the respiratory system dr. dalia fouad. the respiratory system the primary function of the...

THE RESPIRATORY SYSTEM DR. DALIA FOUAD

THE RESPIRATORY SYSTEM The primary function of the respiratory system is to allow oxygen from the air to enter the blood and carbon dioxide from the blood to exit into the air. During inspiration, or inhalation (breathing in), and expiration, or exhalation (breathing out), air is conducted toward or away from the lungs by a series of cavities, tubes, and openings

The respiratory tract extends from the nasal cavities to the lungs, which are composed of air sacs called alveoli. Gas exchange occurs between the air in the alveoli and the blood within a capillary network that surrounds the alveoli. Notice in the blow-up that the pulmonary arteriole is colored blueit carries O 2 - poor blood away from the heart to the alveoli. Then carbon dioxide leaves the blood, and oxygen enters the blood. The pulmonary venule is colored redit carries O 2 -rich blood from the alveoli toward the heart.

The respiratory system also works with the cardiovascular system to accomplish these four respiratory events: 1. breathing, the entrance and exit of air into and out of lungs; 2. external respiration, the exchange of gases (oxygen and carbon dioxide) between air and blood; 3. internal respiration, the exchange of gases between blood and tissue fluid; 4. transport of gases to and from the lungs and the tissues.

Cellular respiration, which produces ATP, uses the oxygen and produces the carbon dioxide that makes gas exchange with the environment necessary. Without a continuous supply of ATP, the cells cease to function. The four events listed here allow cellular respiration to continue.

THE RESPIRATORY TRACT Upper Respiratory Tract Nasal cavitities Pharynx Glottis Larnyx Lower Respiratory Tract Trachea Bronchi Bronchioles Lungs

PATH OF AIR

THE NOSE The nose, a prominent feature of the face, is the only external portion of the respiratory system. Air enters the nose through external openings called nostrils. The nose contains two nasal cavities, which are narrow canals separated from one another by a septum composed of bone and cartilage Mucous membrane lines the nasal cavities.

The nasal conchae are bony ridges that project laterally into the nasal cavity. They increase the surface area for moistening and warming air during inhalation and for trapping water droplets during exhalation. Odor receptors are on the cilia of cells located high in the recesses of the nasal cavities.

THE PHARYNX The pharynx is a funnel-shaped passageway that connects the nasal and oral cavities to the larynx. Consequently, the pharynx, commonly referred to as the throat, has three parts: the nasopharynx, where the nasal cavities open posterior to the soft palate the oropharynx, where the oral cavity joins the pharynx; the laryngopharynx, which opens into the larynx.

The soft palate has a soft extension called the uvula that can be seen projecting into the oropharynx. The tonsils form a protective ring at the junction of the oral cavity and the pharynx. The tonsils contain lymphocytes that protect against invasion of inhaled pathogens. In this way, the respiratory tract assists the immune system in maintaining homeostasis. In the pharynx, the air passage and the food passage cross because the larynx, which receives air, is anterior to the esophagus, which receives food. The larynx lies at the top of the trachea. The larynx and trachea are normally open, allowing air to pass, but the esophagus is normally closed and opens only when a person swallows.

THE LARYNX The larynx is a cartilaginous structure that serves as a passageway for air between the pharynx and the trachea. The larynx is called the voice box because it houses the vocal cords. The vocal cords are mucosal folds supported by elastic ligaments, and the slit between the vocal cords is an opening called the glottis When air is expelled past the vocal cords through the glottis, the vocal cords vibrate, producing sound. When food is swallowed, the larynx moves upward against the epiglottis, a flap of tissue that prevents food from passing through the glottis into the larynx.

THE TRACHEA commonly called the windpipe, is a tube connecting the larynx to the primary bronchi. The trachea lies ventral to the esophagus and is held open by C-shaped cartilaginous rings. The open part of the C-shaped rings faces the esophagus, and this allows the esophagus to expand when swallowing. The mucosa that lines the trachea has a layer of pseudostratified ciliated columnar epithelium. The cilia that project from the epithelium keep the lungs clean by sweeping mucus, produced by goblet cells, and debris toward the pharynx:

THE BRONCHIAL TREE The trachea divides into right and left primary bronchi which lead into the right and left lungs. The bronchi branch into a great number of secondary bronchi that eventually lead to bronchioles. The bronchi resemble the trachea in structure, but as the bronchial tubes divide and subdivide, their walls become thinner, and the small rings of cartilage are no longer present. Each bronchiole leads to an elongated space enclosed by a multitude of air pockets, or sacs, called alveoli. The components of the bronchial tree beyond the primary bronchi, including the alveoli, compose the lungs.

THE LUNGS The lungs are paired, cone-shaped organs that occupy the thoracic cavity. The right lung has three lobes, and the left lung has two lobes. A lobe is further divided into lobules, and each lobule has a bronchiole serving many alveoli.

THE ALVEOLI With each inhalation, air passes by way of the bronchial tree to the alveoli. An alveolar sac is made up of simple squamous epithelium surrounded by blood capillaries. Gas exchange occurs between the air in the alveoli and the blood in the capillaries. Oxygen diffuses across the alveolar and capillary walls to enter the bloodstream, while carbon dioxide diffuses from the blood across these walls to enter the alveoli. The alveoli must stay open to receive the inhaled air if gas exchange is to occur. Gas exchange takes place across moist cellular membranes,

Gas exchange in the lungs. The lungs consist of portions of the bronchial tree leading to the alveoli, each of which is surrounded by an extensive capillary network. Notice that the pulmonary artery and arteriole carry O2-poor blood (colored blue), and the pulmonary vein and venule carry O2-rich blood (colored red).

MECHANISM OF BREATHING During breathing, air moves into the lungs from the nose or mouth (called inspiration, or inhalation), and then moves out of the lungs during expiration, or exhalation. A free flow of air from the nose or mouth to the lungs and from the lungs to the nose or mouth is vitally important.

Internal respiration refers to the exchange of gases in the tissues. Specifically, during internal respiration, gases are exchanged between the blood in systemic capillaries and the tissue fluid. Blood that enters the systemic capillaries is a bright red color because the blood is O2-rich. Tissue fluid, on the other hand, has a low concentration of O2. Why? Because the cells are continually consuming O2 during cellular respiration. Therefore, O2 diffuses from the blood into the tissue fluid. Tissue fluid has a higher concentration of CO2 than does the blood entering the tissues. Why? Because CO2 is an end product of cellular respiration. Therefore, CO2 diffuses from the tissue fluid into the blood.

External and internal respiration. During external respiration in the lungs, CO2 leaves the blood and O2 enters the blood passively by diffusion. During internal respiration in the tissues, O2 leaves the blood and CO2 enters the blood passively by diffusion.

External Respiration External respiration is the exchange of gases in the lungs. Specifically, during external respiration, gases are exchanged between the air in the alveoli and the blood in the pulmonary capillaries. Blood that enters the pulmonary capillaries is dark maroon because it is relatively O 2 -poor. Once inspiration has occurred, the alveoli have a higher concentration of O 2 than does blood entering the lungs. Therefore, O 2 diffuses from the alveoli into the blood. The reverse is true of CO 2. The alveoli have a lower concentration of CO 2 than does blood entering the lungs. Therefore, CO 2 diffuses out of the blood into the alveoli. This CO 2 exits the body during expiration. Another way to explain gas exchange in the lungs is to consider the partial pressure of the gases involved. Gases exert pressure, and the amount of pressure each gas exerts is its partial pressure, symbolized as P O2 and P CO2. Alveolar air has a much higher PO2 than does blood. Therefore, O2 diffuses into the blood from the alveoli. The pressure pattern is the reverse for CO2. Blood entering the pulmonary capillaries has a higher PCO2 than the air in the alveoli. Therefore, CO2 diffuses out of the blood into the alveoli.

Internal Respiration Internal respiration refers to the exchange of gases in the tissues. Specifically, during internal respiration, gases are exchanged between the blood in systemic capillaries and the tissue fluid. Blood that enters the systemic capillaries is a bright red color because the blood is O2-rich. Tissue fluid, on the other hand, has a low concentration of O2. Why? Because the cells are continually consuming O2 during cellular respiration. Therefore, O2 diffuses from the blood into the tissue fluid. Tissue fluid has a higher concentration of CO2 than does the blood entering the tissues. Why? Because CO2 is an end product of cellular respiration. Therefore, CO2 diffuses from the tissue fluid into the blood. We can explain exchange in the tissues by considering the partial pressure of the gases involved. In this case, oxygen diffuses out of the blood into the tissues because the PO2 in tissue fluid is lower than that of the blood. And the carbon dioxide diffuses into the blood from the tissues because the PCO2 in tissue fluid is higher than that of the blood.

Gas Transport The mode of transport of oxygen and carbon dioxide in the blood differs, although red blood cells are involved in transporting both of these gases. Oxygen Transport After O2 enters the blood in the lungs, it enters red blood cells and combines with the iron portion of hemoglobin, the pigment in red blood cells. Hemoglobin is remarkably suited to the task of transporting oxygen because it both combines with and releases oxygen. The higher concentration of oxygen in the alveoli, plus the slightly higher pH and slightly cooler temperature, causes hemoglobin to take up oxygen and become oxyhemoglobin (HbO2). The lower concentration of oxygen in the tissues, plus the slightly lower pH and slightly warmer temperature in the tissues, causes hemoglobin to release oxygen and become eoxyhemoglobin (Hb). This equation summarizes our discussion of oxygen transport:

Carbon Dioxide Transport Transport of CO 2 to the lungs involves a number of steps. After CO 2 diffuses into the blood at the tissues, it enters the red blood cells, where: 1. A small amount is taken up by hemoglobin, forming carbaminohemoglobin. 2. Most of the CO 2 combines with water, forming carbonic acid (H 2 CO 3 ). The carbonic acid dissociates to hydrogen ions (H) and bicarbonate ions (HCO 3 ). The release of these hydrogen ions explains why the blood in systemic capillaries has a lower pH than the blood in pulmonary capillaries. 3. The difference in pH is slight because the globin portion of hemoglobin combines with excess hydrogen ions and becomes reduced hemoglobin (HHb). Bicarbonate ions are carried in the plasma because they diffuse out of red blood cells and go into the plasma. Most of the carbon dioxide in blood is carried as HCO 3, the bicarbonate ion. As bicarbonate ions diffuse out of red blood cells, chloride ions (Cl) diffuse into them. This so-called chloride shift maintains the electrical balance between the plasma and the red blood cells. In pulmonary capillaries, a reverse reaction occurs. Bicarbonate combines with hydrogen ions to form carbonic acid, which this time splits into CO 2 and H 2 O, and the CO 2 diffuses out of the blood into the alveoli. The following equation summarizes our discussion of carbon dioxide transport:

Regulation of respiration: Since the rates of O2 uptake and CO2 production by body cells vary widely with changing metabolic demand, respiration has to be controlled so as to maintain appropriate levels of O2 and CO2 (and H+) in the tissues. This depends on the regulation of ventilation through the interaction of neurological and chemical control mechanisms so that mean alveolar gas pressures remain constant. Since pulmonary blood normally equilibrates with alveolar conditions before entering the systemic circulation, systemic arterial blood gases can also be controlled by changes in ventilation.

NEUROLOGICAL CONTROL FROM RESPIRATORY CENTERS IN THE BRAIN : ALTHOUGH VENTILATION MAY BE CONSCIOUSLY CONTROLLED IT IS NORMALLY REGULATED VIA INVOLUNTARY NERVOUS MECHANISMS.BREATHING IS REGULAR AND CYCLICAL, INSPIRATION ALTERNATING WITH EXPIRATION, AND THIS RHYTHMICAL ACTIVITY OWTHMICAL ACTIVITY IS DEPENDENT ON THE ACTIVITY OF NEURONES WITHIN THEBRAINSTEM. THE NEURONES WHICH ACTIVATE THE INSPIRATORY AND EXPIRATORY MUSCLES ARE LOCATED IN THE MEDULLA OBLONGATA BUT ARE INFLUENCED BY CENTRES IN THE PONS,WHICH MODIFY THEIR ELECTRICAL ACTIVITY, AND THUS ALTER THE PATTERN OF VENTILATION.

- Inspiratory respiratory neurones in the medulla oblongata demonstrate spontaneous, rhythmical activiTy firing regular bursts of action potentials separated by periods of inactvity. These neurones stimulate the motoneurones which pass out from the spinal cord to the diaphragm and external intercostal muscles and so activate contraction in these inspiratory muscles.During the pauses in inspiratory neurone activity, the inspiratory muscles relax and expiration occurs passively. in quiet respiration, therefore, it is the inspiratory centre which plays the major role in sumulating ventilation. Expiratory respiratory neurones in the medula are normally quiescent and only become active during episodes of increased ventilation involving active, or forced, expiration. Under these conditions, bursts of expiratory neurone activity coinciding with the lulls I n inspiratory neurone activity may be recorded. These stimulate motoneurones causing the internal intercostal and abdominal muscles to contract (Section 4.1).

.Respiratory cellsin the porns are not essential for respiration but can modify the pattern of breathing.Stimulating the pneumotaxic centre tends to inhibit the inspiratory neurones and so shortens inspiration.Damage to this pneumotaxic area, by comparison can lead to apneuss, in which inspiration is prolonged and is only interrupted by short, expiratory gasps. chemical factors modifying respiratory center activity: Arterial blood gases and pH: Chemical control by the gases in arterial blood, as monitored by the respiratory chemoreceptors, is the dominant factor in the regulation of ventilation. Arterial Pco2 is most important with PH playing a secondary role allowing respiratory compensation for metabolic acid-base disturbances surprisingly arterial Po2 plays little or no role in the normal regulation of ventilation, although abnormally low levels can powerfully stimulate ventilation.

Elevated arterial Pco2. Any elevation of arterial Pco2 stimulates ventilatio leading to compensatory reduction.in alveolar pco2 as excess CO2 is blown off. This is the primary mechanism responsible for the regulation of breathing, and ventilation is adjusted to keep the arterial pco2 close to 5.3 kPa (40 mmHg) Close regulation of CO2 levels is important since any rise in Pco2 increases the CO2 content of the blood (see ue co dissocation curve, fig 72) and promotes acidosis (Eq. 25) Depressed arteria ph.Any,fall in arterial pH (increase in[ H+] leads to an increase n ventilation. This reduces alveolar and arterial Pao, and so elevates the systemic ph by driving the carbonic acid dissociation reactions towards the left, removing H+ (protons) from the extracellular fluid (Eq. 25). Respiratory changes can compensate for an acidosis (low pH) caused by a non respiratory problem (a metabolic acidosis) in this way (Section 5.9). Similarly, ventilation may decrease in response to a metabolic alkalosis, favouring Co2 accumulation and a reduction of the pH back toward normal.

Depressed arterial Po2.ventilation is also stimulated by low leves of arterial po2.This only happens, however, if the Po2 drops well below the normal value (13 kpa,98 mmHg): increased respiratory drive is only significant when the Po2 is less than about 8 kPa (60 mmH) The shape of the O2 dissociation curve removes any need to regulate Po2 more tightly than this since haemoglobin remains 85% saturated with o2 even at this relatively low pressure (Fig. 68B). Oxygen delivery tissues is not greatly compromised unless arterial po2 falls below these levels and only then is ventilation stimulated. Oxygen is not directy involved, therefore, the normal regulation of ventilation. Nevertheless, the arterial po2 remains relatively constant under physiological conditions, as a secondary consequenceof the close Control of Pco2 levels.

Chemoreceptors: The changes in arterial Co2, O2 and H+ levels which the respiratory changes described above are detected y respiratory chemoreceptors which regulate ventilation through their connections with the respiratory centres. These receptors are divided into two main groups based on their anatomical location and each has its own pattern of sensitivity to changes in arterial blood gases and pH levels.

.Central chemoreceptors. These are located within the CNS itself, dose othe respiratory centre in the medulal These receptors are particularly sensitive to changes in the arterial Pco, and are less affected by changes in arterial pH or Po Experiments on the mechanism of the CO2-stimulatory effect suggest that the chemoreceptor cells are actually sensitive to H+. Carbon dioxide rapidly diffuses from the blood into the brain where it reacts with water to produce H+ (Eq. 25), and it is the resulting drop in pH that directly stimulates the central chemoreceptors. An acidosis of the arterial blood itself, however,has little immediate effect on central chemoreceptors, because H+ cannot easily cross the blood-brain barrier.

Peripheral chemoreceptors. These are located within the carotid bodies, close to the bifurcation of the common carotid arteries, and in the aortic bodies along the aortic arch. These receptors are less important than central chemoreceptors in the responses to an increase in pco2 but they are sensitive to changes in arterial pH) stimulating or inhibiting ventilation in response to arterial acidosis and alkalosis, respectively.peripheral chemoreceptors play a further role as part of the fail-safe response to very Low o2 level. They are only activated when the Po2 falls well below the physiological range (ie at or below 8 kpa;60 mmHg), however, and this mechanism is not important an in normal ventilatory.

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the respiratory system dr. dalia fouad. the respiratory system the primary function of the...

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