Transport of carbon dioxide in a chemically modified form accounts for nearly 70% of total carbon dioxide transport and is achieved by carbon dioxide conversion to bicarbonate. Transport of carbon dioxide in a protein bound form accounts for nearly 20% of total carbon dioxide transport and is achieved by reversible binding of carbon dioxide to haemoglobin (carbaminohaemoglobin).
So, the correct answer is 'As carbaminohaemoglobin and as carbonic acid.'

Transport of Carbon Dioxide in the Blood

Carbon dioxide (CO2) is a waste product produced by cells during cellular respiration. It needs to be transported from the tissues to the lungs for elimination. The process of carbon dioxide transport in the blood involves three main methods: dissolved in plasma, as bicarbonate ions, and bound to hemoglobin.

1. Dissolved in Blood Plasma:
- Carbon dioxide is highly soluble in water, and a small amount of it dissolves directly into the blood plasma.
- This dissolved CO2 exists in equilibrium with carbonic acid (H2CO3), which is formed when CO2 combines with water (H2O).
- The equation for this reaction is: CO2 + H2O ⇌ H2CO3
- Dissolved CO2 accounts for a small fraction (about 5%) of the total carbon dioxide transported in the blood.

2. Formation of Bicarbonate Ions:
- The majority of carbon dioxide is transported in the form of bicarbonate ions (HCO3-) via the bicarbonate buffer system.
- Inside red blood cells (RBCs), an enzyme called carbonic anhydrase catalyzes the reaction between carbon dioxide and water to form carbonic acid.
- Carbonic acid then dissociates into bicarbonate ions and hydrogen ions (H+).
- The bicarbonate ions are transported out of the RBCs into the plasma in exchange for chloride ions (Cl-) through a protein channel called the chloride shift.
- This process helps maintain the electrochemical balance in the RBCs and prevents the accumulation of excess CO2.

3. Binding to Hemoglobin:
- A small portion of carbon dioxide binds directly to hemoglobin molecules, forming carbaminohemoglobin.
- This binding occurs mainly in the deoxygenated form of hemoglobin, which has a higher affinity for CO2 than oxygenated hemoglobin.
- The binding of CO2 to hemoglobin helps to stabilize the T-form of hemoglobin, promoting the release of oxygen in the tissues (the Bohr effect).
- This process accounts for only a small fraction of the total carbon dioxide transported in the blood.

In summary, carbon dioxide is transported in the blood through various mechanisms. It is dissolved in the plasma, converted to bicarbonate ions, and bound to hemoglobin as carbaminohemoglobin. These mechanisms ensure efficient removal of carbon dioxide from the tissues and its safe transport to the lungs for elimination.