Question

The partial pressure of oxygen is maximum in (a) alveolar air (b) arterial blood (c) venous blood (d) expired air-

Short answer

The partial pressure of oxygen is maximum in (a) alveolar air.

Step-by-step solution

Understanding Partial Pressure

The partial pressure of a gas in a mixture is the pressure the gas would exert if it alone occupied the volume of the mixture at the same temperature. In the context of the human body, this relates to the concentration of the gas in different parts of the respiratory system and the bloodstream.

Comparing Locations of Oxygen Partial Pressures

Oxygen's partial pressure is highest where the gas exchange rate is greatest and the concentration of oxygen is highest. This occurs in the alveolar air where oxygen is transferred from the lungs to the blood.

Elimination Process

We can eliminate other options by understanding their roles in gas exchange: arterial blood has just received oxygen so while it is high, there is some loss due to binding to hemoglobin; venous blood is deoxygenated after delivering oxygen to tissues; and expired air has less oxygen as some has been absorbed into the bloodstream.

Key concepts

Understanding Alveolar Air

Alveolar air is the air found in the alveoli, the tiny sacs within our lungs. Alveolar air plays a crucial role in the respiratory system. The air we inhale isn't pure oxygen but a mixture of gases, primarily nitrogen and oxygen. The partial pressure of a gas is a measure of that gas's concentration within a mixture, and because alveoli are the main site for the gas exchange between the air and our blood, the partial pressure of oxygen is highest here.

Within the alveoli, the partial pressure of oxygen is approximately \( P_{O2} = 104 \ mmHg \) which is conducive to transferring oxygen into the blood. This process is vital for the oxygenation of our blood and eventually for the supply of oxygen to the entire body, which is essential for metabolism at the cellular level. The high partial pressure in the alveoli facilitates this transfer efficiently.

Arterial Blood and Oxygen Transport

Following the oxygen exchange in the alveoli, oxygenated blood is transported via the arterial blood system. Arterial blood carries oxygen from the lungs throughout the body, delivering it to various tissues. The partial pressure of oxygen in arterial blood (\( P_{aO2} \) ) averages about \( 95 \ mmHg \). While this value is high, it's slightly less than in alveolar air. This difference accounts for the oxygen which has been used for the metabolism in body tissues and the oxygen that binds to hemoglobin in red blood cells.

The arterial blood predominantly carries oxygen bound to hemoglobin, which is an efficient method of transport. However, the binding to hemoglobin also means oxygen is not all 'free' in the bloodstream, which is why partial pressure doesn’t equate to the total amount of oxygen carried.

Venous Blood in Oxygen Depletion

By the time blood reaches the venous blood system, it has circulated through the body and delivered much of its oxygen content to various tissues. The partial pressure of oxygen in venous blood is substantially lower than in arterial blood, with an average value of \( P_{vO2} = 40 \ mmHg \). This lower partial pressure reflects the deoxygenated state of the venous blood.

It's crucial to understand that venous blood is not devoid of oxygen; rather, it has a reduced quantity due to the necessary delivery of oxygen to body cells. This state also prepares the blood for the upcoming reoxygenation when it returns to the lungs' alveoli.

Gas Exchange in the Respiratory System

The gas exchange in the respiratory system primarily occurs in the alveoli and involves both oxygen and carbon dioxide. This exchange is governed by the partial pressures of these gases on either side of the alveolar-capillary membrane. Taking oxygen as an example, its partial pressure is considerably higher in the alveolar air compared to the blood in the pulmonary capillaries.

As a result of this pressure gradient, oxygen diffuses into the blood, while carbon dioxide diffuses out of the blood into the alveolar air to be exhaled. Efficient gas exchange is critical for maintaining the body's homeostasis. Ensuring adequate oxygen delivery and carbon dioxide removal. The entire process is indicative of the body's genius ability to utilize gradients for nutrient exchange and waste removal.