Blood: Structure, Function, and Components – Study Notes for Anatomy & Physiology

Blood: Structure, Function, and Components

A. Functions of Blood

Blood is a specialized connective tissue with multiple essential functions in the human body. It is responsible for transportation, regulation, and protection.

• Transportation

  • Transports oxygen, carbon dioxide, nutrients, electrolytes, waste, hormones, and other substances throughout the body.

  • Maintains and protects the internal environment by carrying substances necessary for homeostasis.

• Regulation

  • Homeostasis: Maintains a stable internal environment.

  • Buffers: Substances that resist changes in pH. Normal blood pH is 7.35 to 7.45.

  • Blood pH is monitored and regulated by the lungs (CO2 elimination) and kidneys (bicarbonate retention).

  • Regulates fluid and electrolyte balance, body temperature, and tissue maintenance.

• Protection

  • Immune system defends against microorganisms and toxins.

  • Blood clotting mechanisms prevent excessive blood loss.

B. Physical Properties of Blood

• Blood is thicker and more viscous than water.

• Temperature: ~38°C (100.4°F).

• pH: Slightly alkaline (7.35–7.45).

• Blood volume: ~8% of body weight.

• Blood Volume by Sex:

  • Male: 5–6 liters (~1.5 gallons)

  • Female: 4–5 liters (~1.2 gallons)

C. Organization of Blood

Blood consists of two main portions: plasma and formed elements.

• Plasma (55%):

  • Watery liquid containing dissolved substances.

  • Straw-colored, contains plasma proteins, antibodies, waste, and other solutes.

  • 91.5–92% water, 7% proteins, 1% solutes (mainly Na+).

• Formed Elements (45%):

  • Cells and cell fragments, mainly red blood cells (RBCs), white blood cells (WBCs), and platelets.

Plasma Proteins

• Albumins: 54–60% of plasma proteins; maintain colloid osmotic pressure.

• Globulins: 36–38% of plasma proteins; include:

  • Alpha globulins: Transport lipoproteins and fat-soluble vitamins (A, D, E, K).

  • Beta globulins: Transport fat (lipoproteins) and fat-soluble vitamins.

• Fibrinogen: Important for blood clotting.

Other Plasma Solutes

• Electrolytes, nutrients, waste products, gases, enzymes, hormones, and regulatory substances.

Serum

• Serum is plasma minus fibrinogen (yellowish in color), contains serum antibodies.

Whole Blood Centrifugation

Component	Percentage	Description
Plasma	55%	Straw-colored, contains proteins, antibodies, waste, etc.
Buffy Coat	~1%	White blood cells (WBCs) and platelets
Formed Elements	45%	Mainly RBCs

D. Formed Elements of Blood

• Erythrocytes (Red Blood Cells, RBCs)

• Leukocytes (White Blood Cells, WBCs)

  • Granular leukocytes (granulocytes):

    • Neutrophils

    • Eosinophils

    • Basophils

  • Agranular leukocytes (agranulocytes):

    • Lymphocytes (T cells, B cells)

    • Monocytes

• Thrombocytes (Platelets)

E. Production of Formed Elements (Hematopoiesis)

Hematopoiesis is the process by which blood cells are formed.

• Embryonic sites: Yolk sac, liver, spleen, thymus gland, lymph nodes, bone marrow.

• After birth: Red bone marrow (myeloid tissue) in the proximal epiphyses of long bones and flat bones (sternum, ribs, cranial bones, vertebrae, pelvis).

• Hemocytoblasts (pluripotent hematopoietic stem cells): Give rise to all blood cell types:

  • Proerythroblasts → RBCs

  • Myeloblasts → Neutrophils, eosinophils

  • Monoblasts → Monocytes

  • Lymphoblasts → Lymphocytes

  • Megakaryoblasts → Thrombocytes (platelets)

• Erythropoietin (EPO): Hormone produced by kidneys and liver, stimulates RBC production.

• Thrombopoietin: Stimulates formation of platelets.

• Cytokines: Glycoproteins produced by bone marrow cells, regulate blood cell formation.

F. Erythrocytes (Red Blood Cells)

RBCs are the most abundant formed element in blood and are responsible for oxygen transport.

• 99% of formed elements are RBCs.

• Contain hemoglobin, which gives blood its red color and carries oxygen.

RBC Structure

• Biconcave discs (~8 microns diameter).

• Mature RBCs lack a nucleus and most organelles.

• Cannot reproduce or carry out extensive metabolic activities.

• Hemoglobin is ~33% of cell weight.

Hemoglobin (Normal Values)

Group	Hemoglobin (gms/100 mL blood)
Infants	14–20
Females	12–15/16
Males	13–18

RBC Function

• Oxyhemoglobin: Hemoglobin bound to oxygen (formed in lungs).

• Deoxyhemoglobin: Hemoglobin after oxygen is released.

• Globin: Protein portion of hemoglobin, composed of four polypeptide chains (two alpha, two beta).

• Each hemoglobin molecule contains four iron (Fe2+) atoms, each binding one oxygen molecule.

• Each RBC contains about 280 million hemoglobin molecules.

• RBCs lack mitochondria and generate ATP anaerobically.

• Hemoglobin also transports about 23% of carbon dioxide as carbaminohemoglobin.

Buffer System in Blood

Blood contains buffers that help maintain pH stability.

• Example buffer reaction: $H_2O + CO_2 \rightarrow H_2CO_3 \rightarrow H^+ + HCO_3^-$ $H_2O + NaOH \rightarrow Na^+ + OH^- + H_2O$ $H_2O + NaHCO_3 \rightarrow Na^+ + HCO_3^- + H_2O$

RBC Characteristics

• Life span: ~120 days.

• Old RBCs are removed by phagocytic macrophages in the spleen and liver.

• RBCs become more fragile with age and may burst.

Hemoglobin Breakdown

• Hemoglobin is recycled: globin is split into amino acids for protein synthesis.

• Heme portion is broken down into:

  • Iron: Stored as ferritin or hemosiderin.

  • Bilirubin: Pigment that does not contain iron; processed by the liver and excreted in bile.

• Iron is transported by transferrin to make new hemoglobin.

• Bilirubin is converted to urobilinogen and stercobilin (gives feces brown color).

Red Blood Cell Deficiency

• Anemia: Lower than normal number of RBCs or hemoglobin quantity, causing oxygen deficiency.

• Causes:

  • Lack of iron

  • Lack of certain amino acids

  • Lack of vitamin B12

• Intrinsic factor: Needed for vitamin B12 absorption; produced by stomach parietal cells.

• Hypoxia: Cellular oxygen deficiency due to insufficient oxygen entering the blood.

RBC Shape and Count Abnormalities

• Anisocytosis: Abnormal variation in RBC size.

• Poikilocytosis: Abnormal variation in RBC shape.

• Reticulocyte Count: Measures rate of erythropoiesis (0.5–1.5% of RBCs).

• Hematocrit: Percentage of RBCs in blood; females: 38–46%, males: 40–54%.

G. Leukocytes (White Blood Cells, WBCs)

WBCs are involved in immune defense and protection against pathogens.

• WBCs can live for several months or years (lymphocytes), but most live only a few days.

• Phagocytic WBCs may live only a few hours during infection.

• Normal WBC count: 5,000–10,000 cells/mm3.

• Leukocytosis: Increase in WBC count.

• Leukopenia: Decrease in WBC count (below 5,000/mm3).

WBC Functions

• Phagocytosis and immune response against pathogens.

• Neutrophils and macrophages ingest bacteria and dispose of dead matter.

Additional info:

• Blood is classified as a connective tissue due to its origin and function.

• Blood cell formation (hematopoiesis) is regulated by hormones and cytokines.

• Blood pH is tightly regulated to maintain enzyme function and metabolic processes.

• Disorders such as anemia, sickle cell anemia, and leukopenia can significantly impact oxygen transport and immune defense.