Introduction to Blood Physiology

Blood is a specialized fluid connective tissue that performs several critical functions for survival. It consists of both liquid and cellular components working together to maintain your body's internal environment.

The key functions of blood include transport of oxygen and nutrients to cells, removal of waste products, delivery of hormones to target tissues, and protection against pathogens. Without these functions, your body couldn't maintain proper temperature, pH balance, or defend itself against diseases.

Blood has distinctive physical characteristics—it's a viscous, opaque fluid with a metallic taste and a color that varies from bright red $oxygen-rich$ to dark red $oxygen-poor$. For healthy adults, the pH of blood ranges from 7.35-7.45, and it maintains a temperature of about 38°C.

Did you know? Blood accounts for approximately 8% of your total body weight. The average volume is 5-6 liters in males and 4-5 liters in females.

Composition of Blood

Your blood consists of two main components: plasma (55%) and formed elements (45%). This composition allows blood to perform its many essential functions.

Plasma is the yellowish liquid portion that serves as the transport medium. It's mostly water (90%), with proteins (7%), and other substances like nutrients, hormones, electrolytes, and waste products making up the remainder. The most important plasma proteins include albumin, globulins, and fibrinogen.

Formed elements include three types of cells:

• Red blood cells (erythrocytes): Oxygen carriers that give blood its color
• White blood cells (leukocytes): Defenders against infection
• Platelets (thrombocytes): Cell fragments crucial for clotting

These components work together to maintain homeostasis and protect your body from harm. The percentage of blood volume occupied by red blood cells is called the hematocrit, which is an important indicator of your overall health.

Key insight: When a blood sample is centrifuged, it separates into layers based on density—plasma at the top, a thin "buffy coat" of white cells and platelets in the middle, and red blood cells at the bottom.

Blood Cell Formation (Hematopoiesis)

All blood cells begin life from a single type of cell called the pluripotential hematopoietic stem cell in your bone marrow. This remarkable process of blood cell formation is called hematopoiesis.

These stem cells branch into two main lineages: myeloid stem cells and lymphoid stem cells. Myeloid stem cells develop in red bone marrow and give rise to red blood cells, platelets, and several types of white blood cells (monocytes, neutrophils, eosinophils, and basophils). Lymphoid stem cells begin development in bone marrow but complete it in lymphatic tissues, becoming T and B lymphocytes.

The production of blood cells is carefully regulated by chemical messengers called cytokines, which include:

• Colony stimulating factors (CSFs)
• Interleukins $IL-1, IL-2, IL-3, etc.$
• Erythropoietin (EPO)
• Growth hormone and thyroid hormones

These regulators ensure your body maintains the right balance of blood cells for optimal functioning.

Remember: A healthy diet containing proteins, iron, vitamin B12, and folic acid is essential for proper blood cell formation. Deficiencies can lead to various types of anemia.

Red Blood Cells (Erythrocytes)

Red blood cells (RBCs) are the most abundant cells in your blood, with males having about 5.2 million/mm³ and females having 4.6 million/mm³. Their unique biconcave shape (like a donut with a thin center) maximizes surface area for gas exchange.

Unlike other cells, mature RBCs lack a nucleus and organelles, allowing them to pack in more hemoglobin (Hb)—the iron-containing protein that transports oxygen. Each RBC contains approximately 280 million hemoglobin molecules, making up about 97% of the cell's dry content.

The production of RBCs (erythropoiesis) is primarily regulated by the hormone erythropoietin (EPO), which is released by your kidneys in response to low oxygen levels. This process requires proper nutrition, particularly iron, vitamin B12, and folic acid.

Red blood cells have a lifespan of approximately 120 days. When they become old or damaged, they're broken down primarily in your spleen. The iron from hemoglobin is recycled for new RBC production, while the remainder is converted to bilirubin and eventually excreted.

Clinical connection: Jaundice, the yellowing of skin and eyes, occurs when bilirubin levels exceed 1.5 mg/dl due to excessive RBC destruction or liver dysfunction.

Hemoglobin and Blood Indices

Hemoglobin (Hb) is the oxygen-carrying protein in your red blood cells. Normal Hb levels are 13-18 g/dl for men and 12-15 g/dl for women. This remarkable molecule consists of four protein chains (globins) each containing a heme group with iron at its center.

Different types of hemoglobin exist in your body:

• Adult hemoglobin (HbA): The predominant form in adults
• Fetal hemoglobin (HbF): Present during fetal development
• Sickle hemoglobin (HbS): A genetic variant that can cause disease

When measuring blood health, doctors calculate several important indices:

1. Mean Corpuscular Volume (MCV) measures the average size of RBCs $normal: 80-100 fL$
2. Mean Corpuscular Hemoglobin (MCH) indicates the average amount of Hb per RBC $normal: 27-33 pg$
3. Mean Corpuscular Hemoglobin Concentration (MCHC) represents Hb concentration in packed RBCs $normal: 32-36 g/dL$

These values help diagnose different types of anemia. For example, low MCV suggests iron deficiency, while high MCV points to vitamin B12 or folate deficiency.

Quick tip: You can estimate hematocrit by multiplying the hemoglobin value by 3—a useful clinical shortcut!

White Blood Cells (Leukocytes)

White blood cells (WBCs) are your body's primary defenders against disease. Unlike red blood cells, they contain nuclei and organelles. Though far less numerous than RBCs, with normal counts ranging from 5,000-11,000/mm³, they're critical for your immune system.

WBCs are classified into two main categories:

Granulocytes (containing visible granules):

• Neutrophils (60%): First responders that engulf bacteria
• Eosinophils (2-4%): Target parasites and participate in allergic responses
• Basophils (0.5%): Release histamine and other chemicals during inflammatory reactions

Agranulocytes (without visible granules):

• Lymphocytes (30%): B cells produce antibodies; T cells directly attack infected cells
• Monocytes (5%): Large cells that can leave circulation to become tissue macrophages

When infection strikes, your WBC count often increases (leukocytosis), with specific types rising depending on the pathogen. For example, bacterial infections typically cause neutrophil counts to spike, while viral infections increase lymphocyte numbers.

Health note: Abnormal white blood cell production can lead to leukemia, a cancer where immature WBCs multiply uncontrollably, crowding out healthy blood cells and impairing normal immune function.

The Immune System

Your immune system is a complex network that protects your body from harmful invaders. This protection comes in two main forms: innate immunity and adaptive immunity.

Innate immunity provides immediate, non-specific defense and includes:

• Physical barriers like skin and mucous membranes
• Chemical defenses such as stomach acid and antimicrobial proteins
• Phagocytic cells that engulf foreign particles
• Inflammatory responses that isolate and destroy pathogens

Adaptive immunity develops throughout your lifetime and provides targeted protection against specific threats. The two types of adaptive immunity are:

1. Humoral immunity: Mediated by B lymphocytes that produce antibodies (immunoglobulins)
2. Cell-mediated immunity: Orchestrated by T lymphocytes that directly attack infected cells

The adaptive immune response has a remarkable memory—once your body encounters a pathogen, it remembers it and responds more quickly and effectively if exposed again. This is the principle behind vaccination.

Fascinating fact: Your body can produce millions of different antibodies, each designed to recognize and bind to a specific antigen (foreign substance). This incredible diversity allows your immune system to combat virtually any pathogen.

Antibodies and Immunoglobulins

Antibodies (immunoglobulins) are Y-shaped proteins produced by plasma cells that specifically target foreign substances. When B cells encounter an antigen, they differentiate into plasma cells that can produce up to 2,000 antibodies per second!

There are five classes of immunoglobulins, each with distinct functions:

1. IgG: The most abundant antibody in blood; crosses the placenta to protect newborns
2. IgA: Found in secretions (tears, saliva, breast milk); guards body surfaces
3. IgM: The largest antibody; first to respond to initial infection
4. IgE: Triggers allergic reactions; helps fight parasitic infections
5. IgD: Present on B cell surfaces; function not fully understood

Antibodies protect you by:

• Neutralizing toxins
• Marking pathogens for destruction (opsonization)
• Activating complement proteins that punch holes in bacterial cell membranes
• Causing agglutination (clumping) of pathogens

Your immunity can be acquired actively (by infection or vaccination) or passively (by receiving antibodies from another source, like a mother's breast milk).

Important distinction: Active immunity develops slowly but lasts longer, while passive immunity provides immediate protection but is temporary.

Platelets and Hemostasis

Platelets (thrombocytes) are small, nucleus-free cell fragments that play a crucial role in stopping bleeding. With normal counts ranging from 150,000-300,000/mm³, these tiny structures contain contractile proteins and chemicals essential for blood clotting.

When blood vessels are damaged, a process called hemostasis occurs in three overlapping phases:

1. Vascular spasm: Injured blood vessels constrict immediately to reduce blood flow
2. Platelet plug formation: Platelets adhere to the damaged site, change shape, and stick together
3. Coagulation: A cascade of clotting factors converts liquid blood to a solid clot

The coagulation cascade involves numerous clotting factors (numbered I through XIII), most of which are produced by the liver. This complex process ultimately converts fibrinogen (Factor I) to fibrin, which forms a mesh to strengthen the platelet plug.

Several substances can prevent or slow clotting:

• Heparin inhibits thrombin and several other factors
• Warfarin blocks vitamin K-dependent clotting factor synthesis
• EDTA and citrate bind calcium, preventing its role in clotting

Medical application: Measuring bleeding time $normally 1-6 minutes$ and clotting time $normally 6-10 minutes$ helps diagnose bleeding disorders like hemophilia and thrombocytopenia.

Blood Groups and Transfusions

Your blood type is determined by antigens (proteins and glycoproteins) present on the surface of your red blood cells. The two major blood group systems are ABO and Rh.

The ABO system has four blood types based on the presence or absence of A and B antigens:

• Type A: Has A antigen; produces anti-B antibodies
• Type B: Has B antigen; produces anti-A antibodies
• Type AB: Has both A and B antigens; produces no antibodies
• Type O: Has neither antigen; produces both anti-A and anti-B antibodies

The Rh system classifies blood as either positive $Rh+$ or negative $Rh-$ based on the presence of the D antigen.

Understanding blood types is critical for safe transfusions. If a patient receives incompatible blood, the recipient's antibodies attack the donor's red blood cells, causing a potentially fatal reaction.

A special case occurs during pregnancy when an Rh- mother carries an Rh+ baby. During delivery, fetal blood can enter the mother's circulation, triggering antibody production. In subsequent pregnancies, these antibodies can cross the placenta and attack the fetus's red blood cells, causing erythroblastosis fetalis. This condition can be prevented by administering RhoGAM to the mother.

Universal donor and recipient: Type O- individuals are considered "universal donors" for red blood cells, while type AB+ individuals are "universal recipients."