In the human circulatory system, most of the oxygen transported by the blood from the lungs to the tissues is carried in which form?

Introduction / Context:
Transport of respiratory gases in the blood is a core topic in human physiology. Oxygen has limited solubility in water, so the body uses a special mechanism to carry enough oxygen from the lungs to active tissues. Understanding how oxygen is transported in the blood helps explain concepts such as oxygen saturation, anaemia, and the effect of carbon monoxide poisoning. This question asks which form carries most of the oxygen in human blood.

Given Data / Assumptions:

• The focus is on oxygen transport in human blood, not carbon dioxide.
• The options include hemoglobin binding, dissolved oxygen in plasma, dissolved oxygen in water around cells, and ionic forms.
• We assume normal physiological conditions without special medical interventions.
• We focus on the predominant form that carries the majority of oxygen.

Concept / Approach:
Oxygen is only slightly soluble in plasma. If oxygen were transported only as a dissolved gas in plasma, the blood could carry far less oxygen than tissues require. To solve this problem, red blood cells contain hemoglobin, a pigment with iron containing heme groups that can reversibly bind oxygen. Most of the oxygen in the blood, typically over 95 percent under normal conditions, is bound to hemoglobin forming oxyhemoglobin. A small fraction is dissolved in plasma and contributes to the partial pressure of oxygen, but this fraction is minor compared to the amount carried by hemoglobin.

Step-by-Step Solution:
Step 1: Recognise that each hemoglobin molecule can bind up to four oxygen molecules at its heme groups. Step 2: Note that red blood cells are very numerous and packed with hemoglobin, maximising the oxygen carrying capacity of blood. Step 3: Understand that the amount of oxygen dissolved in plasma is small because oxygen is not very soluble in water. Step 4: Observe that there is no physiologically important form where oxygen is transported as an oxygen ion in plasma; oxygen transport is mainly as molecular oxygen bound to hemoglobin. Step 5: Therefore, the majority of oxygen is carried in blood bound to hemoglobin inside red blood cells, forming oxyhemoglobin.

Verification / Alternative check:
Physiology textbooks often provide a breakdown of oxygen transport: about 97 percent of oxygen is carried bound to hemoglobin, and about 3 percent is dissolved in plasma. Diagrams show oxygen entering red blood cells in the lungs, binding to hemoglobin, and being released in tissues where partial pressure is lower. Clinical measures such as oxygen saturation specifically refer to the percentage of hemoglobin binding sites occupied by oxygen, not to dissolved oxygen alone. These details confirm that hemoglobin binding is the principal form of oxygen transport.

Why Other Options Are Wrong:
Dissolved directly in the plasma as free oxygen gas is wrong as the primary mode, because only a small fraction of oxygen is transported this way. Dissolved in water in the tissue fluid around cells does not describe transport in blood but rather the diffusion step from blood to tissues, and even there hemoglobin release is crucial. In ionic form as an oxygen ion solute in plasma is incorrect because oxygen is not carried in blood as ions; it is mainly carried as molecular oxygen bound to hemoglobin.

Common Pitfalls:
Students sometimes confuse the concept of partial pressure of oxygen, which depends on dissolved oxygen, with the total amount of oxygen carried. They may think that because partial pressure is important for diffusion, dissolved oxygen must carry the majority of oxygen. In reality, dissolved oxygen determines how much oxygen loads and unloads from hemoglobin, but hemoglobin binding is responsible for the bulk of transport. Remembering that red blood cells and hemoglobin are the main oxygen carriers will help you avoid this confusion.

Final Answer:
The correct choice is Bound to hemoglobin inside red blood cells, because this form accounts for most of the oxygen transported by the blood from lungs to tissues.