Composition and Function of Blood

Unlike most other cells, mature erythrocytes are "stripped down" for performance. They lack a nucleus, mitochondria, and most organelles. Because they lack mitochondria, they cannot perform aerobic respiration and must rely on anaerobic glycolysis to produce energy.

The Hemoglobin Molecule

The functional heart of the erythrocyte is hemoglobin (Hb). Each hemoglobin molecule consists of four polypeptide chains known as globins, each associated with an iron-containing heme group. This structure allows a single hemoglobin molecule to bind multiple oxygen molecules. In fact, approximately $98.5%$ of the body's oxygen is transported bound to hemoglobin, while only $1.5%$ is dissolved directly in the plasma.

Exam Tip: Remember that hemoglobin doesn't just carry oxygen. It also binds to carbon dioxide, hydrogen ions ($\text{H}^+$), and carbon monoxide ($\text{CO}$)!

Internal Support and Enzymes

To maintain their shape during the turbulent journey through narrow capillaries, erythrocytes utilize a structural protein called spectrin. Furthermore, they contain the vital enzyme carbonic anhydrase, which is essential for the conversion of $\text{CO}_2$ and water into bicarbonate, a key step in both gas transport and pH regulation.

Hematocrit and Blood Viscosity

The hematocrit is the measurement of the percentage of total blood volume occupied by erythrocytes. Normal ranges are typically $42\text{--}52\%$ for men and $37\text{--}47\%$ for women.

Several factors can influence these levels:

• High Altitude: Low oxygen levels stimulate the kidneys to produce more red blood cells to compensate.
• Exogenous Hormones: The use of certain drugs can artificially spike RBC production.
• Dehydration: A decrease in plasma volume can lead to a higher relative hematocrit.

While more red blood cells mean more oxygen-carrying capacity, there is a dangerous upper limit. If the hematocrit becomes too high (e.g., $>60\%$), the blood's viscosity (thickness) increases significantly. This makes the blood harder to pump, greatly increasing the risk of blood clots, heart attacks, and strokes.

Erythropoiesis: The Life Cycle of a Red Cell

Because mature erythrocytes lack DNA and organelles, they cannot divide or repair themselves. Consequently, they have a limited lifespan of approximately $120$ days.

The process of producing new erythrocytes is called erythropoiesis. This occurs in the red bone marrow, where stem cells differentiate into various blood cell types. The primary driver of this process is the hormone erythropoietin (EPO), which is secreted by the kidneys in response to low oxygen levels. As the demand for oxygen increases, the kidneys release more EPO, which in turn stimulates the bone marrow to produce more RBCs.

Once an erythrocyte reaches the end of its life, it is filtered out of circulation and recycled by the spleen and liver. During this recycling process, vital components like iron from the heme group are recovered to be used in the production of new hemoglobin.

Oxygen and Carbon Dioxide Transport Mechanisms

The transport of gases relies on complex chemical equilibria and pressure gradients.

Oxygen Transport

Oxygen moves from the lungs to the tissues primarily via hemoglobin. The binding of oxygen to hemoglobin exhibits positive cooperativity: as one oxygen molecule binds to a heme group, the hemoglobin molecule changes shape, making it easier for subsequent oxygen molecules to bind.

Carbon Dioxide Transport

Carbon dioxide is transported in three main ways:

1. Dissolved in Plasma: Approximately $5\text{--}6\%$.
2. Bound to Hemoglobin: Approximately $20\text{--}25\%$ (forming carbaminohemoglobin).
3. As Bicarbonate ($\text{HCO}_3^-$): The most significant method, accounting for $75\text{--}80\%$ of transport.

The conversion to bicarbonate is facilitated by carbonic anhydrase through the following reaction: $\begin{aligned} \text{CO}_2 + \text{H}_2\text{O} &\rightleftharpoons \text{H}_2\text{CO}_3 \\ \text{H}_2\text{CO}_3 &\rightleftharpoons \text{H}^+ + \text{HCO}_3^- \end{aligned}$

To maintain electrical neutrality during this process, a mechanism known as the chloride shift occurs. When bicarbonate moves out of the erythrocyte into the plasma, chloride ions ($\text{Cl}^-$) move into the cell to balance the charge.

Clinical Connection: The Bohr effect describes how increased $\text{CO}_2$ and decreased pH (increased acidity) cause hemoglobin to have a lower affinity for oxygen. This is a brilliant physiological adaptation: when tissues are metabolically active, they produce more $\text{CO}_2$ and acid, which signals the blood to "drop off" more oxygen exactly where it is needed most.