Question

Compare \(\mathrm{CO}_{2}\) and \(\mathrm{O}_{2}\) transport in the blood. Include the source and destination of each, and what physiological process makes gas exchange necessary.

Short answer

CO2 is mainly generated during cellular respiration as a waste product and is transported to the lungs, where it is exchanged for O2 and exhaled. O2 is sourced from the air and transported to cells throughout the body, where it is used for ATP production in cellular respiration. Hemoglobin in red blood cells binds and transports O2, while CO2 is transported as dissolved gas, bound to hemoglobin, and as bicarbonate ions. Gas exchange occurs in the lung's alveoli and in the body's tissue capillaries, facilitated by the differences in partial pressures of O2 and CO2. Cellular respiration makes gas exchange necessary for maintaining cellular function and homeostasis in the body.

Step-by-step solution

Sources and destinations of CO2 and O2

CO2 is mainly generated during cellular respiration, a process where cells generate ATP (energy molecules) by breaking down glucose and other organic molecules. In this process, CO2 is produced as a waste product and needs to be removed from the cells. The main destinations for CO2 are the lungs, where it is exchanged for O2 and expelled as we breathe. O2 is mainly sourced from the air we breathe, which contains around 21% oxygen. The major destination for O2 is cells throughout the body, where it is used in cellular respiration to produce ATP.

Transportation of CO2 and O2 in blood

The transportation of CO2 and O2 in the blood involves two main components: hemoglobin (a protein found in red blood cells) and blood plasma. About 98.5% of O2 is transported in the blood bound to hemoglobin, which picks up O2 in the lungs and releases it into tissues. Only 1.5% of oxygen is dissolved in the plasma. CO2 transport is more complex. Approximately 10% of CO2 is dissolved in plasma, while 30% is bound to hemoglobin. The remaining 60% is transported as bicarbonate ions (HCO3-) that form through the following reaction: \[ CO_{2} + H_{2}O \rightleftharpoons H_{2}CO_{3} \rightleftharpoons H^{+} + HCO_{3}^{-} \] This reaction is accelerated by carbonic anhydrase, an enzyme found inside red blood cells.

Gas exchange in the lungs and tissues

Gas exchange primarily occurs in the lung's alveoli and the surrounding capillaries. Oxygen diffuses from the alveoli into the blood, where it binds to hemoglobin in red blood cells. Simultaneously, CO2 diffuses from the blood into the alveoli to be exhaled. This process is facilitated by the difference in partial pressures of O2 and CO2 between the air in the alveoli and the blood. In the body tissues, gas exchange happens at the capillaries, where O2 unbinds from hemoglobin and diffuses into the cells. At the same time, CO2 produced during cellular respiration diffuses out of the cells and into the blood. This process relies on the difference in partial pressures of O2 and CO2 between the blood and the tissues.

The physiological process making gas exchange necessary

Gas exchange is necessary for maintaining cellular function and overall homeostasis in the body. The main physiological process that makes gas exchange necessary is cellular respiration. In this process, cells use O2 to generate ATP and produce CO2 as a waste product. It is essential that the cells continuously receive O2 and get rid of CO2 to maintain their metabolism and cellular functions. Gas exchange ensures that the blood continuously transports O2 from the lungs to the cells and CO2 from cells to the lungs. This process is vital for maintaining proper oxygenation and pH balance in the body. Without adequate gas exchange, cells would not be able to generate energy or perform essential functions, leading to tissue damage and ultimately organ failure.

Key concepts

Oxygen Transport

Oxygen transport is essential to keeping our bodies functioning optimally. Oxygen ( O_2 ) is primarily sourced from the air, comprising about 21% of the atmosphere. When we breathe in, oxygen flows down through the respiratory system into the alveoli in the lungs. Here, it diffuses across thin membranes into the capillaries.

• Once in the bloodstream, oxygen mostly hitches a ride on hemoglobin, a protein in red blood cells (RBCs). In fact, a whopping 98.5% of oxygen is carried this way.
• Only a smaller fraction, about 1.5%, is dissolved directly within the blood plasma.

As RBCs circulate through the body, they deliver oxygen to cells in tissues. This delivery is crucial because cells need oxygen to produce energy through cellular respiration.

Carbon Dioxide Transport

Transporting carbon dioxide ( CO_2 ) from the body’s tissues back to the lungs where it can be expelled is a bit more involved than transporting oxygen. As a waste product of cellular respiration, carbon dioxide accumulates within cells and needs to exit the body. In the bloodstream, carbon dioxide travels in three forms:

• About 10% of it dissolves in plasma.
• Approximately 30% binds with hemoglobin to form carbaminohemoglobin.
• The majority, around 60%, transforms into bicarbonate ions ( HCO_3^- ) through a reaction catalyzed by an enzyme called carbonic anhydrase.

The reaction looks like this: CO_2 + H_2O ↔ H_2CO_3 ↔ H^+ + HCO_3^- . As blood reaches the lungs, these processes reverse, allowing carbon dioxide to be released and exhaled.

Cellular Respiration

At the heart of why we need gas exchange is the process of cellular respiration. Our cells use this chemical process to convert glucose and oxygen into energy in the form of adenosine triphosphate (ATP). This energy is essential for all cellular activities. In cellular respiration:

• Oxygen serves as a crucial reactant, and it must be supplied continuously.
• The process generates carbon dioxide as a byproduct.
• This carbon dioxide must be expelled from the body to prevent it from building up in tissues.

Without cellular respiration, our cells would be unable to function or survive, highlighting the importance of continuous and efficient gas exchange in the body.

Hemoglobin

Hemoglobin is a versatile and crucial protein found within red blood cells that plays an essential role in gas transport. It is responsible for the transportation of both oxygen and carbon dioxide between the lungs and various tissues in the body.

• Each hemoglobin molecule can bind up to four oxygen molecules, facilitating efficient oxygen transport from the lungs to tissues.
• Hemoglobin also assists in carrying carbon dioxide from the tissues back to the lungs for exhalation.

Its ability to change shape and affinity based on the different chemical environments encountered as it travels through the blood allows hemoglobin to effectively release oxygen where it is needed and pick up carbon dioxide where it is produced.

Partial Pressure

Partial pressure is a key concept that drives the exchange of gases such as oxygen and carbon dioxide during respiration. It refers to the pressure exerted by a single type of gas in a mixture of gases. In the context of human physiology:

• Partial pressures help us understand how oxygen moves from the air into the blood and how carbon dioxide moves from the blood into the air.
• Oxygen is at a higher partial pressure in the alveoli compared to the blood, allowing it to diffuse efficiently into blood plasma.
• Conversely, carbon dioxide is at a higher partial pressure in the blood than in the alveoli, facilitating its diffusion out of the bloodstream.

By capitalizing on differences in partial pressures, our body efficiently exchanges oxygen and carbon dioxide in the lungs and tissues, supporting essential cellular functions through this delicate balance.