All of the following statements are correct, axpect

Options

A. RCBs of human are not capable of metabolising glucose to carbon dioxide aerobically
B. No mixing of blood occurs in heart of a fish
C. With increase in heart rate, duration of a single cardiac cycle decreases
D. Pacemaker of human heart is a specialised neural tissue which maintains rhythmic activity of heart

Answer

To solve the question, we need to analyze each of the provided statements regarding their correctness. Let's break it down step by step.

### Step 1: Analyze the First Statement
- **Statement:** RBCs (red blood cells) of humans are not capable of metabolizing glucose to carbon dioxide aerobically.
- **Analysis:** This statement is correct. Human RBCs lack mitochondria, which are necessary for aerobic respiration, and therefore rely on anaerobic respiration for their energy needs.

### Step 2: Analyze the Second Statement
- **Statement:** No mixing of blood occurs in the heart of fish.
- **Analysis:** This statement is also correct. Fish have a two-chambered heart (one atrium and one ventricle), which allows for the separation of oxygenated and deoxygenated blood.

### Step 3: Analyze the Third Statement
- **Statement:** With an increase in heart rate, the duration of a single cardiac cycle decreases.
- **Analysis:** This statement is true. As the heart rate increases, the time for each cardiac cycle (which includes both systole and diastole) decreases.

### Step 4: Analyze the Fourth Statement
- **Statement:** The pacemaker of the human heart is a specialized neural tissue which maintains the rhythmic activity of the heart.
- **Analysis:** This statement is partially correct. The pacemaker, known as the sinoatrial (SA) node, does maintain the rhythmic activity of the heart, but it is not a neural tissue; it is a specialized group of cardiac muscle cells.

### Conclusion
Based on the analyses:
- The first three statements are correct.
- The fourth statement is partially correct but contains an inaccuracy regarding the nature of the pacemaker tissue.

### Final Answer
The statement that is incorrect (or partially correct) is the fourth one regarding the pacemaker of the human heart.

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