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

Blood & Connective Tissue

Overview of Blood as a Connective Tissue

Blood is a specialized connective tissue that plays a vital role in the transport of substances, regulation of physiological parameters, and protection against disease. It constitutes about 8% of total body weight and has distinct properties based on oxygen content.

• Volume: Males: 5-6 liters; Females: 4-5 liters

• Color: Varies with oxygen content

  • High O2: Scarlet (brighter color)

  • Low O2: Dark red

• pH: 7.35 – 7.45

Function of Blood

Blood performs several essential functions in the body:

• Distribution: Transports gases, hormones, and nutrients throughout the body.

• Regulation: Maintains pH, distributes heat, and regulates fluid balance.

• Protection: Provides immune defense against pathogens.

Blood Composition

Plasma and Formed Elements

Blood consists of plasma (the non-living matrix) and formed elements (living cells).

• Plasma: Makes up 55% of total blood volume

  • Over 90% water

  • Contains 100+ dissolved solutes: nutrients, gases, hormones, wastes, proteins, inorganic ions

  • Plasma proteins (albumin, globulins, fibrinogen) are most abundant (>7%)

• Formed Elements:

  • Erythrocytes: Red blood cells (RBCs)

  • Leukocytes: White blood cells (WBCs)

  • Platelets: Cell fragments involved in clotting

Formed Elements

Leukocytes (White Blood Cells)

Leukocytes are complete cells that function in defense against disease. They make up less than 1% of total blood volume.

• Normal count: 4,800 – 10,800 WBCs/μL

• Abnormal count: >11,000/μL indicates possible infection

• Diapedesis: Ability to move out of blood vessels into tissues

• Lifespan: Can live hours, days, or decades

• Categories:

  • Granulocytes: Visible cytoplasmic granules

    • Neutrophils: Most abundant, phagocytic, multilobed nucleus

    • Eosinophils: Lysosome-like granules, involved in allergies and general immune response

    • Basophils: Release histamine (vasodilator), involved in inflammation

  • Agranulocytes: No visible cytoplasmic granules

    • Lymphocytes: T cells (cell-mediated immunity), B cells (antibody production)

    • Monocytes: Leave circulation, become macrophages, activate lymphocytes

Platelets

Platelets are fragments of megakaryocytes and play a key role in blood clotting.

• Granules: Contain serotonin, Ca2+, enzymes, ADP, and platelet-derived growth factor (PDGF)

• Normal count: 150,000 – 400,000 platelets/μL

• Lifespan: Degenerate in about 10 days

• Formation: Regulated by thrombopoietin

Erythrocytes (Red Blood Cells)

Erythrocytes are simple biconcave discs specialized for gas transport.

• No protein synthesis: No nucleus or organelles

• Lifespan: 100-120 days

• Hemoglobin: Main protein for O2 transport

• Count: 4.2 to 6.1 million cells/μL

Hemoglobin Structure and Function

Hemoglobin (Hb)

Hemoglobin is a protein in RBCs that binds reversibly with oxygen and is responsible for oxygen transport.

• Structure: Four polypeptide chains (globin), each with a heme group containing iron

• O2 loading in lungs: Produces oxyhemoglobin (ruby red)

• O2 unloading in tissues: Produces deoxyhemoglobin (dark)

• CO2 loading in tissues: 20% of CO2 in blood binds to Hb → carbaminohemoglobin

Erythrocyte Structure & Function

• Biconcave shape: Increases surface area for gas exchange

• RBCs: >97% hemoglobin, not counting water

• No mitochondria: ATP produced anaerobically

Hematopoiesis

Blood Cell Formation

Hematopoiesis is the process of blood cell formation, occurring in red bone marrow.

• Location in adults: Axial skeleton, girdles, proximal epiphyses of humerus and femur

• Hematopoietic stem cells (hemocytoblasts): Give rise to all formed elements

Hematocrit

Packed Cell Volume

Hematocrit is the percentage of blood volume that is RBCs.

• Normal values: 47% ± 5% for males, 42% ± 5% for females

• Formula: $\text{Hematocrit} = \frac{\text{Volume RBC}}{\text{Volume total blood}} \times 100$

• Low hematocrit: Anemia (not enough healthy RBCs)

• High hematocrit: Dehydration, polycythemia vera, lung/heart disease

Hemostasis

Mechanisms of Hemostasis

Hemostasis is the process that stops bleeding through a series of reactions involving platelets and tissue factors.

1. Vascular spasm: Vasoconstriction reduces blood flow at injury site

2. Platelet plug formation: Platelets adhere to exposed collagen fibers, become activated, and release chemical messengers (ADP, serotonin, thromboxane A2)

3. Coagulation: Blood clotting via fibrin formation

Platelet Plug Formation

• Von Willebrand factor: Plasma protein that helps platelets adhere to collagen

• Activated platelets: Swell, become sticky, and release chemical messengers

Blood Vessel Response

• Intact vessel: Endothelial cells release NO and PGI2 to prevent platelet aggregation

• Ruptured vessel: Exposed collagen fibers trigger platelet adhesion and activation

Key Table: Leukocyte Types and Functions

Type	Granules	Main Function
Neutrophils	Yes	Phagocytosis of bacteria
Eosinophils	Yes	Combat parasites, modulate allergic responses
Basophils	Yes	Release histamine, mediate inflammation
Lymphocytes	No	Immune response (T cells, B cells)
Monocytes	No	Phagocytosis, become macrophages

Example: Hemostasis in Action

When a blood vessel is injured, platelets rapidly adhere to exposed collagen fibers, forming a temporary plug. Chemical messengers released by platelets recruit more platelets and initiate the coagulation cascade, resulting in a stable clot that prevents blood loss.

Additional info: These notes expand on the original content by providing definitions, examples, and a summary table for leukocyte classification. All major topics from the provided materials are covered and organized for exam preparation.