Blood: Structure, Function, and Composition (Anatomy & Physiology Chapter 10 Study Notes)

Blood: Structure, Function, and Composition

Introduction to Blood

Blood is a vital connective tissue in the human body, responsible for the transport of essential substances and the maintenance of homeostasis. It circulates through blood vessels, delivering nutrients, removing wastes, transporting hormones, and distributing body heat.

• Transport Functions: Blood carries nutrients, wastes, hormones, and body heat throughout the body.

Components of Blood

Main Components

Blood is the only fluid tissue in the body and is classified as a type of connective tissue. It consists of two main components:

• Formed elements: The living cells, including erythrocytes (red blood cells), leukocytes (white blood cells), and platelets.

• Plasma: The nonliving fluid matrix in which the formed elements are suspended.

Separation of Blood Components

When blood is centrifuged, its components separate based on density:

• Plasma: Rises to the top and makes up about 55% of blood volume.

• Erythrocytes (Red Blood Cells): Sink to the bottom, accounting for about 45% of blood volume. This percentage is known as the hematocrit.

• Buffy coat: A thin, whitish layer between plasma and erythrocytes, containing leukocytes (white blood cells) and platelets (less than 1% of blood volume).

Table: Composition of Blood

Component	Percentage	Main Functions
Plasma	~55%	Transport of nutrients, wastes, hormones, and heat
Erythrocytes (RBCs)	~45%	Oxygen and carbon dioxide transport
Buffy coat (Leukocytes & Platelets)	<1%	Immunity (WBCs), blood clotting (platelets)

Physical Characteristics and Volume of Blood

Physical Properties

Blood exhibits several unique physical characteristics:

• Texture: Sticky, opaque fluid

• Density: Heavier and thicker than water

• Color: Varies with oxygen content

  • Oxygen-rich blood: Scarlet red

  • Oxygen-poor blood: Dull red or purple

• Taste: Metallic and salty

• pH: Slightly alkaline, between 7.35 and 7.45

• Temperature: Slightly higher than body temperature, at 38°C (100.4°F)

Blood Volume

• Average adult volume: 5–6 liters (about 6 quarts)

• Percentage of body weight: Blood makes up approximately 8% of total body weight

Plasma

Composition of Plasma

Plasma is the straw-colored, liquid portion of blood, making up about 90% water. It serves as the medium for transporting dissolved substances.

• Water: Acts as a solvent and absorbs heat

• Dissolved substances:

  • Nutrients (e.g., glucose, amino acids)

  • Salts (electrolytes such as sodium, potassium, calcium)

  • Respiratory gases (oxygen, carbon dioxide)

  • Hormones

  • Plasma proteins (albumin, clotting proteins, antibodies)

  • Waste products (urea, uric acid, creatinine)

Plasma Proteins

• Albumin: Most abundant plasma protein; important for maintaining osmotic pressure and acting as a blood buffer.

• Clotting proteins: Essential for blood clotting (e.g., fibrinogen, prothrombin).

• Antibodies (Immunoglobulins): Help protect the body from pathogens.

Most plasma proteins are synthesized by the liver.

Functions of Plasma

• Transports nutrients, hormones, and waste products

• Helps maintain blood pH and osmotic balance

• Distributes body heat

Additional info: The composition of plasma can change as cells exchange substances with the blood. The liver and kidneys play key roles in maintaining plasma composition and pH balance.