Blood Histology General Information

Blood is a fluid connective tissue.  This means that blood contains specialized cellsThe basic structural and functional units of life., proteinsLarge molecules made of amino acids with various functions in the body., and a ground substance.  However, unlike the other gelatinous and solid connective tissues, blood is a fluid connective tissue.  Therefore, the name that refers to its components is slightly modified.  Instead of specialized cells, we refer to the cellular components of blood as formed elementsThe cellular components of blood, including red blood cells, white blood cells, and platelets..  Formed elements contain white blood cells or leukocytes, red blood cells or erythrocytes(RBCs): Red blood cells responsible for oxygen transport., and platelets.  Leukocytes fit the definition of a cell.  Erythrocytes and their identity as a cell is largely debated.  Because erythrocytes do not contain organellestructures within a cell that perform specialized functions., their identity as a cell is questionable.  However, in their formation, they are derived from a developing cell that performed exocytosisThe process of expelling materials from a cell via vesicles that fuse with the plasma membrane. of its organelles.  Similarly, platelets are derived from the explosion of a larger cell called a megakaryocyte.  The debated erythrocytes and platelets lead to the name of formed elements for the three, cellularly derived formed elements of blood.

The ground substance for blood is waterThe universal solvent essential for life..  Blood is a suspension of solids in a fluid baseA substance that accepts hydrogen ions (H⁺) or releases hydroxide ions (OH⁻)..  That base is water.  Plasma is 99% water, however it contains important chemicals and substances such as glucoseA simple sugar that is the main source of energy for cells., lipidsOrganic molecules including fats, oils, and steroids., hormones, transport proteinsBind hormones for transport in the blood., and carbon dioxide (oxygen is carried in the erythrocytes).  Plasma contains proteins, the most abundant being albuminA plasma protein that helps maintain osmotic pressure and transport substances., the protein that gives both plasmaThe liquid component of blood. and egg yolks their yellow color. Together, the water, dissolved chemicals, and proteins create the matrix of blood.  Of course, when histological slides of blood are made, no matrix is present as it evaporates in the drying process.  Only the formed elements are represented.

Erythrocytes or red blood cells (RBCs) are one of the three formed elements of blood.  Erythrocytes are specialized for carrying oxygen.  They have what is referred to as a bioconcave shape.  This shape maximizes the surface area-to-volume ratio for this shape.  Bigger is not better in this instance.  Only part of the job of an erythrocyte is to carry oxygen.  The other part is to quickly drop off and take on oxygen via diffusionPassive movement of molecules from areas of high to low concentration..  Red blood cells are like a train that doesn’t slow down at the train station. As they pulse through capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues. in tissues oxygen must be able to quickly diffuse into or out of a red blood cell much like trying to get off a train that doesn’t slow down. The bioconcave shape ensures that, for the volume of oxygen that is being carried, there is sufficient surface area over which it can be exchanged.

Red blood cells have other specializations for carrying oxygen.  They are devoid of organelles, which makes them unable to repair themselves when damaged.  However, this lack of organelles also means that they have more room for the oxygen-binding proteins named hemoglobinThe oxygen-carrying protein in red blood cells that gives blood its red color..