Why is a nucleus not present in human RBC?

Well RBC or erythrocytes are the blood carriers or the cells in human body that carry haemoglobin .Even in early stages they contain a nucleus when they are produced from the bone marrow . read more

If nucleus present in RBC it make difficult for the body to survive as the nucleus use oxygen for itself which is collected by blood from lungs. Therefore nucleus is not present in RBC which helps in carrying oxygen from lungs to other part of the body and to their cells. read more

I think the most important reason why red blood cells don't have nuclei or organelles is efficiency. Not only to make room for hemoglobin, but to not consume the oxygen they are supposed to be carrying to the tissues. The organelle system uses up a lot of oxygen. For a nucleus to function, you also need mitochondria. read more

The absence of a nucleus is an adaptation of the red blood cell for its role. It allows the red blood cell to contain more hemoglobin and, therefore, carry more oxygen molecules. It also allows the cell to have its distinctive bi-concave shape which aids diffusion. read more

Red blood cell of birds, frogs and fishes has nucleus while mammal's mature red blood cell has no nucleus. While there are many theories about how erythrocytes without nucleus is evolutionary advantaged, it might just because a mutation during the early stage of mammal's evolution. read more