1. Chapter 15: Respiratory System

2. The Respiratory System
The respiratory system works with the cardiovascular system to exchange gases between the air and blood (external respiration) and between blood and tissue fluids (internal respiration).
Inspiration and expiration move air in and out of the lungs during breathing.
Cellular respiration is the final destination where ATP is produced in cells.

3. The respiratory tract
The respiratory tract extends from the nose to the lungs, which are composed of air sacs called alveoli. Gas exchange occurs between air in the alveoli and blood within a capillary network that surrounds the alveoli. Notice that the pulmonary arteriole is colored blue – it carries O2-poor blood away from the heart to the alveoli. The pulmonary venule is colored red – it carries O2-rich blood from alveoli toward the heart.

4. The Respiratory Tract
Air is cleansed, warmed, and moistened as it passes the cilia and mucus in the nostrils and nasal cavity.
In the nose, the hairs and the cilia act as a screening device.
In the trachea, the cilia beat upward, carrying dust and mucus into the pharynx.
Exhaled air carries out heat and moisture.

5. The Nose The two nasal cavities are divided by a septum.
They contain olfactory cells, receive tear ducts from eyes, and communicate with sinuses.
The nasal cavities empty into the nasopharynx.
Auditory tubes lead from the middle ears to the nasopharynx.

6. The path of air
This drawing shows the path of air from the nose to the trachea.

7. The Pharynx
The pharynx (throat) is a passageway from the nasal cavities to oral cavities and to the larynx.
The pharynx contains the tonsils; the respiratory tract assists the immune system in maintaining homeostasis.
The pharynx takes air from the nose to the larynx and takes food from the oral cavity to the esophagus.
The pharynx has three parts: the nasopharynx, the oropharynx, and the laryngopharynx.
The tonsils contain lymphocytes that protect against invasion of foreign antigens that are inhaled.

8. The Larynx
The larynx is a cartilaginous structure lying between the pharynx and the trachea.
The larynx houses the vocal cords.
A flap of tissue called the epiglottis covers the glottis, an opening to the larynx.
In young men, rapid growth of the larynx and vocal cords changes the voice.
Vocal cords are folds of mucous membrane supported by elastic ligaments, which are stretched across the glottis. Pitch of the voice is controlled by regulating the tension on the vocal cords. Volume of the voice depends on the amplitude of vibrations of the vocal cords.

9. Placement of the vocal cords
a. Frontal section of the larynx shows the location of the vocal cords inside. b. Viewed from above, it can be seen that the vocal cords are stretched across the glottis. When air passes through the glottis, the vocal cords vibrate, producing sound. The glottis is narrow when we produce a high-pitched sound (top), and it widens as the pitch deepens (bottom).

10. The Trachea
The trachea, supported by C-shaped cartilaginous rings, is lined by ciliated cells, which sweep impurities up toward the pharynx.
Smoking destroys the cilia.
The trachea takes air to the bronchial tree.
Blockage of the trachea requires an operation called a tracheostomy to form an opening.

11. Cilia in the trachea

12. The Bronchial Tree
The trachea divides into right and left primary bronchi which lead into the right and left lungs.
The right and left primary bronchi divide into ever smaller bronchioles to conduct air to the alveoli.
An asthma attack occurs when smooth muscles in the bronchioles constrict and cause wheezing.

13. The Lungs
Lungs are paired, cone-shaped organs that lie on either side of the heart and within the thoracic cavity.
The right lung has three lobes, and the left lung has two lobes, allowing for the space occupied by the heart.
The lungs are bounded by the ribs and diaphragm.

14. The Alveoli
Alveoli are the tiny air sacs of the lungs made up of squamous epithelium and surrounded by blood capillaries.
Alveoli function in gas exchange, oxygen diffusing into the bloodstream and carbon dioxide diffusing out.
Infant respiratory distress syndrome occurs in premature infants where underdeveloped lungs lack surfactant (thin film of lipoprotein) and collapse.
Surfactant lowers the surface tension within the alveoli and prevents them from collapsing.

15. Gas exchange in the lungs
The lungs consist of alveoli surrounded by an extensive capillary network. Notice that the pulmonary artery carries O2-poor blood (colored blue), and the pulmonary venule carries O2-rich blood (colored red).

16. Mechanism of Breathing
During breathing, air moves into the lungs during inspiration (inhalation) from the nose or mouth, then moves out again during expiration (exhalation).
A spirometer allows measurement of the components of air during breathing.

17. Respiratory Volumes
Tidal volume, the normal amount of air moved in and out of the lungs when relaxed, is usually 500 ml.
Inspiratory reserve volume is the maximum amount of forcibly inspired air.
Expiratory reserve volume is the maximum amount of forcibly expired air.
Vital capacity is the maximum amount of air moved in and out on deep breathing, and is the sum of tidal, inspiratory reserve, and expiratory reserve volumes.
Air that remains in the lungs is residual volume.
Some of the inhaled air never reaches the lungs; instead, it fills the nasal cavities, trachea, bronchi, and bronchioles. These passages are not used for gas exchange, and therefore they are said to contain dead-air space.

18. Vital capacity
A spirometer measures the amount of air inhaled and exhaled with each breath. During inspiration, the pen moves up, and during expiration, the pen moves down. Vital capacity (red) is the maximum amount of air a person can exhale after taking the deepest inhalation possible.

19. Inspiration and Expiration
There is a continuous column of air from the pharynx to the alveoli, and the lungs lie within the sealed-off thoracic cavity.
The thoracic cavity is bounded by the rib cage and diaphragm.
Pleural membranes line the thoracic cavity and lungs and the intrapleural pressure is lower than atmospheric pressure, keeping the lobules of the lungs from collapsing.
A infection of the pleural membranes is called pleurisy.

20. Inspiration
When we inhale (inspiration) impulses from the respiratory center in the medulla oblongata cause the rib cage to rise and the diaphragm to lower, causing the thoracic cavity to expand.
The negative pressure or partial vacuum in the alveoli causes the air to come in.
Changing amounts of blood of CO2 and H+ increase breathing rate.
Chemoreceptors in the carotid bodies, located in the carotid arteries, and in the aortic bodies, located in the aorta, are sensitive to the level of oxygen in blood. Decreasing oxygen causes these bodies to communicate with the respiratory center to increase the rate and depth of breathing.
Carbon dioxide (CO2) and hydrogen ions (H+) are the primary stimuli that cause changes in the activity of the medullary respiratory center; the center itself is not affected by low oxygen levels.

21. Nervous control of breathing
During inspiration, the respiratory center stimulates the external intercostal (rib) muscles to contract via the intercostal nerves and stimulates the diaphragm to contract via the phrenic nerve. Should the tidal volume increase above 1.5 liters, stretch receptors send inhibitory impulses to the respiratory center via the vagus nerve. In any case, expiration occurs due to lack of stimulation from the respiratory center to the diaphragm and intercostal muscles.

22. Inspiration
During inspiration, the thoracic cavity and lungs expand so that air is drawn in.

23. Expiration
When we exhale (expiration), lack of impulses from the respiratory center allow the rib cage to lower and diaphragm to resume dome shape.
Expiration is passive, while inspiration is active.
The elastic recoil of the lungs causes expiration.
A deep breath causes alveoli to stretch; stretch receptors then inhibit the respiratory center.

24. Expiration
During expiration, the thoracic cavity and lungs resume their original positions and pressures. Now air is forced out.

25. Gas Exchanges in the Body
External Respiration
Individual gases exert pressure proportional to their portion of the total in a mixture of gases; this is called “partial pressure”.
External respiration is the diffusion of CO2 from pulmonary capillaries into alveolar sacs and O2 from alveolar sacs into pulmonary capillaries.
The principles of diffusion alone govern whether O2 or CO2 enters or leaves the blood in the lungs and in the tissues.

26. In both cases, diffusion occurs because the partial pressures are higher causing diffusion (from higher to lower concentrations) across the capillary wall.
Most CO2 is carried as bicarbonate ions.
The enzyme carbonic anhydrase, in red blood cells, speeds up the conversion of bicarbonate and H+ to H2O and CO2; CO2 enters alveoli and is exhaled.
Hemoglobin (Hb) takes up oxygen from alveoli and becomes oxyhemoglobin (HbO2).

27. Internal Respiration
Internal respiration is the diffusion of O2 from systemic capillaries into tissues and CO2 from tissue fluid into systemic capillaries.
Oxyhemoglobin gives up O2, which diffuses out of the blood and into the tissues because the partial pressure of O2 of tissues fluid is lower than that of the blood.

28. After CO2 diffuses from tissue cells into the blood, it enters red blood cells where a small amount is taken up by hemoglobin, forming carbaminohemoglobin.
Most of the CO2 combines with water to form carbonic acid (H2CO3), which dissociates to release hydrogen ions (H+) and bicarbonate ions (HCO3-); the enzyme carbonic anhydrase speeds this reaction.

29. The globin portion of hemoglobin combines with excess hydrogen ions to become reduced hemoglobin or HHb; this helps maintain a normal blood pH.
Blood leaving capillaries is a dark maroon color because red blood cells contain reduced hemoglobin.

30. External and internal respiration
During external respiration in the lungs, CO2 leaves the blood and O2 enters the blood. During internal respiration in the tissues, O2 leaves the blood and CO2 enters the blood.
External Respiration: At the pulmonary capillaries, O2 enters red blood cells where it combines with hemoglobin (Hb) to form oxyhemoglobin (HbO2). Also, bicarbonate (HCO3-) is converted inside red blood cells to H2O and CO2. CO2 leaves red blood cells and capillaries and diffuses into the lungs to be exhaled.
Internal Respiration: At the systemic capillaries, oxyhemoglobin (HbO2) inside red blood cells gives up its oxygen and becomes Hb and O2. Hemoglobin (Hb) now combines with H+ to form reduced hemoglobin (HHb). O2 leaves red blood cells and capillaries and enters tissue cells. At the same time, CO2 enters red blood cells. Some combines with Hb to form carbaminohemoglobin (HbCO2). Most CO2 is converted to bicarbonate (HCO3-), which is carried in the plasma.

31. Binding Capacity of Hemoglobin
The partial pressure of gases, temperature, and pH affect binding capacity of hemoglobin.
The high pressure of oxygen, the low temperature and low pH aid the binding of oxygen to hemoglobin in the lungs; the opposite is true in the tissues.
In both cases, environmental conditions are favorable to the uptake of the appropriate gases.
Hemoglobin is about % saturated in the capillaries of the lungs and about 60-70% saturated in the tissues.
During exercise, muscle contraction raises temperature (up to 103oF) and lowers pH (due to accumulation of lactic acid), decreasing hemoglobin saturation further in the tissues.

32. Saturation of Hb relative to temperature
The partial pressure of oxygen (PO2) in pulmonary capillaries is about mm Hg, but in tissue capillaries is only about 40 mm Hg. Hemoglobin is about 98% saturated in the lungs because of PO2, and also because the temperature is cooler (and pH is higher in the lungs). On the other hand, hemoglobin is only about 60% saturated in the tissues because of PO2, and also because the temperature is warmer (and pH is lower) in the tissues.

33. Saturation of Hb relative to pH
Hemoglobin is about 98% saturated in the lungs because of PO2, and also because pH is higher in the lungs (and temperature is cooler). On the other hand, hemoglobin is only about 60% saturated in the tissues because of PO2, and also because the pH is lower (and temperature is warmer) in the tissues.

34. Respiration and Health
The presence of disease in the upper or lower respiratory tract means that homeostasis is threatened.
Upper Respiratory Tract Infections
These infections involve the nasal cavities, pharynx, or larynx.
Some infections, such as “strep throat”, can lead to systemic body infection.

35. Sites of upper respiratory infections
A nasal infection, more commonly called rhinitis, is the usual symptom of a common cold due to a viral infection, but rhinitis can also be due to a bacterial infection. Secondary to an URI, the sinuses, middle ear, tonsils, and vocal cords can become infected. Allergies also cause runny nose, blocked sinuses, and laryngitis.

36. Sinusitis
Sinusitis is infection of the cranial sinuses within the facial skeleton that drain into nasal cavities.
It occurs when nasal congestion blocks the sinus openings and is relieved when drainage is restored.
Pain and tenderness over the lower forehead and cheeks, and toothache, accompany this condition.

37. Otitis Media Otitis media is bacterial infection of the middle ear.
Children suffer when a nasal infection spreads to the middle ear by way of the auditory tube and antibiotics are usually used to clear the infection.
Sometimes drainage tubes (called tympanostomy tubes) are inserted into the eardrums of children with recurrent infections.

38. Tonsillitis Laryngitis
Tonsillitis is infection of tonsils and recurrent infections that make breathing or swallowing difficult may be relieved by a tonsillectomy.
Laryngitis
Laryngitis is an infection of the larynx and usually results in a loss of voice.
Persistent hoarseness is a warning sign of cancer.

39. Lower Respiratory Tract Disorders
Lower respiratory infections include:
acute bronchitis, an infection of primary and secondary bronchi;
pneumonia involving a bacterial or viral infection of the lungs; and
pulmonary tuberculosis (infection caused by tubercle bacillus).
Acute bronchitis is usually preceded by a viral URI that has led to a secondary bacterial infection.
Pneumonia causes the lungs to fill with fluid; high fever and chills, headache, and chest pain are symptoms of pneumonia.
When tubercle bacilli invade the lung tissue, the lung cells build a protective capsule about the foreign cells, isolating them from the rest of the body. This capsule is called a tubercle (hence the name tuberculosis). If the person’s immune system is functioning well, the bacteria are killed. If not, the bacteria are eventually liberated from their capsules. Tuberculosis was a major killer in the United States before the middle of the twentieth century, after which antibiotic therapy brought it under control. New, antibiotic-resistant strains, plus the rise of tuberculosis among AIDS patients, the homeless, and the rural poor are contributing to the new increase in tuberculosis cases.

40. Restrictive Pulmonary Disorders
In restrictive pulmonary disorders, vital capacity is reduced because the lungs have lost their elasticity due to inhaled particles such as silica, coal dust, or asbestos.
Fibrous connective tissue builds in the lungs in pulmonary fibrosis, caused by exposure to inhaled particles, including those of fiberglass.
It has been projected that two million deaths caused by asbestos exposure – mostly in the workplace – will occur in the United States between 1990 and Asbestos exposure is also associated with the development of lung cancer. Asbestos was formerly widely used as a fireproofing and insulating material.

41. Obstructive Pulmonary Disorders
In obstructive pulmonary disorders, air does not flow freely in the airways, and inhalation and exhalation are difficult.
Chronic bronchitis with inflamed airways, emphysema where alveolar walls break down, and asthma with constricted bronchioles obstruct the airways and tend to get progressively worse or recur.

42. Lower respiratory tract disorders
Exposure to infectious pathogens and/or air pollutants, including cigarette and cigar smoke, can cause the diseases and disorders shown here.
Pneumonia: Alveoli fill with thick fluid, making gas exchange difficult.
Pulmonary fibrosis: Fibrous connective tissue build up in the lungs, reducing lung elasticity.
Pulmonary tuberculosis: Tubercles encapsulate bacteria, and the elasticity of the lungs is reduced.
Emphysema: Alveoli burst and fuse into enlarged air spaces. Surface area for gas exchange is greatly reduced.
Asthma: Airways are inflamed due to irritation, and bronchioles constrict due to muscle spasms.
Bronchitis: Airways are inflamed due to infection (acute) or an irritant (chronic). Coughing brings up mucus and pus.

43. Lung Cancer
Lung cancer follows this sequence of events: thickening of airway cells, loss of cilia on the lining, cells with atypical nuclei, tumor development, and finally metastasis.
Removal of a lobe or lung, called pneumonectomy, may remove the cancer.
Smoking, whether active or passive, is a major cause of lung cancer.

44. Normal lung versus cancerous lung
On the left is a normal lung with the heart in place. Note the healthy red color. On the right are the lungs of a heavy smoker. Notice how black the lungs are except where cancerous tumors have formed.

45. Chapter Summary
Air passes through a series of tubes before gas exchange takes place across an extensive moist surface in the alveoli of the lungs.
Respiration comprises breathing, external and internal respiration, and cellular respiration.

46. During inspiration, the pressure in the lungs decreases and air comes rushing in; during expiration, increased pressure in the thoracic cavity causes air to leave the lungs.
External respiration occurs in the lungs where oxygen diffuses into the blood and carbon dioxide diffuses out of the blood.
Internal respiration occurs in the tissues where oxygen diffuses out of the blood into tissue cells and carbon dioxide diffuses into the blood.

47. The respiratory pigment hemoglobin transports oxygen from the lungs to the tissues and aids in the transport of carbon dioxide from the tissues to the lungs.
The respiratory tract is especially subject to disease because it is exposed to infectious agents; also, cigarette smoking contributes to two major lung disorders—emphysema and cancer.