The Circulatory System I: Blood – Structure, Function, and Composition

The Circulatory System I: Blood

Introduction to the Circulatory System

The circulatory system is essential for transporting substances throughout the body. It consists of the heart, blood vessels, and blood. The cardiovascular system is a subset, including only the heart and blood vessels.

• Circulatory system: Heart, blood vessels, and blood

• Cardiovascular system: Heart and blood vessels only

Functions of Blood

Blood performs several vital functions necessary for maintaining homeostasis and supporting life.

• Regulation of pH: Maintains blood pH between 7.35 and 7.45, crucial for enzyme function and metabolic processes.

• Temperature regulation: Distributes heat throughout the body to maintain a stable internal temperature.

• Immune defense: Fights infections by transporting white blood cells and antibodies.

• Clot formation: Forms clots to prevent excessive blood loss after injury.

Transport Functions of Blood

Blood is the primary transport medium in the body, moving essential substances to and from cells.

• Oxygen transport: Carries oxygen from the lungs to body tissues.

• Carbon dioxide removal: Picks up carbon dioxide from tissues and carries it to the lungs for exhalation.

• Nutrient distribution: Picks up nutrients from the digestive tract and distributes them to all tissues.

• Waste removal: Transports metabolic waste products to the kidneys for excretion.

• Hormone delivery: Carries hormones from endocrine glands to target cells.

Characteristics and Composition of Blood

Physical and Chemical Properties

Blood is a specialized connective tissue with unique physical and chemical characteristics.

• Type: Connective tissue

• Texture: Sticky and more viscous than water

• Color: Determined by oxygen content (bright red when oxygenated, darker red when deoxygenated)

• Volume: Males typically have 5–6 liters; females 4–5 liters

• Temperature: Slightly higher than the rest of the body (about 38°C or 100.4°F)

Major Components of Blood

Blood is composed of plasma and formed elements. Each component has distinct functions and properties.

• Plasma: The liquid matrix, making up about 55% of total blood volume

• Formed elements: Cellular components, making up about 45% of total blood volume

Blood Composition Table

Component	Percentage of Blood	Main Constituents
Plasma	~55%	91% water, 7% proteins (albumin, globulins, fibrinogen), 2% other solutes (ions, nutrients, waste, gases, hormones)
Formed Elements	~45%	Red blood cells (erythrocytes), white blood cells (leukocytes), platelets (thrombocytes)

Plasma

Plasma is the pale, yellow liquid portion of blood that acts as a solvent and transport medium for many substances.

• Percentage of total blood: 55%

• Composition:

  • 91% water

  • 7% proteins (albumin, globulins, fibrinogen)

  • 2% other solutes (ions, nutrients, waste products, gases, regulatory substances)

• Functions: Maintains osmotic balance, transports substances, and aids in clotting and immune responses

Major Plasma Proteins Table

Protein	Percentage of Plasma Proteins	Function
Albumin	~58%	Maintains osmotic pressure, transports substances
Globulins	~38%	Immune functions (antibodies), transport hormones
Fibrinogen	~4%	Blood clot formation

Formed Elements

The formed elements are the cellular components of blood, each with specialized roles.

• Red blood cells (erythrocytes): Transport oxygen and carbon dioxide

• White blood cells (leukocytes): Defend against infection and remove debris

• Platelets (thrombocytes): Involved in blood clotting

Formed Elements Table

Element	Number per mm3	Main Function
Red blood cells	4.2–6.2 million	Oxygen and carbon dioxide transport
White blood cells	5,000–9,000	Immune defense
Platelets	250,000–400,000	Blood clotting

Additional info: The provided notes and diagrams are highly relevant to a college-level Anatomy & Physiology course, focusing on the structure, function, and composition of blood as part of the circulatory system.