Blood Components and Hemoglobin Lecture

Differentiation and Cellular Lineage

All blood cells originate from pluripotent cells, which serve as the common ancestors for the various components of the blood. These pluripotent cells undergo a process of differentiation in which they commit to either the myeloid cell line or the lymphoid cell line. This critical step determines the eventual specialized function and type of the resulting blood cells.

Red Blood Cells (Erythrocytes) and Their Physical Properties

Red blood cells, also referred to as erythrocytes, are the most numerous cells found within the blood. They are characterized by their distinct biconcave disc shape, which is specifically not spheroidal; this unique architecture allows the cells to bend and deform as they navigate through narrow capillaries. Erythrocytes are anuclear, meaning they lack a nucleus, which provides more space for internal components. These cells have a finite life cycle, lasting in circulation for approximately $120$ days before completing their life span. In a healthy individual, erythrocytes make up about $45\%$ of the total blood volume. Quantitatively, there are roughly $5\text{ million}$ red blood cells per $\mu L$ (microliter) of blood. Red blood cells are essential for waste transport, as they carry $23\%$ of the body's $CO_2$ via amino acids.

Molecular Structure and Function of Hemoglobin

Hemoglobin is the vital oxygen-carrying protein found within erythrocytes, and it is responsible for the characteristic red color of the cells. Approximately $33\%$ of the red blood cell's volume is filled with hemoglobin. The molecule is composed of a protein portion called globin and a nonprotein portion. The globin consists of four separate polypeptide chains, specifically two alpha chains and two beta chains. Attached to this protein structure is a ringlike nonprotein pigment known as heme. Hemoglobin molecules are highly efficient, with four $O_2$ molecules carried per hemoglobin molecule. In the center of each heme pigment resides an iron ($Fe$) molecule, which is the specific site where oxygen binds; this mechanism is responsible for transporting $99\%$ of the oxygen in the blood. For adults, the normal concentration value of hemoglobin is listed as $12-18\,mg/dL$ (milligrams per deciliter).

Hemoglobin Recycling and Degradation Pathways

Once red blood cells reach the end of their $120$-day life cycle, they are recycled within the liver and spleen. During this recycling process, the hemoglobin molecule is broken down into its constituent parts. The globin portion is digested into its component amino acids, which are then released to be reused by other cells to synthesize new proteins. The iron ($Fe$) molecules are not discarded; instead, they are transported back to the bone marrow to be incorporated into new blood cells. This transport is facilitated by specialized carrier proteins known as ferritin or transferrin. The remaining heme group undergoes a chemical degradation process where it is first converted into a green pigment called biliverdin and subsequently transformed into a yellow-orange pigment called bilirubin.