Internal respiration is the process of gas exchange that occurs between the blood and the body’s cells and tissues. This process represents the second step in the body’s respiratory sequence, following the external respiration that happens in the lungs. It ensures every living cell receives the oxygen necessary to function while simultaneously removing metabolic waste. The exchange is driven purely by passive movement, transferring oxygen from the bloodstream into the tissues and carbon dioxide from the tissues back into the blood for transport to the lungs.
Mechanism of Gas Exchange in Tissues
The exchange of oxygen and carbon dioxide takes place across the walls of the systemic capillaries, the body’s smallest blood vessels. These capillaries form dense networks throughout nearly all tissues, bringing blood into close contact with the trillions of cells. The walls of these vessels are composed of a single layer of flattened endothelial cells, which creates an extremely thin barrier for gases to cross.
As blood enters the systemic capillaries, oxygen is carried primarily bound to the protein hemoglobin inside red blood cells. To reach the tissue cells, oxygen must first dissociate from the hemoglobin molecule. It then diffuses out of the capillary, crosses the interstitial fluid, and finally enters the cell.
At the same time, the tissue cells are constantly producing carbon dioxide as a byproduct of metabolism. This carbon dioxide moves in the opposite direction, diffusing out of the tissue cells and into the interstitial fluid. From there, it quickly crosses the capillary wall to enter the bloodstream. This simultaneous, bidirectional movement ensures a continuous supply and removal of waste. The blood leaving the systemic capillaries is now oxygen-poor and carbon dioxide-rich, ready to be returned to the lungs.
The Role of Partial Pressure Gradients
The physical force that drives gas exchange is the difference in partial pressure. Partial pressure refers to the pressure exerted by a single gas within a mixture of gases, such as those dissolved in blood. Gas molecules naturally move from an area where their partial pressure is higher to an area where it is lower, a movement known as a concentration gradient.
In the arterial blood arriving at the tissues, the partial pressure of oxygen (PO2) is high, typically around 100 millimeters of mercury (mmHg). Conversely, the metabolically active tissue cells are constantly consuming oxygen, which keeps their PO2 low, often around 40 mmHg. This steep difference in pressure creates a gradient that forces oxygen to rapidly diffuse out of the capillary blood and into the cells.
A similar but opposite gradient governs the movement of carbon dioxide (CO2). Tissue cells continuously produce CO2, resulting in a high partial pressure of carbon dioxide (PCO2) in the tissues, generally about 45 to 46 mmHg. The blood entering the systemic capillaries has a lower PCO2, around 40 mmHg. This sufficient gradient causes the CO2 to diffuse out of the cells and into the blood, ensuring its efficient removal.
Connecting Internal Respiration to Cellular Energy
The purpose of internal respiration is to sustain cellular respiration, the process that generates the energy required for every bodily function. Cellular respiration occurs within the mitochondria of cells, where nutrient molecules are broken down to produce adenosine triphosphate (ATP). This production of ATP requires a steady and reliable supply of oxygen.
Oxygen plays a specific role in the final stage of aerobic cellular respiration, known as the electron transport chain. It acts as the final electron acceptor, collecting electrons and hydrogen ions to form water. Without oxygen to accept these electrons, the energy-producing pathway would quickly halt.
The breakdown of nutrient molecules generates carbon dioxide as a waste product. This production of CO2 creates the high partial pressure inside the cells, which establishes the gradient necessary for its removal into the bloodstream. Internal respiration is the direct link between the air breathed in and the energy produced within the cells, facilitating oxygen delivery and waste product disposal.
Methods of Carbon Dioxide Transport
Once carbon dioxide diffuses from the tissues into the systemic capillary blood, it is transported back to the lungs via three distinct chemical methods.
Only a small fraction, approximately 7% of the total CO2, remains dissolved directly in the blood plasma. Carbon dioxide is twenty times more soluble in water than oxygen, which allows this small percentage to be carried effectively in the liquid component of the blood.
Another portion, about 23% of the total CO2, binds directly to the amino groups of hemoglobin within the red blood cells, forming a compound known as carbaminohemoglobin. This binding is distinct from oxygen binding, which occurs at the iron atom of the hemoglobin molecule.
The majority of carbon dioxide, however, is transported in the form of bicarbonate ions, accounting for about 70% of the total volume. This conversion to bicarbonate is catalyzed by the enzyme carbonic anhydrase inside the red blood cells, which rapidly combines CO2 with water to form carbonic acid. The carbonic acid then quickly dissociates into hydrogen ions and bicarbonate ions. The bicarbonate ions are then moved out of the red blood cell and into the plasma in exchange for a chloride ion, a process called the chloride shift, which allows the blood to carry large amounts of CO2 efficiently while helping to maintain the blood’s pH balance.