Chapter 17 Mechanics of Breathing

Transcription

1 1 Chapter 17 Mechanics of Breathing Running Problem COPD: Chronic Obstructive Pulmonary Disease (impaired air exchanged) - Chronic Bronchitis: (Blue Bloaters) Bluish tinge of skin and tendency to be overweight - Emphysema: (Pink puffers) Breathe shallow rapid breaths, tendency to be thin I. LEAD-UP a. Four primary functions of the respiratory system i. Exchange of gases between atmosphere and the blood ii. Homeostatic regulation of body ph iii. Protection from inhaled pathogens and irritating substances iv. Vocalization b. Respiratory system is a significant source of water loss and heat loss from the body c. Flow takes place from regions of higher pressure to regions of lower pressure d. A muscular pump creates pressure gradients e. Resistance to air flow is influenced primarily by the diameter of the tubes through which the air is flowing II. THE RESPIRATORY SYSTEM a. Ventilation (breathing): the exchange of air between the atmosphere and the lungs i. Inspiration (inhalation): is the movement of air into the lungs ii. Expiration (exhalation): is the movement of air out of the lungs

2 2 b. Respiratory system consists of structures involved in ventilation and gas exchange i. Conducting system (airways) ii. Alveoli (exchange surface O 2 In and CO 2 Out) iii. Bones/Muslces of Thorax (chest cavity) 1. Respiratory Diaphragm: Dome-shaped sheet of skeletal muscle 2. Intercostal Muscles a. Internal: Expiration b. External: Inspiration 3. Sternocleidomastoids: Head to sternum (inspiration) 4. Scalenes: Head to first two ribs (inspiration) 5. Abdominal Muscles: Expiration c. Respiratory System can be divided into Upper and Lower Respiratory tracts i. Upper Tract: Mouth/Nasal Cavity/Pharynx/Larynx 1. Pharynx: Common passageway for food, liquids and air 2. Larynx ii. Lower Tract: Trachea, Two primary bronchi/bronchi branches/lungs (Thoracic Portion) 1. Larynx: contains vocal folds and C.T. that tighten to make sound when air moves past them 2. Trachea: Semi-flexible tube help upon by C-shaped cartilage rings 3. Primary Bronchi: One bronchus to each lung; semi-rigid tubes supported by cartilage 4. Bronchioles: Smallest bronchi branch; collapsible passageways with walls of smooth muscle a. Contain Respiratory bronchioles form a transition between airways and the exchange epithelium of the lung d. Pleural Sacs Enclose the Lungs i. Each lung is surrounded by a double-walled pleural sac whose membranes line the inside of the thorax and cover the outer surface of the lungs 1. Pleural membrane (pleura) contains elastic C.T. and capillaries 2. Opposing membranes are held together by pleural fluid a. P.F creates slippery surface so opposing membranes can slide across on another as lungs move within thorax b. Holds lungs tight against thoracic wall

3 3 ii. Air filled balloon (the lung) surrounded by Water filled balloon (Pleural Sac) e. Alveoli site of Gas Exchange i. Clustered at the end of terminal bronchioles ii. Primary function is exchange of gases between themselves and blood iii. Type 1 Alveolar Cells 1. Very thin so gases can diffuse rapidly through them 2. Thin walls do not contain muscle because muscle fibers would block rapid gas exchange 3. IMPORTANT: Lung tissue itself cannot contract; C.T. between alveolar epithelia cells contain elastin fibers that create elastic recoil when lung tissue is stretched iv. Type 2 Alveolar Cells 1. Synthesize and Secrete Surfactant 2. Surfactant aids the lungs as they expand during breathing v. Blood vessels cover 80-90% of alveolar surface f. The Pulmonary Circulation is a High-Flow : Flow-Pressure System i. The rate of blood flow through the lungs is much higher than the rate in other tissues ii. Pulmonary blood pressure is low 1. Normal BP: 120/80 2. Lung/Pulmonary Arterial Pressure: 25/8 3. Resistance of pulmonary circulation is low so the right ventricle does not have to pump as forcefully to create blood flow to lungs 4. Low resistance is from shorter total length of pulmonary blood vessels and large crosssectional area s of pulmonary arterioles GAS LAWS I. The total pressure of a mixture of gases is the sum of the pressures of the individual gases (Dalton s law) II. Gases, singly or in a mixture, move from areas of higher pressure to areas of lower pressure III. If the volume of a container of gas changes, the pressure of the gas will change in an inverse manner (Boyle s law) IV. AIR IS A MIXTURE OF GASES a. The pressure of a single gas in a mixture is known as partial pressure b. Partial pressure is equal to atmospheric pressure (P atm ) times the gas s relative contribution (%) to P atm V. GASES MOVE FROM HIGH PRESSURE AREAS TO LOW PRESSURE AREAS a. Air flow occurs when pressure gradient present

4 4 VI. BOYLE S LAW DESCRIBES (pressure-volume) RELATIONSHIP of GASES a. P 1 V 1 =P 2 V 2 b. Chest volume increases, the alveolar pressure falls, and air flows into the respiratory system c. Chest volume decreases, the alveolar pressure increases, and air flows out of the respiratory system VENTILATION I. First exchange in respiratory physiology is ventilation (breathing) II. Lung Volumes Change During Ventilation III. Lung Volumes a. The air moved during breathing can be divided into four lung volumes i. Tidal Volume 1. Breathe quietly 2. Volume of air that moves during a single inspiration or expiration ii. Inspiratory Reserve Volume 1. Additional volume you inspire above the tidal volume; amount of additional air you can forcefully inhale after normal inhalation iii. Expiratory Reserve Volume 1. Amount of air forcefully exhaled after the end of normal expiration iv. Residual Volume 1. Volume of air in the respiratory system after maximal exhalation 2. Residual volume exists because the lungs are held stretched against the thoracic wall by pleural fluid 3. Amount of air always in your lungs IV. Lung Capacities a. Capacity: the sum of two or more lung volumes b. Vital Capacity i. Sum of the inspiratory reserve volume, expiratory reserve volume, and tidal volume ii. Represents the maximum amount of air that can be voluntarily moved into or out of the respiratory system with one breath iii. To test: Instruct person to take in as much air as possible then blow out as much air as possible

5 5 iv. Decreases with each age V. During Ventilation Air Flows Because of Pressure Gradients a. Air flows into the lungs because o the pressure gradients created by a pump b. The muscles of the thoracic cage and diaphragm function as the pump, because most lung tissue is thin for gas exchange and thick muscle tissue would block this process c. Breathing is an active process that uses muscle contraction to create pressure gradient d. Air moves into lungs when you inhale; air moves out of lungs when you exhale e. Respiratory Cycle = Inspiration followed by Expiration VI. Inspiration Occurs When Alveolar Pressure Decreases a. For air to move into the lungs, pressure inside the lungs must become lower than atmospheric pressure outside of the lungs b. Inspiration: i. Thoracic volume increases ii. Skeletal muscles of rib cage and diaphragm contract iii. Alveolar pressure decreases iv. External intercostals and scalene muscles contract and pull the ribs upward and out [pump handle (muscles) lift the ribs upwards and out increases volume/decreasing pressure] VII. Expiration Occurs When Alveolar Pressure Increases (exceeds atmospheric pressure) a. At the end of inspiration the elastic recoil of the lungs and thoracic cage returns the diaphragm and rib cage to relaxed positions b. Passive Expiration: Expiration that involves passive elastic recoil rather then muscle contraction (passive expiration) c. Active Expiration: During exercise or forced heaving breathing; occurs when ventilation exceeds breaths per minutes (normal ventilation BPM) i. Uses internal intercostals muscles ii. Uses abdominal muscles 1. Internal intercostals and abdominal muscles are collectively called expiratory muscles 2. When Internal intercostals contract they pull ribs inwards decrease volume increasing pressure [Internal/External intercostals are antagonistic pair]

6 6 3. Abdominal contraction decreases abdominal volume, displacing the intestines and liver upward, decreasing the thoracic volume even more because of the organs pushing against the diaphragm Pneumothorax Air in the pleural cavity breaks the fluid bond holding the lung to the chest wall Chest wall expands outward while elastic lung collapses to unstretched state (deflated balloon) Obstructive Lung Disease Air flow during expiration is diminished because of narrowing of bronchioles Air whistling through narrowed lower airway during exhalation creates wheezing sound o o Includes Asthma Treatments for Asthma β2 adrenergic agonsits Anti-inflammatory drugs Leukotriene antagonists Comment [HAW1]: Bronchoconstrictors that are released during inflammatory responses VIII. Lung Compliance and Elastance a. The ability of the lung to stretch is called compliance i. High-compliance lung stretches easily just as a highly compliant person is willing to obey ii. Low-compliance lung requires more force from inspiratory muscles as a outcast who doesn t want to obey iii. A decrease in lung compliance affects ventilation because more work must be expended to stretch a stiff lung (restrictive lung disease) 1. Energy expenditure required to stretch less-compliant lung can far exceed normal work of breathing 2. Causes: a. Inelastic scar tissue formed by Fibriotic Lung Disease b. Inadequate alveolar production of surface (chemical that facilitates lung expansion) b. Elastance (elasticity) i. Ability to return to resting volume after being forcefully stretched ii. High compliance DOES NOT mean High Elastance iii. Ex: Comment [HAW2]: Makes Inhalation harder/more active process because lung CAN NOT stretch;

7 7 1. Old gym shorts; after many washes the elastic waste band is easy to stretch (high compliance), but lacking in elastance, making it impossible for the shorts to stay up on your waste iv. Ex: Emphysema is a disease in which elastin fibers which are normally found in the lung tissue are destroyed; resulting in lungs that exhibit high compliance and stretch easily during inspiration, however they have decreased elastance so they do not recoil to their resting position during expiration Comment [HAW3]: Elastin broken down by Elastase which is released when lungs are filled with smoke or irrantants; creates difficulty exhaling (turning a passive process into a active process/most PUSH air out of lungs) IX. Surfactant Decreases the Work of Breathing a. Lungs secrete surfactant that reduces surface tension b. Molecules that disrupt cohesive forces between water molecules c. More concentrated in smaller alveoli, making their surface tension less than that in larger alveoli i. Lower surface tension helps equalize pressure among alveoli of different sizes and makes inflating smaller alveoli easier ii. Lower surface tension = less worked need with each breath iii. PROBLEM 1. Newborn Respiratory Distress Syndrome (RDS) 2. Low-compliance because no surfactant production X. Airway Diameter is the Primary Determinant of Airway Resistance a. Three parameters contribute to resistance i. Systems length ii. Viscosity of substance flowing through system iii. Radius of the tubes in the system b. Length/Viscosity are essential constant for the respiratory system, the radius (or diameter) of the airways becomes primary determinant of airway resistance c. CARBON DIOXIDE is the primary Paracrine that affects bronchiolar diameter i. BRONCHOCONSTRICTION 1. Increases resistance to air flow and decreases the amount of fresh air that reaches the alveoli 2. Parasympathetic Neurons (Muscarinic receptors) used to protect lower respiratory system from inhaled irritants 3. Histamine (released by mast cells in response to tissue damage or allergenic causing antigens) 4. Leukotrienes ii. BRONCHODILATION

8 8 1. Increased CO 2 in expire air relaxes bronchiolar smooth muscle and causes dilation of bronchioles (as during hyperventilation; increasing amount of carbon dioxide release; decreasing amount of CO 2 in blood and increasing O 2 in blood; alkalosis condition) 2. Epinephrine Agonists (β2 Receptors) XI. Rate and Depth of Breathing Determine Efficiency of Breathing a. Estimate effectiveness of ventilation by calculating TOTAL PULMONARY VENTILATION; the volume of air moved into and out of the lungs each minutes (also known as minute volume) i. Total pulmonary ventilation = ventilation rate X tidal volume ii. Represents the physical movement of air into and out of the respiratory tract (not a good indicator though to how much fresh air reaches alveolar exchange) iii. Part of every breath remains in conducting airways (trachea/bronchi), spots known as anatomic dead space (which holds average 150mL) iv. Maximum voluntary ventilation (breathing as deeply and quickly as possible) increases total pulmonary ventilation b. More accurate indicator of ventilation efficiency is AVEOLAR VENTILATION i. The amount of fresh air that reaches the alveoli each minute ii. Aveolar Ventilation Rate = Ventilation rate X (tidal volume dead space) XII. Gas composition in Alveoli varies little during normal breathing a. Hyperventilation: (Alveolar Ventilation Increases) Alveolar PO 2 rises to 120mmHg while PCO 2 falls to 20mHg b. Hypoventilation: (Alveolar Ventilation Decreases) Alveolar PO 2 decreases while PCO 2 increases Types and Patterns of Ventilation Eupnea Normal quiet breathing Hypernea Increased respiratory rate and or Exercise volume in response to increased metabolism Hyperventilation Increased respiratory rate and or volume without increased metabolism Emotional hyperventilation; blowing up balloon Hypoventilation Decrease alveolar ventilation Shallow breaths; asthma; restrictive lung disease Tachypnea Rapid breathing; usually increased Panting respiratory rate with decreased epth Dyspnea Diffiuctly breathing (air hunger) Pathologies/extreme exercise Apnea Cesation of breathing Voluntary breath holding; depression of CNS control centers Local Control of Arterioles and Bronchioles by O and CO 2 Gas Consumption Bronchioles Pulmonary Arterioles Systemic Arterioles PCO 2 Increases Dilate Constrict Dilate PCO 2 Decreases Constrict Dilate Constrict PO 2 Increases Constrict Dilate Constrict PO 2 Decreases Dilate Constrict Dilate Hypoventilation Decreased O 2 Bronchioles Dilate Increased CO 2 Bronchioles Dilate Hyperventilation Increased O 2 Bronchioles Constrict Decreased CO 2 Bronchioles Constrict

9 9 [Body attempts to match air flow and blood flow in each section of the lung by regulating diameters of arterioles and bronchioles] KEY: Decreased tissue PO 2 around under-ventilated alveoli constricts their arterioles, diverting blood to better ventilated alveoi