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Blood: Structure, Function, and Clinical Relevance

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Blood and the Cardiovascular System

Main Components of the Cardiovascular System

The cardiovascular system is essential for transporting substances throughout the body and maintaining homeostasis. It consists of three main components:

  • Heart: Pumps blood and maintains blood pressure.

  • Blood Vessels: Arteries carry blood away from the heart, capillaries permit exchange between blood and interstitial fluids, and veins return blood to the heart.

  • Blood: The fluid connective tissue that transports substances.

Key Functions of Blood

  • Transport: Dissolved gases (oxygen, carbon dioxide), nutrients, hormones, and metabolic wastes.

  • Regulation: pH and ion composition of interstitial fluids; absorbs/neutralizes acids and balances ion concentrations.

  • Restriction of Fluid Loss: Initiates clotting at injury sites to prevent excessive blood loss.

Blood Properties

Physical and Chemical Properties

  • Blood is a fluid connective tissue.

  • Whole blood includes all components: plasma (liquid matrix) and formed elements (cells and cell fragments).

  • Temperature: About 38°C (100.4°F), slightly above normal body temperature.

  • Viscosity: Five times as viscous as water.

  • pH: Slightly alkaline, between 7.35 and 7.45 (average: 7.4).

  • Volume: 5–6 liters in males, 4–5 liters in females.

Blood Composition

Plasma

Plasma is the liquid part of blood, making up about 55% of blood volume. It is mostly water (about 90%), with proteins, electrolytes, nutrients, hormones, and wastes dissolved in it.

  • Plasma: Inside blood vessels.

  • Interstitial fluid: Outside cells, in tissues.

Plasma Proteins

  • Albumin (~65%): Maintains osmotic pressure.

  • Globulins (~30%): Includes antibodies for immune defense.

  • Fibrinogen (~5%): Involved in blood clotting.

  • Liver synthesizes >90% of plasma proteins.

Plasma Solutes

  • Electrolytes: Na+, K+, Ca2+, Mg2+, Cl-, etc. Essential for cellular activities.

  • Organic Nutrients: Lipids, carbohydrates, amino acids for cell ATP production, growth, and maintenance.

  • Organic Wastes: Urea, uric acid, creatinine; carried to sites of breakdown or excretion.

Formed Elements

  • Platelets (<0.1%): Small fragments involved in clotting.

  • White Blood Cells (WBCs, leukocytes) (<0.1%): Body defense; five classes with different functions.

  • Red Blood Cells (RBCs, erythrocytes) (99.9%): Oxygen transport.

Blood Composition Table

Component

Percentage

Main Function

Plasma

55%

Transport of nutrients, hormones, wastes

Formed Elements

45%

Oxygen transport, defense, clotting

RBCs

99.9% of formed elements

Oxygen transport

WBCs

<0.1% of formed elements

Immune defense

Platelets

<0.1% of formed elements

Clotting

Hemopoiesis (Hematopoiesis)

Development of Formed Elements

Hemopoiesis is the process of forming blood cells from stem cells in red bone marrow.

  • Hemocytoblasts: Stem cells that produce two types of stem cells:

    • Lymphoid stem cells: Produce lymphocytes (type of WBC).

    • Myeloid stem cells: Produce RBCs, other WBCs, and platelets.

Myeloid Stem Cell Differentiation

  • Monoblasts → Promonocytes → Monocytes

  • Myeloblasts → Myelocytes → Band Cells → Neutrophils, Eosinophils, Basophils

  • Megakaryocytes (large nuclei) → Platelets

  • Proerythroblasts → Erythroblasts (shed nuclei) → Reticulocytes → Erythrocytes (RBCs)

Erythropoietin (EPO)

  • Released in response to low tissue oxygen (hypoxia).

  • Stimulates stem cells and developing RBCs in red bone marrow.

  • Stimulus for EPO release: anemia, reduced blood flow to kidneys, decreased O2 in lungs, lung damage.

Hematology

Study of Blood and Blood-Forming Tissues

  • Provides important information about a person's health.

  • Detects disorders: anemia, infection, clotting disorders.

  • Dyscrasias: Blood disorders with systemic effects.

Blood Tests

  • Complete Blood Count (CBC): Measures RBC count, WBC count, hemoglobin, hematocrit, and other indices.

  • WBC Differential Count: Identifies numbers of each type of WBC.

  • RBC Tests: Assess number, size, shape, maturity of RBCs.

RBC Test Table

Test

Determines

Elevated

Depressed

Hematocrit

Percentage of formed elements in whole blood

Polycythemia

Anemia

Hemoglobin Concentration

Concentration of hemoglobin in blood

Polycythemia

Anemia

Mean Corpuscular Volume (MCV)

Average size of RBC

Macrocytic

Microcytic

RBC Count

Number of RBCs per microliter

Erythrocytosis/polycythemia

Anemia

Reticulocyte Count

Percentage of circulating reticulocytes

Reticulocytosis

Reticulocytopenia

Red Blood Cells (RBCs)

Characteristics and Functions

  • Most common formed element (~99.9%).

  • Roughly one-third of all cells in the body.

  • Single drop of whole blood contains ~260 million RBCs.

  • Average adult: ~25 trillion RBCs.

  • Adult males: 4.5–6.3 million RBCs/μL; females: 4.2–5.5 million RBCs/μL.

Functional Aspects

  • Packed with hemoglobin (protein that carries oxygen).

  • Large surface area-to-volume ratio allows more oxygen exchange.

  • Form stacks (rouleaux) to facilitate transport in small vessels.

  • Flexible—can move through narrowest capillaries.

Cell Characteristics

  • Lose most organelles during development.

  • Mature RBCs lack nuclei (anucleate) and ribosomes; cannot divide or repair.

  • Lifespan <120 days.

  • 95% of RBC intracellular proteins are hemoglobin molecules.

  • Hemoglobin (Hb) content: 14–18 g/dL males, 12–16 g/dL females.

Hemoglobin Structure and Function

Structure

  • Complex quaternary structure.

  • Each Hb molecule has:

    • Two alpha chains

    • Two beta chains

  • Each chain has a single heme molecule containing an iron ion.

  • Iron interacts with oxygen to form oxyhemoglobin (HbO2), making oxygenated blood bright red.

Function

  • Oxygen binding is reversible.

  • Deoxyhemoglobin: Hemoglobin not bound to oxygen; blood is dark red.

  • Each RBC has ~280 million Hb molecules; each Hb has four heme units.

  • Each RBC can carry >1 billion oxygen molecules.

  • ~98.5% of oxygen in blood is bound to Hb; rest is dissolved in plasma.

RBC Production and Breakdown

Production

  • ~1% of circulating RBCs replaced each day.

  • ~3 million new RBCs enter circulation each second.

  • Occurs in red bone marrow (myeloid tissue).

  • Fatty yellow bone marrow can convert to red bone marrow in cases of severe, sustained blood loss.

Breakdown

  • Plasma membrane ruptures (hemolysis) or RBC is engulfed by macrophages in spleen, liver, or bone marrow.

  • Macrophages monitor and remove old RBCs before rupture.

  • Heme units stripped of iron; iron stored or transported by transferrin.

  • Heme → biliverdin → bilirubin → bloodstream → liver.

  • Globular proteins disassembled; amino acids recycled.

  • Hemoglobin not phagocytized breaks down and is excreted in urine.

  • Breakdown of large numbers of RBCs results in red or brown urine (hemoglobinuria).

Check Your Understanding

  • List the three components of the cardiovascular system and one function of each.

  • Identify the main functions of blood (transport, regulation, protection).

  • Explain how hematopoiesis produces the formed elements of blood.

  • Describe what is measured in a CBC, and why it's clinically important.

  • Discuss one key feature of RBCs that relates to their function.

  • Explain how hemoglobin's structure allows it to carry oxygen efficiently.

Additional info: These notes are based on lecture slides and class notes for Chapter 17: Blood, which is directly relevant to Anatomy & Physiology college courses.

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