Chapter 13: The Respiratory System
Lecture Notes taken from:
Marieb,E.N. 2009. Essentials of Human Anatomy and Physiology. PBC
2 major functions that maintain homeostasis:
· Bring O2 to the blood and tissues
· Remove CO2 from the tissues and blood
The respiratory system works intimately with the cardiovascular system to remove carbon dioxide waste and deliver oxygen throughout the body. This all depends on sufficient nourishment from the digestive system and removal of metabolic waste by the urinary system which cannot take place without functioning respiratory and cardiovascular systems. Everything, in short, must be working pretty well to keep us alive.
Major anatomical features of the respiratory system include:
1. Nasal Cavity – lined with mucous membranes, the nasal cavity warms, filters, and humidifies incoming air. Blood vessels near the surface which help warm the air are susceptible to drying and friction = nose bleeds.
2. Oral cavity – mouth, can serve as alternative airway if nasal cavity is blocked, very limited filtering capacity though.
3. Pharynx – throat, serves as both an airway and a pathway for food.
4. Epiglottis – protects the airway from food, swallowing causes the larynx to lift upward which forces the epiglottis to fold down over the opening to the larynx, this seals off the airway as food is passed down the esophagus.
5. Larynx – voice box, houses folds of skin known as the glottis, the glottis vibrates to generate our voices, cartilage rings keep the larynx open for free air passage.
6. Trachea – windpipe inferior to the larynx, cartilage here and in the larynx prevent the airways from ever collapsing, cartilage in the trachea only extends part way around so that food is free to pass down the esophagus on the dorsal side of the trachea. The trachea is also lined with ciliated mucosa (mucous membrane with hair-like cilia that constantly sweep mucus toward the throat and away from the deeper airways). The action of the cilia moves particles filtered by the mucus away from the lungs. After smoking a cigarette, cilia are inactive for an hour or more; no wonder smokers tend to get more respiratory infections!!
7. Bronchial tree – the trachea branches into right and left bronchi that then branch further in to smaller and smaller bronchioles that terminate at alveolar sacs where gas exchange tales place. This branched system of airways is known as the bronchial tree.
8. Lungs – we have a right lung with 3 lobes; left lung with 2 lobes; both represent the major respiratory organs.
9. Pleural membrane – pleural membranes cover each lung and line the chest cavity to form a double membrane around each lung known as the pleural cavity. The serous fluid inside the pleural cavity allows the lungs to easily move within the chest cavity and also keeps the lungs tight against the chest wall.
10. Alveoli – these are tiny air sacs within the lungs that serve as the site of gas exchange.
Respiration = gas exchange. The body relies on diffusion for this to happen. Diffusion is the tendency of molecules to move from areas of high concentration to low concentration (you could also say, ‘along a concentration gradient’).
In the lungs:
Respiratory cycle = inspiration phase and expiration phase.
During inspiration, the contraction of the diaphragm and rib muscles (intercostals) contract. This flattens diaphragm and intercostals lift and expand the chest cavity; this creates a vacuum that draws air into the lungs. Under normal conditions, alveoli are filled with fresh, O2 rich air.
During normal expiration, diaphragm and intercostals simply relax and air is forced out as chest cavity compresses and diaphragm moves up into chest cavity. Elasticity of lungs also helps push air out.
Muscles connecting neck to sternum and upper ribs can contract to force more air into lungs for a deep breath. Abdominal muscles will contract during exertion to force air out and speed exhalation (like during strenuous workouts).
Pleural cavity maintains a slight vacuum between lungs and chest wall. Lungs cling to the chest wall as a result. Infections or injuries that result in air entering the pleural cavity = collapsed lung (a.k.a. atalectosis); normal lung expansion is prevented and breathing becomes difficult.
Some normal lung capacity values and terms:
Tidal volume (TV) – amount of air inhaled during a normal breath = 500ml (1/2 liter); 350ml actually reach alveoli
Inspiratory reserve volume (IRV) – amount of air that can be forcefully inhaled = 3.1 liters; expiratory reserve volume (ERV) = amount that can be forcibly exhaled (about 1.2 liters). All together, TV + ERV + IRV = Vital Capacity = the amount of air we can actively process in one deep breath cycle.
Even after complete forced exhale, another 1.2 liters of air remain in the airways = residual volume. The residual volume helps ensure some level of gas exchange continuously (even when exhaling).
Gas Exchange physiology:
External gas exchange = gas exchange of the lung; O2 absorbed into blood and CO2 released into the air at the alveoli. O2 must pass from air into liquid across the respiratory membrane in the alveoli. This depends on the pressure gradient (concentration gradient) and the surface area of the respiratory membrane (from 50 to 70m2 in a healthy adult male!).
Once in the blood, O2 will bind to hemoglobin in red blood cells. Oxyhemoglobin (hemoglobin bound with O2) helps maintain the pressure gradient so that O2 keeps entering the blood stream. Blood can carry 65 – 70X more O2 with hemoglobin than without. A small amount of O2 also dissolves directly into the blood plasma.
Internal gas exchange = gas exchange between body tissues and the blood. O2 levels surrounding tissues are lower than that of oxygenated blood from the lungs. Along with higher temperatures and higher acidity near tissues, this promotes the release of O2 from hemoglobin into the tissues (i.e. O2 is unloaded).
CO2 moves along high concentration in tissues into the low concentration of the blood. Once in the blood, 7% of CO2 is dissolved directly into the plasma, 23% binds with hemoglobin, and 70% is converted to bicarbonate = HCO3-. As in O2 transport, the conversion of CO2 into carbonic acid/bicarbonate and bound to hemoglobin helps maintain pressure gradient so that CO2 continues to dissolve into the bloodstream. The reverse reactions take place in the alveoli due to the reversal of the pressure gradient (i.e. more CO2 in the blood than in the air).
Breathing rates are controlled automatically by the nervous system:
The ‘pacemaker’ of the lungs is found in the brain stem, namely the medulla and pons regions. Increased levels of CO2 in the blood (surprisingly, O2 not directly involved) raise H+ concentrations (because of increased levels of bicarbonate) that are detected by the brain; result is deeper and faster breathing (hyperventilation). Low CO2 levels result in slowed breathing (hypoventilation) to bring CO2 levels back up. Prolonged or abnormal hyperventilation can lead to alkalosis (blood too basic due to low CO2); abnormal hypoventilation = acidosis (blood too acidic due to high CO2)
Secondarily, O2 sensors do exist near the heart (aorta and carotid artery) but they do not typically control breathing rates. However, in patients with emphysema or chronic bronchitis that causes chronically high levels of CO2 in the blood, the sensors in the medulla stop responding to CO2 levels. As a result, these secondary O2 sensors control breathing. If pure oxygen is used to treat these patients, breathing will slow or even stop as a result of super high O2 levels. Thus, only low levels are used for treatment.
Malfunctions in the control systems can deteriorate (especially with age) and lead to apnea – momentary cessation of breathing lasting a few seconds to minutes and occurring up to 500 times a night in severe cases.
Some common or well-known disorders:
Tobacco use is a respiratory system killer. Know the consequences.
Lung cancer – #1 among cancer deaths; 80% from tobacco.
Chronic Obstructive Pulmonary Diseases (COPDs) include: emphysema – lungs become less elastic due to build up of scar tissue; and chronic bronchitis – lower respiratory passages become severely and chronically inflamed, excess mucus production clogs airways. All type of COPDs can result in dypnea (difficult, labored breathing) and eventual respiratory failure.