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Blood Composition
and Hemostasis

Explore the cellular and fluid components of blood, understand their functions, and learn how hemostasis stops bleeding through a remarkable cascade of events.

Anatomy & Physiology

What Is Blood?

Blood is a specialized connective tissue composed of cells suspended in a liquid matrix called plasma. An average adult has approximately 5 liters of blood, which accounts for roughly 8% of total body weight.

Blood Composition Overview

Plasma ~55% <1% RBCs ~45% Water, proteins, nutrients, gases WBCs & Platelets (Buffy coat) Erythrocytes (Hematocrit) Centrifuged Blood Sample

The Formed Elements

Erythrocytes
Red blood cells: biconcave discs lacking a nucleus. Transport \( \text{O}_2 \) via hemoglobin. ~4.8-5.4 million per \( \mu\text{L} \).
Leukocytes
White blood cells: nucleated cells of the immune system. Five types, 4,500-11,000 per \( \mu\text{L} \). Defense against pathogens.
Thrombocytes
Platelets: cell fragments from megakaryocytes. Essential for clot formation. 150,000-400,000 per \( \mu\text{L} \).

Plasma: The Liquid Matrix

Plasma is approximately 92% water and contains dissolved proteins, electrolytes, nutrients, gases, and waste products. Its three major protein groups play critical roles:

1
Albumins (~60%): Maintain osmotic pressure; transport lipids and steroid hormones.
2
Globulins (~36%): Include antibodies (immunoglobulins) and transport proteins for iron, copper, and lipids.
3
Fibrinogen (~4%): Essential clotting factor converted to fibrin threads during hemostasis.

Hemoglobin: The Oxygen Carrier

Alpha 1 Alpha 2 Beta 1 Beta 2 Fe Fe Fe Fe Hemoglobin Structure 4 polypeptide chains, each with a heme group
\( \text{Heme} + \text{Fe}^{2+} \)
\( \text{Binds 1 O}_2 \)
Each hemoglobin carries up to 4 \( \text{O}_2 \) molecules

What Is Hemostasis?

Hemostasis (from Greek: hemo = blood, stasis = standing still) is the body's rapid, localized response to stop bleeding at a damaged blood vessel. It involves three overlapping phases that occur in sequence.

Phase 1
Vascular Spasm: Smooth muscle in the vessel wall contracts immediately.
Phase 2
Platelet Plug Formation: Platelets adhere, activate, and aggregate at the injury site.
Phase 3
Coagulation Cascade: Clotting factors produce a fibrin mesh that reinforces the plug.

Three Phases of Hemostasis

1. Vascular Spasm Smooth muscle contracts Reduces blood flow 2. Platelet Plug Adhesion, activation, aggregation Temporary seal 3. Coagulation Fibrin mesh formation Stable clot Clot Retraction Then fibrinolysis (clot removal)

The Coagulation Cascade

Intrinsic Pathway Contact with exposed collagen (Factor XII) Extrinsic Pathway Tissue factor (TF) released (Factor III) Common Pathway
\( \text{Factor X} \rightarrow \text{X}_\text{a} \)
\( \text{Prothrombin} \rightarrow \text{Thrombin} \)
\( \text{Fibrinogen} \rightarrow \text{Fibrin Mesh} \)

Hemostasis vs Thrombosis

Hemostasis Thrombosis
Definition Normal clotting at injury site Abnormal clot in intact vessel
Trigger Vessel wall damage Endothelial dysfunction, stasis, hypercoagulability
Location At the wound Anywhere in vasculature
Outcome Protective: stops bleeding Pathological: blocks blood flow
Resolution Fibrinolysis dissolves clot May cause infarction or embolism

Types of Leukocytes

1
Neutrophils (~60-70%): First responders; phagocytize bacteria. Most abundant WBC.
2
Lymphocytes (~20-25%): T cells, B cells, and NK cells. Drive adaptive immunity and produce antibodies.
3
Monocytes (~3-8%): Mature into macrophages in tissues. Phagocytize pathogens and debris.
4
Eosinophils (~2-4%): Combat parasitic worms and modulate allergic responses.
5
Basophils (<1%): Release histamine and heparin. Involved in inflammatory and allergic reactions.

Platelet Plug Formation

1
Adhesion: Platelets bind to exposed collagen in the damaged vessel wall via von Willebrand factor (vWF).
2
Activation: Adhered platelets change shape (become spiny), degranulate, and release ADP, thromboxane A2, and serotonin.
3
Aggregation: Released chemicals recruit and activate more platelets, which bind together via fibrinogen bridges to form the plug.

Fibrinolysis: Clot Removal

Plasminogen tPA Plasmin Fibrin Degraded Fibrin Degradation Products (FDPs)

How to Analyze a CBC Report

1
Check Hemoglobin and Hematocrit: Low values suggest anemia. High values may indicate polycythemia or dehydration.
2
Evaluate WBC Count: Elevated (>11,000/\( \mu\text{L} \)) suggests infection or leukemia. Decreased (<4,500/\( \mu\text{L} \)) suggests immunosuppression.
3
Review the Differential: Which WBC type is elevated? Neutrophilia points to bacterial infection; lymphocytosis to viral.
4
Check Platelet Count: Low (<150,000/\( \mu\text{L} \)) = thrombocytopenia (bleeding risk). High (>400,000/\( \mu\text{L} \)) = thrombocytosis (clotting risk).
5
Correlate with Symptoms: Match lab abnormalities to the patient's clinical presentation for a diagnosis.

Example: Identifying Anemia

A patient presents with fatigue, pallor, and shortness of breath. Her CBC shows:

Hemoglobin
9.2 g/dL
Normal: 12-16 g/dL
Hematocrit
28%
Normal: 37-47%
MCV
68 fL
Normal: 80-100 fL

Example: Coagulation Disorder

A 10-year-old boy experiences prolonged bleeding after a tooth extraction and frequent nosebleeds. Lab results show:

Platelet Count
250,000/\( \mu\text{L} \)
Normal range
PT
12 sec
Normal range
aPTT
65 sec
Normal: 25-35 sec

Common Mistakes

✕ Confusing Hemostasis with Homeostasis
Hemostasis is specifically about stopping bleeding. Homeostasis is the broader concept of maintaining internal stability.
✕ Mixing Up PT and aPTT
PT tests the extrinsic pathway (factors VII, X, V, II). aPTT tests the intrinsic pathway (factors XII, XI, IX, VIII).
✕ Forgetting the Common Pathway
Both intrinsic and extrinsic pathways converge at Factor X. From there, the common pathway leads to fibrin formation.

Blood Components Summary

Component Count / Amount Primary Function
Erythrocytes 4.8-5.4 M / \( \mu\text{L} \) \( \text{O}_2 \) / \( \text{CO}_2 \) transport
Leukocytes 4,500-11,000 / \( \mu\text{L} \) Immune defense
Platelets 150-400K / \( \mu\text{L} \) Clot formation
Plasma ~55% of blood volume Transport medium
Albumin ~60% of plasma proteins Osmotic pressure
Fibrinogen ~4% of plasma proteins Clotting (fibrin precursor)

Example: Leukocytosis

A patient with fever, chills, and a productive cough has a WBC count of 18,000 per \( \mu\text{L} \) with the following differential:

Neutrophils
82%
Normal: 60-70%
Lymphocytes
12%
Normal: 20-25%
Bands
8%
Normal: 0-3%

Question 1 of 5

Which plasma protein is the precursor to fibrin and is essential for the final step of the coagulation cascade?

Question 2 of 5

A patient has a prolonged aPTT but a normal PT. Which coagulation pathway is most likely deficient?

Question 3 of 5

Which type of white blood cell is the most abundant in a normal blood sample and serves as the first responder to bacterial infections?

Question 4 of 5

During platelet plug formation, what molecule acts as a bridge between platelets during the aggregation phase?

Question 5 of 5

A patient's CBC reveals a hemoglobin of 8.5 g/dL, a hematocrit of 26%, and an MCV of 110 fL. This presentation is most consistent with which type of anemia?