respiratory system During inspiration, there is a fall in: atmospheric pressure intraalveolar pressure intrapleural pressure

Transcription

1 respiratory system During inspiration, there is a fall in: atmospheric pressure intraalveolar pressure intrapleural pressure intraalveolar and intrapleural pressure

2 intraalveolar and intrapleural pressure Air flows because of pressure differences between the atmosphere and the gases inside the lungs. Air, like other gases, flows from a region with higher pressure to a region with lower pressure. Muscular breathing movements and recoil of elastic tissues cre ate the changes in pressure that result in ventilation. Pulmonary ventilation involves three different pressures: Atmospheric pressure is the pressure of the air outside the body Intraalveolar (intrapulmonary) pressure is the pressure inside the alveoli of the lungs Intrapleural pressure is the pressure within the pleural cavity Note: In the resting position, the intrapleural pressure is approximately 4 mm Hg less than the atmospheric pressure. Hence, the intrapleural pressure is approximately 756 mm Hg. 3 ". It is subatmospheric, or negative. Inspiration (inhalation) is the process of taking air into the lungs. This is the active phase of ventilation because it is the result of muscle contraction. During inspiration, the diaphragm contracts and the thoracic cavity increases in volume.this decreases the intraalveolar pressure so that air flows into the lungs. Important point: There is a fall in both intrapleural pressure and intraalveolar pressure. Expiration (exhalation) is the process of letting air out of the lungs during the breathing cycle. During expiration, the relaxation of the diaphragm and elastic recoil of tissue decreases the thoracic volume and increases the intraalveolar pressure. Expiration pushes air out of the lungs. Important point: Intrapleural pressure becomes less negative and intraalveolar pressure rises. 1. Following a normal expiration (functional residual capacity, FRC), the alveolar pressure is 760 mm Hg, which is the atmospheric pressure. 2. At functional residual capacity, the expanding forces are equal and opposite to the collapsing pressures. This is the point of rest. Either increasing or decreasing volume from FRC requires muscle contraction.

3 respiratory system The factors that influence the rate of gas diffusion across the respiratory membrane include all of the following EXCEPT one. Which one is the EXCEPT- ION? the thickness of the membrane the surface area of the membrane the temperature of the system the diffusion coefficient of the gas in the substance of the membrane the partial pressure difference of the gas between the two sides of the membrane

4 the temperature of the system The respiratory membranes of the lungs are in the respiratory bronchioles, alveolar ducts and al - veoli. Surrounding each alveolus is a network of capillaries arranged so that air within the alveoli is separated by a thin respiratory membrane from the blood contained within the alveolar capillaries. Factors That Influence the Rate of Gas Diffusion Across the Respiratory Membrane: Thickness of the membrane: the rate of diffusion across the membrane is inversely proportional to the diffusion distance. Surface area of the respiratory membrane: the rate of diffusion is directly proportional to the surface area. Emphysema decreases the surface area, which impedes the exchange of gases. The diffusion coefficient of the gas in the substance of the membrane: the diffusion coefficient is a measure of how easily a gas will diffuse through a liquid or tissue, taking into account the solubility of the gas in the liquid and the size of the gas molecule (molecular weight). Note: The solubility of CO, is approximately 20 times greater than the solubility of 0 2. The partial pressure difference of the gas between the two sides of the membrane: the partial pressure difference of a gas across the respiratory membrane is the difference be tween the partial pressure of the gas in the alveoli and the partial pressure of the gas in the blood of the alveolar capillaries. When the partial pressure of a gas is greater on one side of the respiratory membrane than on the other side, net diffusion occurs from the higher to the lower pressure. Normally, the partial pressure of oxygen (P0 2) is greater in the alveoli than in the blood of the alveolar capillaries and the partial pressure of carbon dioxide (PCO 2) is greater in the blood than in the alveolar air. The partial pressure difference for oxygen and carbon dioxide can be increased by increasing the alveolar ventilation rate. The greater volume of atmospheric air exchanged with the residual volume raises alveolar P0 2, lowers alveolar PCO 2, and promotes gas exchange.

5 respiratory system Alveolar ventilation is expressed as: respiratory rate x (tidal volume + dead air space volume) respiratory rate + (tidal volume + dead air space volume) respiratory rate x (tidal volume - dead air space volume) respiratory rate - (tidal volume - dead air space volume)

6 respiratory rate x (tidal volume - dead air space volume) The exchange of oxygen and carbon dioxide between the lungs and the blood occurs within the alveoli located in respiratory bronchioles, alveolar ducts and alveolar sacs. No gas exchange takes place within the remaining respiratory passageways (nose, pharynx, trachea and conducting bronchioles).these air-filled passageways are called anatomical dead air space. During quiet breathing, the amount of air brought into the lungs is the tidal volume (500 ml). Approximately 150 ml of that volume remains in the dead air space. The volume of atmospheric air that actually reaches the alveoli (either per breath or in one minute) and that can participate in the exchange of gases between the alveoli and the blood is called the alveolar ventilation. 1. Alveolar ventilation is a good criterion for the effectiveness of breathing. 2. Respiratory rate = Breaths/min. 3. Minute ventilation = Tidal volume x Breath/min. Gas exchange can either be perfusion or diffusion-limited. In perfusion-limited gas exchange, the partial pressure of gasses in the alveolar capillaries becomes equal to the partial pressure in the alveoli and the only way to increase gas exchange is to increase the rate of blood flow through the alveolar capillaries. Gas exchange is perfusion-limited in healthy people unless they are vigorously exercising. In vigorous exercise, and in patients with emphysema or fibrosis, gas exchange is diffusion-limited, meaning the partial pressure gradient between the pulmonary blood and alveoli is maintained because gases cannot diffuse through the alveoli before the blood passes through the end of the alveolar capillaries.

7 respiratory system The major factor that influences ventilation is: ph of arterial blood arterial PCO 2 arterial P0 2 arterial albumin content

8 arterial PCO 2 Ventilatory control is composed of the respiratory control center, central chemoreceptors, peripheral chemoreceptors and pulmonary mechanoreceptors/sensory nerves. Important: Arterial PCO 2 is the major factor that influences ventilation. The respiratory control center is composed of the dorsal respiratory group and the ventral respira tory group. Rhythmic breathing depends on a continuous (tonic) inspiratory drive from the dorsal respiratory group and on intermittent (phasic) expiratory input from the cerebrum, thalamus, cranial nerves and ascending spinal cord sensory tracts. Central chemoreceptors are specialized cells on the ventrolateral surface of the medulla.these receptors are sensitive to the ph of the surrounding extracellular fluid. The carotid and aortic bodies are peripheral chemoreceptors that respond to changes in arterial P0 2 (not the 0 2 content), PCO 2 and ph, and they transmit afferent information to the central respiratory control center. The peripheral chemoreceptors are the only chemoreceptors that respond to changes in P0 2. Two situations that will excite the respiratory neurons and increase respiration: (1) An increase in hydrogen ion concentration in the arterial blood (decreased ph) and (2) An increase in the PCO 2 of arterial blood. These two are closely related in the following way. Any time the partial pressure of carbon dioxide increases (PCO 2), this also increases the hydrogen ion concentration (decreases the ph) because carbon dioxide combines with water to form carbonic acid. This carbonic acid then dissociates into hydrogen ions and bicarbonate. These hydrogen ions decrease the ph of the ar terial blood, thus increasing respiration. Important: The hypoxia of high altitude stimulates ventilation. The carotid body senses hypoxia and signals the medulla to stimulate ventilation (hypoxic ventilatory response). Decreased alveolar carbon dioxide allows for an equivalent increase in alveolar oxygen. The cardiovascular system responds to increased catecholamines with a moderate increase in heart rate, blood pressure and cardiac output. Over days to weeks, there are increases in hematocrit and capillar y density with changes in the tissues and cells.

9 respiratory system The volume of air remaining in the lungs after a maximal expiration is called the: vital capacity tidal volume residual volume functional residual capacity

10 residual volume *** Determined by the force generated by the muscles of expiration and the inward elastic recoil of the lungs as they oppose the outward elastic recoil of the chest wall. The residual volume of a healthy 70-kg adult is 1.5 liters. Lung Volumes and Capacities: Total lung capacity (TLC) is the volume of air in the lungs after a maximal inspiratory effort. Determined by the strength of contraction of the inspiratory muscles in opposition to the inward elastic recoil of the lungs and chest wall. This is about 6 liters in a healthy 70-kg adult. Vital capacity (VC) is the volume of air expelled from the lungs during a maximal forced expiration starting after a maximal forced inspiration. VC is about 4.5 liters. VC = TV + IRV + ERV Tidal volume (TV) is the volume of air entering or leaving the nose or mouth per breath. During normal, quiet breathing (eupnea), the tidal volume of a 70 -kg adult is about 500 ml per breath. Note: The normal rate of respiration is about 12 times per minute. Functional residual capacity (FRC) is the volume of gas remaining in the lungs at the end of a normal tidal expiration. This is the balance point between the inward elastic recoil of the lungs and the outward elastic recoil of the chest wall. FRC is about 3 liters in a healthy 70-kg adult. FRC = ERV + RV Inspiratory reserve (IRV) is the volume of gas inhaled into the lungs during a maximal forced inspiration starting at the end of a normal tidal inspiration. IRV is about 2.5 liters. Expiratory reserve (ERV) is the volume of gas expelled from the lungs during a maximal forced expiration that starts at the end of normal tidal expiration. ERV is about 1.5 liters.

11 respiratory system Which of the following factors has no direct effect on pulmonary ventilation? arterial P0 2 arterial PCO 2 arterial [H1 arterial [HCO3-]

12 arterial [HCO 3 - ] 3E.* HCO 3 - does not directly affect pulmonary ventilation. HCO 3 - does have influence, but that is through ph and [H+]. There are no HCO 3 - sensors. Pulmonary ventilation is the total volume of gas per minute, inspired or expired. Sensory information is coordinated in the brain stem. The output of the brain stem controls the respiratory muscles and the breathing cycle. Receptors for CO 2, 0 2, and H+: Central (medullary) chemoreceptors which are located in medulla. Stimuli that increase breathing rate include decreased ph. The medulla has H+ receptors and H+ concentrations are controlled by diffusion of CO 2 through blood-brain barrier and conversion to H+ and HCO - 3, so PCO 2 indirectly stimulates central chemoreceptors. Peripheral chemoreceptors which are located in the carotid and aortic bodies. Stimuli that increase breathing rate include P0 2 (if less than 60 mmhg), PCO 2 and ph. Factors That Stimulate These Receptors: Arterial P0 2 (partial pressure of oxygen in arterial blood) - very low P0 2 in arterial blood increases pulmonary ventilation. Arterial PCO 2 (partial pressure of carbon dioxide in arterial blood) is the major stimulus for the respiratory centers. Elevated arterial PCO 2 increases ventilation. Arterial ph - a low arterial ph (increased hydrogen ion concentration) increases ventilation. *** These various factors interact with one another to regulate breathing. Normal Adult Arterial Values: ph = PCO 2 = mm Hg P0 2 = mm Hg CO 2 = mmol/l Normal Adult Venous Values: ph = PCO 2 = mm Hg PO 2 = 40 mm Hg CO 2 = mmol/l

13 respiratory system The Hering-Breuer inspiratory-inhibitory reflex is mediated by: renal fibers vagal fibers sacral fibers lumbar fibers

14 vagal fibers The Hering-Breuer inspiratory-inhibitory reflex is stimulated by increases in lung volume, especially those associated with an increase in both ventilatory rate and tidal volume. This stretch reflex is mediated by vagal fibers, and when elicited it results in cessation of inspiration by stimulating the off-switch neurons in the medulla. This reflex is inactive during quiet breathing and appears to be most important in newborns. Note: This reflex does not appear to be of great importance in the control of re spiration during normal breathing. This reflex is mainly a protective mechanism that prevents the lungs from overfilling. There are three major types of sensory receptors located in the tracheobronchial tree that respond to a variety of different stimuli and result in changes in the lung's mechanical properties, alterations in the respiratory pattern and the development of respiratory symptoms. Specialized sensory receptors occur in the lung parenchyma and respond to chemical or mechanical stimulation in the lung interstitium. These receptors are called juxta-alveolar or J receptors. They transmit their afferent input through unmyelinated, vagal C fibers. Inhaled dust, noxious gases or cigarette smoke stimulates irritant receptors in the trachea and large airways that transmit information through myelinated vagal afferent fibers. Stimulation of these receptors results in an increase in airway resistance, reflex apnea and coughing. Somatic receptors are also located in the intercostal muscles, rib joints, acce ssory muscles of respiration and tendons. They respond to changes in the length and tension of the respiratory muscles. Remember: The partial pressure of carbon dioxide in arterial blood (PCO 2) is the most important stimulus for the respiratory control center (medulla). An increased PCO 2 increases respiration by stimulating the central 1-1+ chemoreceptors (CO 2 > H+ + HCO 3 - ), while a decrease in PCO 2 inhibits respiration.

15 respiratory system An abnormally low arterial PCO, is called: apnea dyspnea hypercapnea hypocapnea

16 hypocapnea Arterial blood gas abnormalities: Arterial hypoxemia is defined as an arterial P0 2 less than 80 mm Hg in an adult who is breathing room air at sea level. Hypoxia occurs when there is insufficient 0 2 to carry out normal metabolic functions; hypoxia often occurs when arterial P0 2 is less than 60 mm Hg. Hypercapnea is defined as an increase in arterial PCO 2 above the normal range. Hypocapnea is an abnormally low arterial PCO 2 (usually less than 35 mm Hg). Note: Two major mechanisms account for the development of hypercapnea (elevated PCO 2 ) > hypoventilation and wasted ventilation. Ventilation insufficient to maintain normal levels of CO, is called hypoventilation. Hy - poventilation always decreases Pa0 2 (arterial P0 2 in mm Hg) and increases PaCO 2 (arterial PCO 2 in mm Hg). Note: If the Pa0 2 falls to less than 60 mm Hg the aortic and carotid body chemoreceptors respond by causing hyperventilation and increasing car - diac output through sympathetic nervous system stimulation. This normal protective response to hypoxia is reduced by anaesthetic drugs. Remember:The pulmonary circulation is a low-pressure, low-resistance system.the arteries of the pulmonary circulation are thin walled and they have minimal smooth muscle. The pulmonary vessels are seven times more compliant than the systemic vessels. Recruitment of new capillaries is a unique feature of the lung and allows for adjustments in stress, as in the case of exercise. Pulmonary vascular resistance is the change in pressure from the pulmonary artery (E PA ) to the left atrium (P LA ) divided by cardiac output (Q T ). This resistance is about 10 times less than in the systemic circulation.

17 respiratory system The surface tension-reducing and antistick properties of surfactant diminish the work of breathing and help stabilize alveoli. The lung demonstrates anatomic and physiological unity; that is, each unit (bronchopulmonary segment) is structurally identical and it functions just like every other unit. both statements are true both statements are false the first statement is true, the second is false the first statement is false, the second is true

18 both statements are true The lung has two separate blood supplies. The pulmonary circulation brings deoxygenated blood from the right ventricle to the gas-exchanging units for removal of CO 2 and oxygenation before blood is returned to the left atrium for distribution to the rest of the body. The bronchial circulation arises from the aorta and provides nourishment to the lung parenchyma. The circulation to the lung is unique in its duality and ability to accommodate large volumes of blood at low pressure. The alveoli are polygonal in shape and about 250 [inn in diameter. An adult has around 5 x 10 8 alveoli, which are composed of type I and type II epithelial cells. Under normal conditions type I and type II cells exist in a 1:1 ratio. The type I cell occupies 96% to 98% of the surface area of the alveolus and it is the primary site for gas exchange. The type II epithelial cell is small and cuboidal and is usually found in the "corners" of the alveolus, where it occupies 2% to 4% of its surface area. Type II cells synthesize pulmonary surfactant, which reduces surface tension in the alveolar fluid and is responsible for regeneration of the alveolar structure subsequent to injury. Remember: The region of the lung supplied by a segmental bronchus is called a bronchopulmonary segment and is the functional anatomic unit of the lung. The basic physiological unit of the lung is the respiratory or gas-exchanging unit (respiratory unit), which consists of the respiratory bronchioles, the alveolar ducts and the alveoli. Bronchi that contain cartilage and nonrespiratory bronchioles (i.e., lacking alveoli) in which cartilage is absent, serve to move gas from the airways to the alveoli and are referred to as the conducting airways. Inspiration is the active phase of breathing: the muscles of the chest wall, mainly the diaphragm, contract and move down into the abdomen, thereby resulting in negative pressure inside the chest. Gas then flows from higher to lower pressure.