Breathing Mechanisms

Processes of Respiration 1) Pulmonary Ventilation (Air moves in and out of lungs) 2) External Respiration (Exchange of gas between lungs and blood) 3) Respiratory Gas Transport 4) Internal Respiration (Exchange of gas between blood and cells)

Pulmonary Ventilation Breathing, or ventilation, is the movement of air from outside the body into and out of the bronchial tree and alveoli The actions providing these air movements: - Inspiration = inhalation - Expiration = exhalation

Inspiration Atmospheric pressure due to the weight of air is the force that moves air into the lungs The pressure on the inside of the lungs and alveoli (intrapulmonary pressure) and on the outside are about the same If the pressure inside the lungs and alveoli decreases below air pressure, atmospheric pressure will push outside air into the airways.

Fig. 16.12

Cause of decrease in intrapulmonary pressure: Phrenic nerves stimulate contraction of the diaphragm – diaphragm moves downward enlarging thoracic cavity The external intercostal muscles may be stimulated, raising the rib cage and sternum, enlarging thoracic cavity Both of these actions will cause the lungs to expand and decrease intrapulmonary pressure as long as both the pleural membranes move together The water based serous fluid of the pleural cavity ensures that the two membranes are held together

Fig. 16.13

Expiration As the diaphragm and intercostal muscles relax following inspiration, the elastic tissues cause the lungs and thoracic cavity to ‘recoil’ and return to their original shapes. The surface tension of the alveolar linings causes them to decrease in size Both of these actions cause the intrapulmonary pressure to increase (above atmospheric pressure) causing air to be forced out.

Fig. 16.14ab

Forced Expiration If respiratory passageways are narrowed by spasms of the bronchioles (asthma) or clogged with mucus/fluid (pneumonia), respiration becomes an active process.

Collapsed Lung The alveoli are so elastic that they attempt to recoil at the end of exhalation In order to prevent this, there is a small amount of pressure in the pleural cavity that acts as a “suction” to keep the alveoli slightly inflated If air enters the pleural cavity it will cause the lung to collapse

Respiratory volumes Tidal volume (TV) – volume of air that enters/leaves during a single respiratory cycle (resting tidal volume – 500mL) Inspiratory reserve volume (IRV)– the additional air that enters during forced inspiration (3100mL) Expiratory reserve volume (ERV)– the additional air that is expelled during forced expiration (1200mL) Residual volume (RV)– volume of air that continually remains in lungs (approx. 1200mL)

Vital Capacity Total amount of exchangeable air IRV + TV + ERV

Respiration & Transport

Alveoli The aveoli carry on the vital process of exchanging gases between the air and the blood Each aveolus wall consists of simple squamous epithelium, as is the network of capillaries that surrounds each aveolus. The area between each aveolus and capillary is referred to as the respiratory membrane

Diffusion of Gases The gases found in air (Nitrogen, Oxygen, Carbon Dioxide) will diffuse across the respiratory membrane from an area of high pressure to an area of low pressure The gases will then dissolve in the plasma portion of blood.

High Altitude Sickness The efficiency of gas exchange is decreased because of the low atmospheric pressure at high altitudes

Oxygen Transport Over 98% of oxygen that blood transports is carried on the iron-containing protein hemoglobin. Once combined, the two weakly bonded molecules form oxyhemoglobin, which will release oxygen when it reaches areas of low oxygen concentration. It will also release oxygen in areas of extremely high Carbon Dioxide levels

Carbon Dioxide Transport Carbon Dioxide is a waste product of cellular activities, so it will diffuse from the tissues into the blood. It will be transported in one of three ways: 1) dissolved in the plasma 2) bonding with hemoglobin to form carbaminohemoglobin 3) Formation of bicarbonate ions (HCO3-) in the plasma

Carbon Monoxide Poisoning Carbon Monoxide (CO) competes vigorously with oxygen for the same binding sites on hemoglobin In fact, it binds more easily Causes inadequate oxygen delivery (hypoxia)

Control of Respiration

Control of Respiration The basic rhythm of respiration is controlled by groups of neurons in the brain stem. The area from which nerve impulses are sent to respiratory muscles is called the Respiratory Center, which is functionally divided into three areas:

Medulla Centers “Pacemaker” – sets the basic breathing rhythm 1. Self-exciting inspiratory center Impulses travel along the phrenic and intercostal nerves to excite the diaphragm and external intercostal muscles Expiratory center Inhibits the pacemaker in a rhythmic way

Pons centers Neurons of the pons that control the breathing rate by transmitting inhibitory impulses to the inspiratory area so that the lungs to not become too full of air Smooth out the rhythm set by the medulla

Regulation of Respiratory Center Chemosensitive areas – monitor concentration of dissolved gases in blood and CSF The inflation reflex – the stretch of lung tissue indicate the necessity of breathing Emotional upset Conscious control Body movement Temperature (Increase body temperature, increase respiration) Blood pressure (increase in blood pressure will decrease respiration rate)

Regulation of Bronchiole size Bronchoconstriction : caused by parasympathetic NS and mediators of allergic reactions (histamine) Bronchodilation: caused by sympathetic NS and epinephrine Asthma attack – increased mucous and smooth muscle spasms