Respiratory Gas Exchanges

Respiratory gases are exchanged in the alveoli and the capillaries of the body tissues. Oxygen is transferred from the lungs to the blood, and carbon dioxide is transferred from the blood to the alveoli to be exhaled as a wate product. At the tissue level, oxygen is transferred from the blood to tissues and carbon dioxide is transferred from tissues to the blood to return to the alveoli and be exhaled. This transfer is dependent on the profess of diffusion.
As mentioned, diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration. Diffusion of respiratory gases occurs at the alveolocappillary membrane, and the rate of diffusion can be affected bt the thickness of the membrane. Increased thickness of the membrane impedes diffusion because gases take longer to transfer across. Clients with pulmonary edema, pulmonary infiltrates,or a pulmonary effusion have an increased thickness of the alveolocapilary membrane, resulting the slowed diffusion, slowed exchange of respiratory gases, and impaired delivery of oxygen to tissues. The surface area of the membrane can be altered as a result of chronic disease (e.g.,emphysema), an acure disease (e.g.,pneumothorax), or surgical process(e.g.,lobectomy). The alveolocappilary membrane can be destroyed or many thicken, changing the rate of diffusion. When fewer alveoli are functioning, the surface area is decreased.
Oxygen transport. The oxygen transport system consist of the lungs and cardiovascular system. Delivery depends on the amount of oxygen entering the lungs (ventilation), blood flow to the lungs and tissues(perfusion),rate of diffusion,and oxygen-carrying capacity. The capacity of the blood to carry oxygen is influenced by the amount of dissolved oxygen in the plasma, amount of hemoglobin, and tendency of hemoglobin to bind with oxygen. Only a relatively small amount of required oxygen, less than 1%,is dissolved in the plasma. Most oxygen is transported by hemoglobin,which serves as a carrier for oxygen and carbon dioxide. The hemoglobin molecules combines with oxygen to form oxyhemoglobin. The formation of oxyhemoglobin is easily reversible,allowing hemoglobin and oxygen to dissociate,which frees oxygen to enter tissues.
Carbon dioxide transport. Carbon dioxide diffuses into red blood cells and is rapidly hydrated into carbonic acid(H2CO3) because of the presence of carbonic anhydrase. The carbonic acid then dissociates into hydrogen (H+-) and bicarbonate (H2CO3-) ions. The hydrogen ion is buffered by hemoglobin,and the HCO3- diffuses into the plasma. In addition,some of the carbon dioxide in red blood cells reacts with amino acid groups,forming carbamino compounds. This reaction can occur rapidly without the presence of an anzyme. Reduced hemoglobin(deoxyhemoglobin) can combine with carbon dioxide more easily than oxyhemoglobin, and therefore venous blood transports the majority of carbon dioxide.