Blood: Structure, Components, and Functions (Anatomy & Physiology Study Notes)

Blood: Structure, Components, and Functions

Overview of Blood

Blood is a specialized fluid connective tissue that plays a vital role in maintaining homeostasis within the human body. An average adult has about 5 liters of blood, which constitutes approximately 8% of total body weight. Blood circulates continuously through blood vessels, delivering essential substances and removing waste products.

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

• Circulation: Blood is pumped by the heart and flows through arteries, veins, and capillaries.

Components of Blood

Blood consists of two major components: plasma and formed elements. When a blood sample is centrifuged, it separates into three distinct layers:

• Plasma: The top layer, making up about 55% of total blood volume. Plasma is the liquid extracellular matrix of blood, primarily composed of water, plasma proteins, and dissolved solutes.

• Buffy coat: The thin middle layer (about 1% of total volume), containing leukocytes (white blood cells) and platelets.

• Erythrocytes: The bottom layer (about 44% of total volume), consisting of red blood cells. The percentage of blood volume occupied by erythrocytes is called the hematocrit.

Formed elements include:

• Erythrocytes (Red Blood Cells, RBCs): Specialized for oxygen transport.

• Leukocytes (White Blood Cells, WBCs): Involved in immune defense.

• Platelets: Small cell fragments essential for blood clotting.

Table: Layers of Centrifuged Blood Sample

Layer	Composition	Approximate % of Total Volume
Plasma	Water, plasma proteins, solutes	55%
Buffy coat	Leukocytes, platelets	1%
Erythrocytes	Red blood cells	44%

Functions of Blood

Blood performs several critical functions necessary for survival and homeostasis:

• Exchanging gases: Blood transports oxygen (O2) from the lungs to tissues and carbon dioxide (CO2) from tissues to the lungs for exhalation.

• Distributing solutes: Plasma carries ions, nutrients, hormones, and waste products, helping regulate ion concentrations in tissues.

• Performing immune functions: Leukocytes and immune proteins in blood defend the body against pathogens and foreign substances.

• Maintaining body temperature: Blood absorbs and distributes heat throughout the body, helping regulate temperature.

• Functioning in blood clotting: Platelets and certain plasma proteins form clots to seal damaged blood vessels and prevent blood loss.

• Preserving acid-base homeostasis: Blood maintains a stable pH (typically between 7.35 and 7.45) through buffering systems.

• Stabilizing blood pressure: Blood volume is a major determinant of blood pressure, influencing cardiovascular stability.

Key Terms

• Plasma: The liquid component of blood, primarily water, containing dissolved proteins, nutrients, hormones, and waste products.

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

• Hematocrit: The percentage of blood volume occupied by erythrocytes; a key indicator of oxygen-carrying capacity.

• Buffy coat: The thin layer of leukocytes and platelets found between plasma and erythrocytes after centrifugation.

Example: Clinical Application

• Hematocrit Measurement: A low hematocrit may indicate anemia, while a high hematocrit can suggest dehydration or polycythemia.

Additional info: Blood is classified as a connective tissue because it contains cells suspended in an extracellular matrix (plasma). The study of blood and its disorders is known as hematology.