Question

The oxygen dissociation curve is (a) parabola (b) slope (c) sigmoid (d) straight line.

Short answer

The oxygen dissociation curve is sigmoid.

Step-by-step solution

Understanding the Oxygen Dissociation Curve

The oxygen dissociation curve describes the relationship between the partial pressure of oxygen (PO2) and the hemoglobin saturation (SO2). This relationship is not linear but rather a curve that initially rises steeply and then levels off as it approaches 100% saturation.

Identifying the Shape of the Curve

By examining the shape of the oxygen dissociation curve, which starts with a lower slope and then increases steeply before plateauing at higher partial pressures of oxygen, one can determine that the curve has a characteristic 'S' shape.

Matching the Curve to the Correct Option

The 'S' shape of the oxygen dissociation curve is most commonly described as sigmoidal. Thus, the correct option that describes the shape of the oxygen dissociation curve is 'sigmoid'.

Key concepts

Hemoglobin Saturation

Hemoglobin saturation is a measure of how much oxygen is bound to hemoglobin, the oxygen-carrying protein in red blood cells. Understanding hemoglobin saturation is essential for grasping how the body delivers oxygen to various tissues.

Think of hemoglobin as a bus with four seats, where each seat represents a binding site for oxygen molecules. When all four seats are occupied, we say the hemoglobin is 'fully saturated'. Hemoglobin saturation changes depending on the oxygen availability in the environment and the body's demand for oxygen. In the lungs, where oxygen is abundant, hemoglobin tends to be highly saturated.

However, this is a dynamic process – as blood travels to tissues which have used up their oxygen, hemoglobin releases some of its cargo to 'recharge' those tissues. The ability of hemoglobin to pick up or release oxygen is visually represented by the oxygen dissociation curve.

Partial Pressure of Oxygen

The partial pressure of oxygen, often abbreviated as PO2, is a term that quantifies the amount of oxygen gas present in the blood or a given environment. It is part of the broader concept of gas partial pressures, which is essential in understanding how gases like oxygen and carbon dioxide are exchanged in the body.

The PO2 in arterial blood gives us insight into the oxygen content and can indicate how well the lungs are functioning in oxygenating the blood. A key point about PO2 is how it influences hemoglobin saturation: lower PO2 means fewer oxygen molecules are available for hemoglobin to latch onto, while higher PO2 typically leads to higher hemoglobin saturation levels.

The PO2 is directly linked to the shape of the oxygen dissociation curve; at low PO2, the curve is relatively flat, indicating hemoglobin's reluctance to release oxygen. As PO2 increases, so does the curve's steepness, reflecting a greater ease of oxygen binding to hemoglobin.

Sigmoidal Curve

The oxygen dissociation curve takes on a distinct 'S' or sigmoidal shape, which provides a visual representation of hemoglobin's oxygen-binding behavior at different levels of partial pressure of oxygen. The sigmoidal curve is fundamental in understanding the cooperative nature of hemoglobin as it binds with oxygen.

At the initial part of the curve, where the PO2 is low, the hemoglobin has a lower affinity for oxygen and thus, the curve rises slowly. This represents the hemoglobin in the peripheral tissues, where oxygen is released for cellular use. As PO2 increases, hemoglobin's affinity for oxygen also increases – this is represented by the steep middle section of the curve.

Finally, at high PO2 levels, such as those found in the lungs, the curve levels out indicating that hemoglobin becomes almost fully saturated and the gain in oxygen binding is minimal for further increases in PO2. This behavior is crucial for efficient oxygen transport and release within the body.