# Tissue Gas Exchange --- **Tissue gas exchange** is a step in [[respiratory system|respiration]] which involves the exchange of oxygen the blood entering into the bodily [[tissue]] at the [[capillaries]], and at the same time picking up the [[carbon dioxide]] waste from the tissues. It is essentially a reverse of the reactions that happen in the [[alveolar gas exchange|alveoli]]. ![[tissue gas exchange.png]] This process is then essentially reversed during [[alveolar gas exchange]] in the capillaries of the lungs. ## Oxygen Any [[oxygen]] that's dissolved directly into the plasma can freely diffuse across the capillary membrane. Most of the oxygen, however, is bound to [[hemoglobin]]. When the O$_2$ disassociates from the hemoglobin, a free floating [[hydrogen ion|H+]] can bind to it and it becomes **deoxyhemoglobin**. ## Carbon Dioxide About 10% of the [[carbon dioxide]] from the cells enter the bloodstream and then stay dissolved directly in the plasma. Another 20% enters into the [[red blood cells]] and binds to hemoglobin. The remaining 70% reacts with free floating [[water]] and becomes the weak acid [[carbonic acid]], which immediately sheds a H+ and becomes [[bicarbonate]]. Some of this reaction happens directly in the plasma, but it's quite slow. [[carbonic acid|Carbonic anhydrase]] is an enzyme present in RBCs which drastically speeds up this reaction. In order to then kick out the bicarbonate into the plasma, a single [[chloride]] is brought into cell by a process called **chloride shift**. ## Partial Pressures Before the tissue gas exchange happens the blood is "oxygenated": The PO$_2$ in the oxygenated blood is 100 mmHg (technically it's 104) The PCO$_2$ in the deoxygenated blood is 40 mmHg After the tissue gas exchange the blood is "deoxygenated": The PO$_2$ in the deoxygenated blood is 40 mmHg The PCO$_2$ in the deoxygenated blood is 45 mmHg ___