Blood: Structure and Function

Functions of Blood

Blood is a vital fluid in the human body, responsible for multiple essential functions:

• Transport: Blood carries oxygen (O2), carbon dioxide (CO2), nutrients, metabolic wastes, hormones, and heat throughout the body.

• Protection: White blood cells (WBCs), antibodies, and platelets defend against infection and blood loss.

• Regulation: Blood regulates fluid balance, pH buffering, and temperature.

Properties of Blood

Blood possesses unique physical and chemical properties:

• Temperature: 38°C (100.4°F)

• pH: 7.35–7.45 (slightly alkaline)

• Osmolarity: Total molar concentration of dissolved particles (sodium ions, proteins, RBCs). High osmolarity increases fluid absorption and blood pressure; low osmolarity leads to edema and decreased blood pressure.

• Volume: 5–6 liters in males; 4.5 liters in females

• Viscosity: Blood is about 5 times as viscous as water, affecting flow and resistance.

Oncotic Pressure

Oncotic pressure is a form of osmotic pressure exerted by plasma proteins, mainly albumin, which pulls water into the circulatory system. When pathologically low, as in kwashiorkor (starvation), blood pressure drops and edema occurs.

Hematopoiesis: Blood Cell Formation

Embryonic and Fetal Hematopoiesis

Blood cell formation begins in the embryo and fetus in the yolk sac, which produces stem cells that colonize the liver, bone marrow, spleen, and thymus.

Adult Hematopoiesis

After birth, hematopoiesis occurs primarily in the red bone marrow, which contains pluripotent stem cells capable of differentiating into various blood cell types.

Stem Cell Lines

Pluripotent stem cells give rise to two main stem lines:

• Myeloid line: Produces most blood cells in bone marrow

• Lymphoid line: Produces lymphocytes in lymphatic organs

Blood Components

Hematocrit

Hematocrit is the percentage of blood volume occupied by erythrocytes (RBCs). It is a key indicator of blood health and oxygen-carrying capacity.

Plasma Composition

Plasma is the liquid portion of blood, consisting of water, proteins, and other solutes. Serum is plasma without fibrinogen.

• Albumins: Most abundant; contribute to viscosity and osmolarity

• Globulins: Antibodies; provide immune functions

• Fibrinogen: Precursor to fibrin; helps form blood clots

• Nitrogenous compounds: Amino acids and wastes (urea)

• Nutrients: Glucose, vitamins, fats, minerals

• Gases: O2 and CO2

• Electrolytes: Essential ions for cellular function

Formed Elements

The formed elements of blood include:

• Erythrocytes (RBCs): Transport oxygen and carbon dioxide

• Leukocytes (WBCs): Immune defense

• Platelets: Blood clotting

Erythrocytes (Red Blood Cells)

Structure and Function

Erythrocytes are disc-shaped, anucleate cells with a biconcave shape, maximizing surface area for gas diffusion. They transport O2 and some CO2, and have a lifespan of about 120 days.

• Biconcave shape: Increases surface area for gas exchange

• Anucleate: No nucleus or organelles

• Size: 7.5 μm diameter, just large enough to pass through capillaries

Hemoglobin Structure

Hemoglobin (Hb) is the main protein in RBCs, responsible for oxygen transport:

• Composed of four globin chains (2 alpha, 2 beta)

• Each chain has a heme group that binds oxygen to iron

• One hemoglobin molecule can carry four O2 molecules

• 33% of RBC cytoplasm is hemoglobin

Gender Differences in RBCs

There are notable gender differences in hematocrit, hemoglobin, and RBC count, mainly due to androgens, menstrual losses, and body fat:

• Hematocrit: Males 42–52%, Females 37–48%

• Hemoglobin: Males 13–18 g/dL, Females 12–16 g/dL

• RBC count: Males 4.6–6.2 million/μL, Females 4.2–5.4 million/μL

Erythropoiesis: RBC Production

Hormonal Regulation

Erythropoietin (EPO) is the hormone that stimulates erythropoiesis, especially in response to increased exercise, low O2 levels (altitude or emphysema), or blood loss.

Negative Feedback Control

A drop in RBC count causes the kidneys to secrete EPO, which stimulates bone marrow to increase RBC production within 3–4 days.

Stages of Erythrocyte Production

RBC production involves several stages:

• First committed cell: erythrocyte colony-forming unit (CFU) with EPO receptors

• Erythroblasts multiply and synthesize hemoglobin

• Nucleus is discarded to form a reticulocyte

• Development takes 3–5 days; 2.5 million RBCs are produced per second

Nutritional Needs for Erythropoiesis

Key nutrients required for RBC production include:

• Iron: Essential for hemoglobin synthesis; stomach acid converts Fe3+ to absorbable Fe2+

• Vitamin B12: Requires intrinsic factor for absorption; found in animal products

• Folate (folic acid): Important for DNA synthesis; found in leafy greens and liver

The RBC Life Cycle

Lifecycle and Breakdown

RBCs circulate for about 120 days before being broken down in the liver and spleen. Hemoglobin is degraded, and its components are recycled or excreted.

RBC Disorders

Polycythemia

Polycythemia is an excess of RBCs, which can be primary (cancer of erythropoietic cell line) or secondary (dehydration, emphysema, high altitude, physical conditioning). It increases blood volume, pressure, and viscosity, raising the risk of embolism, stroke, or heart failure.

Anemia

Anemia is a deficiency of RBCs or hemoglobin, caused by inadequate erythropoiesis, poor nutrition (vitamin B12 or iron deficiency), kidney failure, or aplastic anemia (complete cessation of blood cell production).

Effects of Anemia

• Tissue hypoxia and necrosis: Shortness of breath, lethargy

• Low blood osmolarity: Tissue edema

• Low blood viscosity: Heart races, blood pressure drops

Sickle-Cell Disease

Sickle-cell disease is a hereditary defect in hemoglobin (HbS) common in African Americans. A single amino acid substitution causes RBCs to become sickle-shaped in low O2 conditions, leading to agglutination, blocked vessels, intense pain, and organ failure.

Sickle-Cell Trait

Individuals heterozygous for HbS (sickle-cell trait) have resistance to malaria, as HbS is indigestible to malaria parasites. This trait persists in populations where malaria is endemic.