why haemoglobin rate is high during infance?

Answer 1

Two main reasons.

First, some back ground.
During pregnancy, the blood of the foetus is mostly foetal haemoglobin (HbF), it is a high affinity Hb, meaning that it grabs oxygen better than adult Hb (HbA). This is the way that oxygen is transferred at the placenta. Oxygen arrives on the HbA, meets up across a very thin barrier with HbF, and has its oxygen stolen from it, and is given CO2 from the baby's circulation. From about 6 months into the pregnancy, some adult Hb starts to be produced, and after the baby is born (for normal pregnancy) very little Hbf is produced, until at about 6 months of age, it drops to less than 1%.
High affinity Hb does not give up their oxygen as readily to the body tissues as HbA, so in order to work normally, and give up enough oxygen, a high affinity Hb has to be in higher concentration.

The second main reason is that children have a higher metabolic and respiration rate, and need more oxygen, resulting in a higher Hb.

Answer 2

labsci is correct