Which one of the following statement is in correct ?

Options

A. the residual air in lungs slightly decrease the efficiency of respiration in mammals
B. the presence of non respiratory air sacs increases the efficiency of respiration in birds
C. in insects circulating body fluids serve to distribute oxygen to tissues
D. the principle of counter current flow facilitates efficient respiration in gills of fishes

Answer

To determine which statement is incorrect, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Analyze Statement A**:
- The statement claims that the residual volume or residual air in lungs slightly decreases the efficiency of respiration in mammals.
- Residual volume refers to the air that remains in the lungs after a forceful exhalation, which is about 1 to 1.5 liters in humans. This air cannot be expelled and is often referred to as "dead space."
- Since this air is not available for gas exchange, it does not contribute to the efficiency of respiration. Therefore, this statement is indeed incorrect.

2. **Analyze Statement B**:
- This statement says that the presence of non-respiratory air sacs increases the efficiency of respiration in birds.
- Birds have a unique respiratory system that includes air sacs which help in maintaining a continuous flow of air through the lungs, even during exhalation. This design enhances the efficiency of gas exchange.
- Thus, this statement is correct.

3. **Analyze Statement C**:
- The statement mentions that in insects, circulating body fluid serves to distribute oxygen to tissues.
- Insects, such as cockroaches, have a fluid called hemolymph that circulates throughout their body. This fluid can directly transport oxygen to tissues.
- Therefore, this statement is also correct.

4. **Analyze Statement D**:
- This statement discusses the principle of counter-current flow facilitating efficient respiration in the gills of fishes.
- In fish gills, water flows in one direction while blood flows in the opposite direction. This counter-current mechanism maximizes oxygen absorption from water, making respiration more efficient.
- Hence, this statement is correct.

### Conclusion:
After analyzing all statements, we find that the incorrect statement is **Statement A**: "The residual volume or residual air in lungs slightly decreases the efficiency of respiration in mammals."