Why is blood called a connective tissue and why is hemoglobin present only in human blood?

Blood is called a connective tissue because it has living cells suspended in an extracellular matrix (plasma). Hemoglobin is not present only in human blood, but in red blood cells of many animals—its presence is not unique to humans.:

• Why blood is a connective tissue: Connective tissues generally consist of cells plus an extracellular matrix (which includes fibres and ground substance) that binds, supports or connects other tissues. Blood meets this definition:

  • Cells: red blood cells (RBCs/erythrocytes), white blood cells (WBCs/leukocytes), platelets
  • Extracellular matrix: plasma – the liquid medium in which cells float.

  Therefore blood is classified as a “fluid connective tissue”.

• Why hemoglobin is present: Hemoglobin (a complex iron-containing protein) is found in the red blood cells of vertebrates and many other animals. It binds oxygen in the lungs/gills and carries it to tissues. It is *not* exclusive to humans. Thus the statement “hemoglobin present only in human blood” is incorrect. Many animals including mammals, birds, fish, some invertebrates have haemoglobin or analogous oxygen-transport pigments.