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

Blood: Structure, Function, and Disorders

Overview of Blood as a Connective Tissue

Blood is a specialized connective tissue that plays a vital role in transport, regulation, and protection within the human body. It develops from mesenchyme during embryonic development.

• Definition: Blood is a fluid connective tissue composed of plasma and formed elements (cells and cell fragments).

• Origin: All connective tissues, including blood, arise from mesenchymal cells.

• Functions: Transport of gases, nutrients, hormones, and waste; regulation of pH and temperature; protection against pathogens and blood loss.

• Example: Blood transports oxygen from the lungs to tissues and removes carbon dioxide for exhalation.

Major Components of Whole Blood

Whole blood consists of two main components: plasma and formed elements. Each has distinct roles and properties.

• Plasma: The liquid matrix of blood, making up about 55% of its volume. Contains water, proteins, electrolytes, nutrients, hormones, and waste products.

• Formed Elements: Includes erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (thrombocytes).

• Table: Major Components of Whole Blood

Component	Percentage	Main Function
Plasma	~55%	Transport of substances, maintenance of osmotic balance
Erythrocytes (RBCs)	~45%	Oxygen and carbon dioxide transport
Leukocytes (WBCs)	<1%	Immune defense
Platelets	<1%	Blood clotting

Composition of Plasma

Plasma is the straw-colored, liquid portion of blood. It serves as the medium for transporting nutrients, hormones, and waste products.

• Water: Makes up about 90% of plasma; acts as a solvent.

• Proteins: Albumin (maintains osmotic pressure), globulins (immune function), fibrinogen (clotting).

• Other Solutes: Electrolytes, nutrients (glucose, amino acids), gases, hormones, metabolic waste.

• Blood pH: Normal range is 7.35–7.45; tightly regulated for enzyme function and metabolic processes.

Structure and Chemical Forms of Hemoglobin

Hemoglobin is the oxygen-carrying protein in red blood cells. It exists in several chemical forms depending on its binding state.

• Structure: Hemoglobin is a tetramer composed of four polypeptide chains, each with a heme group containing iron.

• Chemical Forms:

  • Oxyhemoglobin (HbO2): Hemoglobin bound to oxygen.

  • Deoxyhemoglobin (Hb): Hemoglobin not bound to oxygen.

  • Carbaminohemoglobin (HbCO2): Hemoglobin bound to carbon dioxide.

• Equation: $\text{Hb} + \text{O}_2 \leftrightarrow \text{HbO}_2$

Hormonal Control and Regulation of Erythropoiesis

The production of red blood cells (erythropoiesis) is regulated by the hormone erythropoietin (EPO), primarily produced by the kidneys in response to low oxygen levels.

• Stimulus: Hypoxia (low blood oxygen) triggers EPO release.

• Effect: EPO stimulates bone marrow to increase RBC production.

• Clinical Application: Synthetic EPO is used to treat certain types of anemia.

Destruction of RBCs and Hemoglobin Metabolism

Red blood cells have a lifespan of about 120 days. Their breakdown occurs in the spleen and liver, with hemoglobin metabolized into various pigments.

• Breakdown Products: Hemoglobin is split into heme and globin. Heme is converted to bilirubin (yellow pigment), and globin is broken down into amino acids.

• Pigments Produced: Bilirubin and biliverdin are the main pigments from heme breakdown.

• Clinical Relevance: Excess bilirubin leads to jaundice.

Types and Causes of Anemia

Anemia is a condition characterized by a deficiency of red blood cells or hemoglobin, resulting in reduced oxygen delivery to tissues.

• Types:

  • Iron-deficiency anemia: Caused by insufficient iron intake or absorption.

  • Pernicious anemia: Due to lack of vitamin B12.

  • Hemolytic anemia: Caused by excessive destruction of RBCs.

  • Aplastic anemia: Failure of bone marrow to produce RBCs.

• Symptoms: Fatigue, pallor, shortness of breath.

Steps of Hemostasis

Hemostasis is the process that stops bleeding after blood vessel injury. It involves three main steps.

• Vascular Spasm: Immediate constriction of the blood vessel to reduce blood flow.

• Platelet Plug Formation: Platelets adhere to the site of injury and aggregate to form a temporary plug.

• Coagulation: A cascade of reactions leads to the formation of a stable fibrin clot.

• Equation: $\text{Prothrombin} \xrightarrow{\text{Thrombin}} \text{Fibrinogen} \rightarrow \text{Fibrin (clot)}$

Coagulation and Blood Clotting Factors

Coagulation is a complex process involving multiple clotting factors, most of which are plasma proteins synthesized by the liver.

• Key Factors: Fibrinogen, prothrombin, and factors I–XIII.

• Intrinsic and Extrinsic Pathways: Two pathways converge to activate factor X, leading to fibrin formation.

• Table: Selected Blood Clotting Factors

Factor	Name	Function
I	Fibrinogen	Forms fibrin threads of clot
II	Prothrombin	Converted to thrombin
VIII	Antihemophilic factor	Deficiency causes hemophilia A
X	Stuart-Prower factor	Activates prothrombin

Processes of Clot Retraction and Fibrinolysis

After a clot forms, it retracts to reduce its size and facilitate tissue repair. Fibrinolysis is the process of breaking down the clot once healing is underway.

• Clot Retraction: Platelets contract, pulling the edges of the wound together.

• Fibrinolysis: Plasmin enzyme digests fibrin, dissolving the clot.

• Equation: $\text{Plasminogen} \xrightarrow{\text{tPA}} \text{Plasmin} \rightarrow \text{Fibrin degradation}$

Thromboembolic Disorders

Thromboembolic disorders involve abnormal clot formation within blood vessels, which can obstruct blood flow and cause tissue damage.

• Thrombosis: Formation of a clot (thrombus) within a blood vessel.

• Embolism: A clot (embolus) that travels through the bloodstream and lodges elsewhere.

• Clinical Example: Pulmonary embolism can result from a thrombus in the leg veins.

Bleeding Disorders

Bleeding disorders are conditions that impair the blood's ability to clot, leading to excessive bleeding.

• Hemophilia: Genetic deficiency of specific clotting factors (e.g., factor VIII).

• Thrombocytopenia: Low platelet count, leading to poor clot formation.

• Vitamin K Deficiency: Reduces synthesis of clotting factors.

• Symptoms: Easy bruising, prolonged bleeding after injury.

Blood Typing and ABO Blood Groups

Blood typing is essential for safe transfusions. The ABO system classifies blood based on the presence of antigens on RBC surfaces.

• Types: A, B, AB, and O, determined by the presence or absence of A and B antigens.

• Antibodies: Individuals produce antibodies against antigens not present on their own RBCs.

• Table: ABO Blood Types and Compatibility

Blood Type	Antigens on RBCs	Antibodies in Plasma	Can Receive From
A	A	Anti-B	A, O
B	B	Anti-A	B, O
AB	A and B	None	A, B, AB, O
O	None	Anti-A, Anti-B	O

Additional info: Academic context and expanded explanations have been added to ensure completeness and clarity for exam preparation.