Blood – Internal Transport System

Overview of Blood Functions

Blood is the life-sustaining transport vehicle of the cardiovascular system, responsible for delivering essential substances and removing wastes from the body.

• Transport Functions:

  • Delivering O2 and nutrients to body cells

  • Transporting metabolic wastes to lungs and kidneys for elimination

  • Transporting hormones from endocrine organs to target organs

• Regulation Functions:

  • Maintaining body temperature by absorbing and distributing heat

  • Maintaining normal pH using buffers; alkaline reserve of bicarbonate

  • Maintaining adequate fluid volume in circulatory system

• Protection Functions:

  • Preventing blood loss: Plasma proteins and platelets in blood initiate clot formation

  • Preventing infection: Agents of immunity carried in blood include antibodies, complement proteins, and white blood cells

Composition of Blood

Blood as a Connective Tissue

Blood is the only fluid tissue in the body and is classified as a connective tissue. Its matrix is a nonliving fluid called plasma, and the cells are living blood cells called formed elements.

• A spun tube of blood yields three layers:

  • Erythrocytes (red blood cells) on the bottom (~45% of whole blood)

  • Buffy coat (white blood cells and platelets) in the middle (<1%)

  • Plasma on top (~55%)

• Hematocrit: Percent of blood volume that is RBCs

  • Normal values: Males: 47% ± 5%; Females: 42% ± 5%

Diagram: Blood Separation

• Step 1: Withdraw blood and place in tube.

• Step 2: Centrifuge the blood sample.

• Result: Plasma (top), Buffy coat (middle), Erythrocytes (bottom).

Physical Characteristics and Volume

Blood Color and pH

The color of blood varies with its oxygen content, and its pH is tightly regulated.

• High O2 levels show a bright red color.

• Low O2 levels show a dark red color.

• Blood pH: 7.35–7.45

• Blood volume:

  • Males: 5–6 L

  • Females: 4–5 L

• Blood makes up ~8% of body weight.

Blood pH Levels Table

Blood Plasma

Composition of Plasma

Plasma is the straw-colored, sticky fluid portion of blood, making up about 55% of its volume. It contains water, proteins, nutrients, gases, hormones, and waste products.

• Water: 90% of plasma volume; dissolves and suspends solutes, absorbs heat

• Solutes:

  • Electrolytes: Most abundant by number; help maintain osmotic pressure and blood pH

  • Plasma proteins: 8% by weight; main contributors to osmotic pressure

    • Albumin: 60% of plasma proteins; produced by liver

    • Globulins: 36% of plasma proteins; transport proteins and antibodies

    • Fibrinogen: 4% of plasma proteins; forms fibrin threads of blood clot

  • Nutrients: Glucose, amino acids, fatty acids, vitamins

  • Respiratory gases: Oxygen and carbon dioxide

  • Hormones: Steroid and thyroid hormones carried by plasma proteins

Table: Composition of Plasma

Key Terms

• Plasma: The liquid component of blood in which the blood cells are suspended.

• Formed elements: The cellular components of blood, including erythrocytes, leukocytes, and platelets.

• Hematocrit: The percentage of blood volume that is occupied by red blood cells.

• Albumin: The most abundant plasma protein, important for maintaining osmotic pressure.

• Fibrinogen: A plasma protein essential for blood clotting.

Example: Clinical Application

Blood tests such as hematocrit, complete blood count (CBC), and blood chemistry profiles are used to assess the health and function of the blood and its components. Abnormal results can indicate disorders such as anemia, infection, or clotting abnormalities.

Additional info: Plasma proteins play a critical role in maintaining blood volume and pressure, and their levels can be affected in liver disease, malnutrition, or kidney disorders.