Control of ventilation.pdf

PULMONARY VENTILATION Dr. Shereen Samir ILOs  1- to define pulmonary ventilation  2- to demonstrate the factors affecting pulmonary ventilation  3- to know factors determine the airway resistance  4- What is meant by surfactant and compliance  Pulmonary Ventilation M...

PULMONARY VENTILATION Dr. Shereen Samir ILOs  1- to define pulmonary ventilation  2- to demonstrate the factors affecting pulmonary ventilation  3- to know factors determine the airway resistance  4- What is meant by surfactant and compliance  Pulmonary Ventilation Minute pulmonary ventilation ; is the volume of air breathed in & out /min = Tidal volume x RR= 500 ml x 12 cycles/min = 6 L/min Effective Pulmonary ventilation or alveolar ventilation : is the volume of air exchanged between atmosphere and alveoli /min =Tidal volume –Dead space x RR = 500-150 (350 mL) x 12 breaths /min = 4.2 L/min Factors Affecting Pulmonary Ventilation Surfactant Pressures in Compliance Pulmonary the thoracic Ventilation cavity. Resistance of air passage  I- Resistance to Air flow 1. Tissue resistance to air flow: 20% due to friction from movement of the lung over chest wall & pleura 2. Airway resistance: 80%  Airway Resistance (80%) As the resistance is mostly affected by radius of airway (R α 1/r4) , so Airway radius is the most important determining factor for airway resistance  Factors affecting Airway Diameter I- Nervous 1- Sympathetic adrenergic innervation; Direct symp. stimulation --- bronchodilatation. 2- Parasympathetic cholinergic innervation; rich innervation of airways, so when stimulated → bronchoconstriction & increase in mucus secretion 3- Non adrenergic Non cholinergic(NANC) innervation; e.g. VIP → bronchodilatation. Substance P, Neurokinin A →bronchoconstriction.  Factors affecting Airway Diameter II- Chemical 1. Circulating catechoalmines: adrenaline & noradrenaline act on B2 adrenoreceptors in bronchial smooth muscle → marked bronchodilatation. 2. Local mediators: Histamine (H1 receptors) → bronchoconstriction. Serotonin → bronchoconstriction. Slow reactive substance of anaphylaxis (SRS, allergic response to pollen) → bronchoconstriction. PG F → bronchoconstriction. PG E2 → bronchodilatation  Factors affecting Airway Diameter III Irritants: dust, smoke, air pollution, cold air → Airway constriction.  2- Pressure in thoracic cavity (i) intrapulmonary pressure: -Definition→ pressure inside lung alveoli & it’s connected with the atmosphere. -values → during inspiration= -2 mmHg (air rush into lung) →during expiration = +2 mmHg (air rush out lung) (ii) intrapleural pressure: -Definition → pressure inside pleural cavity which is the space between visceral & parietal pleura (always –ve pr) Values→ –ve pressure→-3 mmHg at end of normal expiration ❑ -6 mmHg at end of normal inspiration ❑-30 mmHg in forced inspiration ❑+40 mmHg in forced expiration  Compliance Compliance (Distensibility): is a measure of the ease of inflation of the lungs the extent to which the lung expands for each unit increase in its transmural pressure. For the lungs, transmural pressure = intrapulmonary pressure – intrapleural pressure. It is called "transpulmonary pressure". Every time, the transpulmonary pressure increase by one centimeter of water, the lungs expand 200 milliliters For the lung and chest wall, transmural pressure = intrapulmonary pressure – atmospheric pressure. It is called "transthorathic pressure". The combined compliance of the lung and thorax is 130 ml/cm water pressure.  Compliance Lungs compliance is reduced by factors that increase resistance to distension e.g. pulmonary fibrosis, edema. Chest compliance is reduced in: obesity, chest wall deformities  Elasticity Elasticity means: Tends to return to its original volume or shape The lung is an elastic structure This elastic property is important for expiration to occur but also it opposes lung inflation Elastic recoil properties are due to elastic fibres in lung parenchyma (elastin & collagen fibres) & surface tension of fluid lining alveoli  Surface Tension The tension of fluid lining alveoli that would lead to instability of alveoli & their collapse This is prevented by presence of Surfactant (Surface Active Agent) Surfactant is a phospholipid and protein mixture secreted by Type II alveolar cells It reduces surface tension of fluid lining alveoli by decreasing the forces attracting water molecules at air- water interface in alveoli The main stimulus for its synthesis is: Stretch of the alveoli (distortion of alveolar type II cells by inspired air) , B adrenergic stimulation and cortisol After its release, it is degraded by phospholipase enzymes present in the alveoli  Function → (1) prevent lung collapse (2) ↓ muscle effort to expand lung (3) Has antibacterial action: stimulant for macrophages, assists ciliary movement Factors decrease surfactant → (1) Resp. distress syndrome (in infants) (2) Cigarette smoking (destruction of type II cells) (3) Heart-lung bypass (prolonged pulmonary artery occlusion) (4) Long term inhalation of 100% O2  Because of ………..the lungs do not usually collapse; a). Negative Intrapleural pressure b). Positive IPP b). Negative Intrapulmonary pressure c). Positive intrapulmonary pressure e). Pulmonary blood pressure