.

C₆H₁₂O₆  +  6O₂    6CO₂  +  6H₂O  +  36ATP

The role of the respiratory system is to provide oxygen so that cellular respiration can occur and to eliminate the carbon dioxide that is produced as a waste product.

78% N₂, 21% O₂, 1% argon, noble gases, CO₂

Diffusion refers to movement of molecules from an area of higher concentration to an area of lower concentration.

Partial pressure is the pressure exerted by one gas in a mixture.

Total atmospheric pressure at sea level = 760 mm Hg.

Partial pressure O₂ = 760 X .21 = 160 mm Hg.

Gasses move by diffusion from areas of higher partial pressure to areas of lower partial pressure.

Introduction

Cellular respiration requires O₂ and produces CO₂. The lungs are designed to absorb O₂ and to eliminate CO₂.

Oxygen must be dissolved in water before animals can take it up. Therefore, the lung surface must always be moist.

The surface area of a human lung is 60 to 80 sq. meters.

pharynx ®  epiglottis (open space is the glottis) ®  larynx with vocal cords ®  trachea ®  bronchi ®  bronchioles ®  alveoli

hair and cilia filter dust and particles.

Blood vessels warm air and mucus moistens air.

To inhale, the diaphragm contracts and flattens.

Muscles move the rib cage which also contributes to expanding the chest cavity.

To exhale, the muscles relax and elastic lung tissue recoils.

Choking results when food enters the trachea instead of the esophagus.

The Heimlich maneuver can force air out of the lungs to dislodge the obstruction.

Hemoglobin is a protein that carries oxygen and is found in the blood of most animals.

It is synthesized by and is contained within erythrocytes (red blood cells).

Oxygen is bound reversibly to the iron portion.

Hemoglobin increases the oxygen-carry capacity of the blood by 70 times. 95% of the oxygen is transported by hemoglobin, 5% in blood plasma.

The bright red color occurs when it is bound with oxygen.

Gas Exchange in humans occurs in alveoli. Gasses must diffuse across the alveolar wall, a thin film of interstitial fluid, and the capillary wall.

Partial pressures

	LUNGS	TISSUES
OXYGEN	high	low
CO2	low	high

The partial pressure of CO₂ is higher in the tissues because respiring tissues produce CO₂ as a result of the breakdown of glucose (C₆H₁₂O₆) during cellular respiration.

1 hemoglobin molecule + 4 oxygen molecules ®  oxyhemoglobin.

The amount of oxygen that combines depends upon the partial pressure. More oxygen is loaded at higher partial pressures of oxygen.

Hemoglobin does not necessarily release (unload) all of its oxygen as it passes through the body tissues. Oxyhemoglobin releases its oxygen when:

the partial pressure of O₂ is low.

the partial pressure of CO₂ is high. High CO₂ causes the shape of the hemoglobin molecule to change and this augments the unloading of oxygen.

the temperature is high.

the pH is low.

Active tissues need more oxygen and all of the conditions listed above are characteristic of actively metabolizing tissues. Therefore, these tissues receive more oxygen from hemoglobin than less active tissues.

CO (carbon monoxide) binds to hemoglobin 200 times faster than O₂ and does not readily dissociate from the hemoglobin. Small amounts of CO can cause respiratory failure.

Carbon dioxide is transported to the lungs by one of the following ways: 

dissolved CO₂

bound to hemoglobin (HbCO₂)

HCO₃^- (bicarbonate ions).

Most is transported as bicarbonate ions because...

CO₂  +  H₂O   «    H₂CO₃   «    HCO₃^-  +  H⁺

     Low     ¬  CO₂ partial pressure ®     High
    (lungs)                                                (tissues)

The equation above moves toward the right when the partial pressure of CO₂ is high. When the partial pressure of CO₂ is low, it moves to the left and CO₂ comes out of solution.

In the active tissues, the CO₂ partial pressure is high, so CO₂ becomes dissolved in water, forming H₂CO₃, which then forms HCO₃^- and H⁺. In the lungs, the partial pressure of CO₂ is low because the concentration of CO₂ in the atmosphere is low. As blood passes through the lungs, HCO₃^- + H⁺ form H₂CO₃ which then forms CO₂ + H₂O.

Carbonic anhydrase (in red blood cells) speeds up this reaction 150 times. HCO₃^- tends to diffuse out of the red blood cells into the plasma.

Control of breathing rate

Eliminating CO₂ is usually a bigger problem than obtaining O₂. The body is therefore more sensitive to high CO₂ concentration than low O₂ concentration.

During inhalation, the diaphragm and intercostal muscles are stimulated. Other neurons inhibit these when exhaling.

Respiration is not under voluntary control.

Monitoring H⁺ and CO₂

Chemoreceptors in the respiratory control center of the brain (medulla oblongata) detect changes in CO₂ by monitoring pH of cerebrospinal fluid. Recall that an acid is a solution with a high H⁺ concentration. High CO₂ therefore lowers the pH as shown below.

CO₂ + H₂O «  H₂CO₃ «  HCO₃^- + H⁺

Chemoreceptors in the aorta and carotid artery are also sensitive to pH and to greatly reduced amounts of O₂.

Bronchiole diameter

The primary bronchus branches extensively into bronchioles. Terminal bronchioles are surrounded by smooth muscle.

The diameter of the bronchioles (and blood vessels) increases or decreases in response to needs. It is adjusted by smooth muscle under the control of the nervous system. The parasympathetic nervous system (discussed in the chapter on nervous systems) stimulates these muscles to contract, reducing the diameter of the airways. This is advantagous when the body is relaxing and breathing is shallow. Narrow bronchioles result in less air remaining within the lungs after each exhalation. 

The sympathetic nervous system relaxes these muscles as a response to stressful situations. This allows a more rapid rate of intake and expulsion of air.

Allergens trigger histamine release which constricts muscles.

Narrower bronchioles result in decreased ventilation of the lungs.

Severe attacks may be life-threatening.

Large particles are filtered out by the nose.

Small particles are filtered out by cilia lining the bronchi and bronchioles.

Bronchitis is an inflammation of the airways that causes mucous to accumulate. The normal cleansing activity of cilia is reduced and not sufficient to remove the mucous. Coughing attempts to clear the mucus.

Smoking and other irritants increase mucus secretion and diminish cilia function.

Emphysema occurs when the alveolar walls lose their elasticity. Damage to the walls also reduces the amount of surface available for gas exchange.

Emphysema is associated with environmental conditions, diet, infections, and genetics. It can result from chronic bronchitis when the airways become clogged with mucous and air becomes trapped within the alveoli.

Cigarette smoke prevents the cilia from beating and stimulates mucus secretion.

Coughing is necessary to expel excess mucous but it contributes to bronchitis and emphysema.

Cigarette smoke also kills phagocytic cells in respiratory epithelium. These cells normally help rid the lungs of foreign particles and bacteria.

Cigarette smoke contains compounds that are modified in the body to form carcinogens.

Smoking causes 80% of lung cancer deaths.