Blood: Structure, Function, and Clinical Relevance (Anatomy & Physiology Study Notes)

Chapter 17: Blood

Introduction

Blood is a vital connective tissue that serves as the internal transport system of the body. It plays essential roles in maintaining homeostasis, defending against disease, and supporting cellular metabolism. Understanding blood's anatomy and physiology is crucial for clinical practice, including advising patients on preventing blood clots during hospital stays.

Functions of Blood

Overview of Blood Functions

• Transport: Blood delivers oxygen (O2), nutrients, metabolic wastes, and hormones throughout the body.

• Regulation: Blood maintains body temperature, pH balance, and fluid volume.

• Protection: Blood prevents blood loss and infection through clotting and immune responses.

Transport Functions

• Delivers oxygen and nutrients to body cells.

• Transports metabolic wastes to lungs and kidneys for elimination.

• Transports hormones from endocrine organs to target tissues.

Regulatory Functions

• Maintains body temperature by absorbing and distributing heat.

• Maintains normal pH using buffers, including the "alkaline reserve" of bicarbonate ions.

• Maintains adequate fluid volume in the circulatory system.

Protective Functions

• Prevents blood loss via plasma proteins and platelets that initiate clot formation.

• Prevents infection by transporting agents of immunity such as antibodies, complement proteins, and white blood cells.

Composition of Blood

Plasma and Formed Elements

Blood consists of cellular (formed elements) and liquid (plasma) components:

• Formed elements: Blood cells and platelets

  • Red blood cells (RBCs): Also called erythrocytes

  • White blood cells (WBCs): Also called leukocytes

  • Platelets: Cell fragments involved in clotting

• Plasma: Fluid extracellular matrix in which formed elements are suspended

Tissue fibers (fibrin) only appear when blood clots.

Blood Sample Layers

• RBCs (hematocrit): Bottom layer, ~45% of whole blood (47% ± 5% in males, 42% ± 5% in females)

• Buffy coat: Middle layer, <1% of blood, consists of WBCs and platelets

• Plasma: Top layer, ~55% of blood

Major Components of Whole Blood (Described Table)

Component	Percentage	Description
Plasma	~55%	Fluid matrix, least dense, contains water, proteins, nutrients, hormones, wastes
Buffy coat	<1%	Leukocytes (WBCs) and platelets
Erythrocytes	~45%	RBCs, most dense component

Physical Characteristics and Volume of Blood

Physical Properties

• Viscous, opaque fluid with metallic taste

• Viscosity is about five times greater than water, mainly due to RBCs

• Color varies with oxygen content: bright red when oxygen-rich, dark red when oxygen-poor

• pH: 7.35–7.45

• Blood volume: 5–6 L in males, 4–5 L in females (~8% of body weight)

Blood Plasma

Composition of Plasma

• Straw-colored, sticky fluid

• ~90% water

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

• Plasma proteins: Most abundant solutes by weight (~8% of plasma)

  • Most produced by the liver (except antibodies and hormones)

  • Functions: blood buffer, carrier, osmotic pressure regulation

  • Albumin: ~60% of plasma proteins; major blood buffer, carrier, and contributor to osmotic pressure

Formed Elements

Overview

• Formed elements include erythrocytes (RBCs), leukocytes (WBCs), and platelets

• RBCs lack nuclei and most organelles; platelets are cell fragments

• Most formed elements survive only a few days in blood

• Most blood cells do not divide; replaced by stem cells in red bone marrow

Summary Table: Formed Elements of Blood

Type	Description	Function
Erythrocytes (RBCs)	Anucleate, biconcave discs	Transport oxygen and carbon dioxide
Leukocytes (WBCs)	Complete cells with nuclei	Defense against disease
Platelets	Cell fragments	Clotting (hemostasis)

Clinical Relevance

Why Blood Matters in Healthcare

• Understanding blood anatomy and physiology is essential for preventing and managing blood clots, infections, and other disorders during hospital stays.

• Blood tests provide critical information about a patient's health, including oxygen-carrying capacity, immune status, and clotting ability.

Example Application

• Advising patients on activities to prevent blood clots during hospital stays, such as early mobilization and hydration.

Additional info: Further chapters would cover detailed mechanisms of erythrocyte and leukocyte function, hemostasis, blood disorders, and transfusion medicine.