BreathingBreathing is the exchange of gases. Therespiratory and cardiovascular systemscombine to provide an efficient deliverysystem that carries oxygen to our body tissuesand removes carbon dioxide from them. Thistransportation involves four separateprocesses:
-1. Pulmonary ventilation (breathing), which is the movement of gases into and out of the lungs2. Pulmonary diffusion, which is the exchange of gases between the lungs and the blood3. Transport of oxygen and carbon dioxide via the blood4. Capillary gas exchange, which is the exchange of gases between the capillary blood and the metabolically active tissue.
-The first two processes are referred to as externalrespiration because they involve moving gasesfrom outside the body into the lungs and thenthe blood. Once the gases are in the blood theymust travel to the tissues. When blood arrives atthe tissues, the fourth step of respiration occurs.This gas exchange between the blood and thetissues is called internal respiration. Thusexternal and internal respiration are linked by thecirculatory system.
Pulmonary VentilationPulmonary ventilation, commonly referred to asbreathing, is the process by which we move airinto and out of our lungs. Air is typically drawninto the lungs through the nose, although themouth must also be used when the demand forair exceedes the amount that can comfortably bebrought in through the nose. Bringing air inthrough the nose has certain advantages overmouth breathing. The air is warmed andhumidified as it swirls through the irregularsurfaces (turbinates) inside the nose.
-Of equal importance, the turbinates churn theinhaled air, causing dust and other particles tocontact and adhere to the nasal mucosa. Thisfilters out all but the tiniest particles, minimizingirritation and the threat of respiratory infections.From the nose and mouth, the air travels throughthe pharynx, larynx, trachea, bronchi, andbronchioles, until it finally reaches the smallestrespiratory units: alveoli. The alveoli are the sitesof gas exchange in the lungs
-• The lungs are not directly attached to the ribs. Rather, they are suspended by the pleural sacs. These sacs envelop the lungs and contain a thin layer of pleural fluid that reduces friction during respiratory movements. In addition, these sacs are connected to the lungs and to the inner surface of the thoracic cage, causing the lungs to take the shape and size of the cage as the chest expands and contracts.• These relationships between the lungs, the pleural sacs, and the thoracic cage determine air flow into and out of the lungs. There are two phases: inspiration and expiration
Inspiration• Inspiration is an active process involving the diaphragm and the external intercostal muscles. The ribs and sternum are moved by the external intercostal muscles. The ribs swing up and out. The sternum swings up and forward. At the same time. the diaphragm contracts, flattening down toward the abdomen. When the lungs are expanded, the air within them has more space to fill, so the pressure within the lungs decreases. As a result, the pressure in the lungs (intrapulmonary pressure) is less than pressure of the air outside the body. Because the respiration tract is open to the outside, air rushes into the lungs to reduce this pressure difference. Thus air is brought into the lungs during inspiration.
Expiration• At rest, expiration is usually a passive process involving the relaxation of the inspiratory muscles and elastic recoil of the lung tissue. As the external intercostal muscles relax, the ribs and sternum lower back into their resting position. This increases the pressure in the thorax, so air is forced out of the lungs. Thus expiration is accomplished. During forced breathing, expiration becomes a more active process.
Pulmonary Diffusion• Gas exchange in the lungs, called pulmonary diffusion, serves two major functions:1. It replenishes the blood oxygen supply that has been depleted at the tissue level where it is used for oxidative energy production.2. It removes carbon dioxide from returning venous blood.
-• Pulmonary diffusion has two requirements: air that brings oxygen into the lungs and blood to receive the oxygen and give up carbon dioxide and give up carbon dioxide. Air was brought into the lungs during pulmonary ventilation; now gas exchange must occur between this air and the blood.
The Respiratory MembraneGas exchange between the air in the alveoli and the blood in the pulmonary capillaries occurs across the respiratory membrane (also called the alveolar-capillary membrane) which is composed ofa) the alveolar wallb) the capillary wall, andc) their basement membranes.
Partial Pressure of Gases• The air we breathe is a mixture of gases. Each exerts pressure in proportion to its concentration in the gas mixture. The individual pressures from each gas in a mixture are referred to as partial pressures. According to Dalton`s law, the total pressure equals the sum of the partial pressures of the individual gases in that mixture.
-• The air we breathe is composed of 79.04% nitrogen (N2), 20.93% oxygen (O2), and 0.03% carbon dioxide (CO2). If the total atmospheric pressure is 760 mmHg, then the partial pressure of nitrogen (PN2) in air is 600.7 mmHg (79.04% of the total 760 mmHg pressure). Oxygen`s partial pressure (PO2) is 159.0 mmHg (20.93% of 760 mmHg ), and carbon dioxide`s partial pressure (PCO2) is 0.03% of 760 mmHg ).
Gas Exchange in the Alveoli• Differences in the partial pressures of the gases in the alveoli and the gases in the blood create a pressure gradient across the respiratory membrane. This forms the basis of gas exchange during pulmonary diffusion
Oxygen Transport• Oxygen is transported by the blood either combined with hemoglobin (Hb) in the red blood cells (>98%) or dissolved in the blood plasma (<2%). Each molecule of hemoglobin can carry four molecules of oxygen. When oxygen binds to hemoglobin, it forms oxyhemoglobin; hemoglobin that is not bound to oxygen is referred to as deoxyhemoglobin. The binding of oxygen to hemoglobin depends on the Po2 in the blood and the bonding strength or affinity, between hemoglobin and oxygen.
-There are other factors which affect this bondingaffinity, such as the level of pH and blood temperature– higher pH and lower blood temperature increase thebonding affinity between hemoglobin and oxygen. TheThe pH in the lungs is generally high, so hemoglobinpassing through the lungs has a strong affinity foroxygen, encouraging high saturation. At the tissuelevel, however, the pH is lower, causing oxygen todissociate from hemoglobin, thereby supplying oxygento the tissues. In the lungs, where the blood might be abit cooler, hemoglobin`s affinity for oxygen isincreased. This encourages oxygen binding.
Carbon Dioxide Transport• Carbon dioxide also relies on the blood for transportation. Once carbon dioxide is released from the cells, it is carried in the blood primarily in three forms:1) Dissolved in plasma (7-10%)2) As bicarbonate ions resulting from the dissociation of carbonic acid (60-70%)3) Bound to hemoglobin (20-30%) – when carbon dioxide binds to hemoglobin, they form a compound called carbaminohemoglobin. Carbon dioxide binding depends on the oxygenation of the hemoglobin and the partial pressure of CO2
The Regulation of Pulmonary Ventilation• The respiratory muscles are under the direct control of motor neurons, which are in turn regulated by respiratory centers located within the brainstem (in the medulla oblongata and pons). These centers establish the rate and depth of breathing by sending out periodic impulses to the respiratory muscles. However, the respiratory centers don`t act alone in controlling breathing. Its regulation is also determined by a changing chemical environment in the body
Voluntary Control• We can exert some voluntary control over our breathing through the cerebral motor cortex. However, this voluntary control can be overridden by the involuntary control of the respiratory centerIf we try to hold our breath, at some point, regardless of our conscious decision to suppress breathing, our carbon dioxide and H+ levels become quite high, our oxygen level drops, and our inspiratory center decides that breathing is imperative and it forces us to inhale,.
Respiratory System – Anatomy and PhysiologyThe respiratory system is made up of the organs involved in the interchanges of gases and consists of the:• nose• mouth (oral cavity)• pharynx (throat)• larynx (voice box)• trachea (windpipe)• bronchi• lungs
Nose and Nasal CavityThe nose is the uppermost part of the respiratorytract. It is made up to two bones and cartilage. Itforms a hollow passage that connects the nostrilsand the top of the throat. This passage is calledthe nasal cavity . It is lined with a mucousmembrane which bears tiny hairs. The function ofthe nose is to filter, warm and moisten the airbefore it moves on to other parts of therespiratory tract. The tiny hairs trap the dustparticles, bacteria and other foreign bodies thatenter the nose. These hairs also induce sneezingto remove foreign bodies lodged in the nose.
Mouth and PharynxThe pharynx is a passageway from the back of themouth (oropharynx) and nose (nasopharynx) tothe upper part of the esophagus(laryngopharynx) and into the voice box, orlarynx. The pharynx acts like a station where thefood tube and the air tube meet. Food beingsswallowed is prevented from entering the airtube by a thin structure, called epiglottis , thatcloses the air tube. This is why we cannot breathewhile we are swallowing.
Larynx and Trachea At the lower end of the pharynx is the larynxwhich forms part of the air tube. It is made ofcartilage. One of its functions is production ofvoice. It does this with the help of either one oftwo pairs of vocal cords. When air from the lungspasses over the stretched vocal cords, vibrationsare produced. The tongue palate and lips modifythe vibrations to produce speech. Anotherfunction of the larynx is to prevent choking. Theelongated space between the vocal cords is calledglottis .
-The epiglottis folds back over the glottis when we swallow food, so the food cannot enter the trachea or air tube. The trachea, or windpipe , begins just below the larynx and ends behind the upper part of the breastbone where it divides to form two tubes. The trachea is made of elastic tissue and smooth muscle. It also has rings of elastic cartilage that keep the trachea open when the neck moves. It is lined with a mucous membrane and hair like projections called cilia . Mucus helps trap dust and bacteria in the incoming air. The cilia move the mucus upward to clear the respiratory tract.
The Lower Respiratory TractThe lower respiratory tract includes the following:• airways (bronchi and bronchioles)• air sacs (alveoli)• lungs
Bronchi, Bronchioles and AlveoliThe trachea branches into two tubes-thebronchi. Each bronchus (singular of bronchi)enters the lung and branches into narrowertubes called bronchioles . The walls of thebronchi and larger bronchioles are supportedby cartilage. Their walls produce mucus whichis moved upward by the cilia to clear the airpassage. Each bronchiole ends in balloon likeair sacs called alveoli .
-The alveoli have thin walls which aresurrounded by blood vessels. The bronchi andbronchioles form the air passage into thelungs. The exchange of oxygen and carbondioxide takes place in the alveoli. There areabout 300 million alveoli in the lungs. Thesealveoli increase the surface area of the lungsand allow many blood vessels to collectoxygen.
LungsThe lungs are a pair of conical organs presentin pleural cavities; both the lungs togetherform an important part of the humanrespiratory system. Left lung is divided into 2lobes (superior and inferior) while the rightone in 3 (superior, inferior and middle). Eachlung has a median slit and hilus (or hilum).