In human blood, there is a compound inside the RBCs called haemoglobin which ensures that the muscle will receive enough oxygen during exercise.
<h3><u>Explanation:</u></h3>
In human blood, the red blood corpuscles contain the haemoglobin. Haemoglobin is a iron chelated compound containing porphyrin ring and a globin tail which can establish co-ordinate covalent bond with both oxygen and carbon dioxide. The bonding element depends on the concentration of these two gases. In lungs, where the oxygen concentration is more than carbon dioxide, the haemoglobin bonds with oxygen and brings it to the tissues where carbon dioxide concentration is more. This makes the haemoglobin to release oxygen and bond with carbon dioxide which is brought back to lungs. This is the process by which each and every tissue including the muscles recieve oxygen.
In muscles there is Myoglobin which is another iron-porphyrin compound which has several times more affinity for oxygen than haemoglobin. This helps to extract more oxygen from haemoglobin in muscles.
The answer that fits the blank provided above is STEROIDS. Steroids are fat-soluble signal molecules.
B. Exchanged during normal breathing
