Physiology and Composition of Blood

Functions of the Blood

Blood serves several critical roles in the body, primarily categorized into transportation, regulation, and protection.

• Transportation Functions:

  • Gases: Blood transports oxygen and carbon dioxide throughout the body.
  • Nutrients: It carries essential nutrients including amino acids, glucose, and fatty acids.
  • Waste Products: Metabolic waste products such as urea, creatinine, and uric acid are transported through the blood for elimination.
  • Hormones: The blood carries numerous hormones from the endocrine system. Specific examples include thyroid simulating hormone and genocorticotropic hormone.

• Regulation Functions:

  • Acid Balance: Blood proteins act as buffers to maintain the body's pH levels. A central mechanism is the carbonic acid reversible reaction. When carbon dioxide levels increase, the reaction proceeds in a direction that creates more hydrogen ions, resulting in a decrease in pH. If carbon dioxide levels are lowered, the reaction shifts to use and bind free hydrogen ions, resulting in an increase in pH. This relates directly to the respiratory system.
  • Temperature Regulation: The body regulates temperature by adjusting blood flow. Vasodilation of blood vessels, particularly in the skin, allows more heat to radiate away from the body to cool it down. Conversely, vasoconstriction helps retain heat and raise the body's internal temperature.
  • Blood Pressure: There is a direct relationship between blood volume and blood pressure. An increase in blood volume lead to an increase in blood pressure, while a decrease in blood volume results in a decrease in blood pressure.

• Protection Functions:

  • Hemostasis: Formed elements in the blood are fundamentally involved in the clotting process to prevent blood loss.
  • Immune Response: The blood contains white blood cells, antibodies, and defensive proteins that identify and protect the body from foreign substances. Antibodies are specialized proteins secreted by certain cells that circulate to provide protection.

General Properties and Physical Characteristics of Blood

Blood has specific physical attributes and normal ranges that indicate health and physiological status:

• Total Body Weight: Blood makes up approximately $8\%$ of the total body weight.
• Volume:
  • Males typically have a volume ranging from $5-6\,\text{liters}$.
  • Females typically have a volume ranging from $4-5\,\text{liters}$. This difference is primarily due to the generally larger size of males.
• Temperature: Normal blood temperature is approximately $100^{\circ}\text{F}$.
• pH Range: The normal pH range is between $7.35-7.45$.
  • Acidosis: If the pH drops below $7.35$, it may be referred to as al acidosis.
  • Alkalosis: If the pH rises above $7.45$, it may be referred to as alkalosis.

Hematocrit and Formed Element Statistics

The hematocrit, also known as the packed cell volume (PCV), refers to the percentage of formed elements in the blood.

• Hematocrit Normals:
  • Males: Range between $45-52\%$.
  • Females: Range between $37-48\%$.
  • The higher hematocrit in males is linked to male sex hormones, specifically testosterone, which stimulates the synthesis of red blood cells.
• Red Blood Cell (RBC) Count:
  • Red blood cells are the most numerous formed element.
  • Males: $4.6-6.2 \times 10^{6}$.
  • Females: $4.2-5.4 \times 10^{6}$.
• Platelet Count: Normal ranges are between $130,000-360,000$.
• White Blood Cell (WBC) Count: Normal ranges are between $5,000-10,000$.

Blood Composition and Centrifugation

A centrifuge is used to spin blood to separate its major components based on density. The resulting separation reveals three clear components:

• Plasma: The liquid portion of blood, accounting for approximately $55\%$ of the total volume.
• Formed Elements (Hematocrit): This area accounts for about $45\%$ of the total volume and consists largely of erythrocytes (red blood cells).
• Buffy Coat: A tiny portion within the centrifuge tube, located between the plasma and the erythrocytes, which contains leukocytes (white blood cells) and platelets.

Clinical Conditions: Anemia and Polycythemia

Specific ratios in the blood can indicate various clinical conditions:

• Anemia: Defined as a low oxygen-carrying capacity in the blood. In a centrifuged sample, this might appear as plasma making up $70\%$ and hematocrit dropping to $30\%$. Since red blood cells carry oxygen, a greatly reduced red blood cell count decreases the total formed elements.
• Polycythemia: This is an increase in the number of red blood cells, which can increase the hematocrit to levels such as $60\%$.
  • Causes include living or training at high elevations where more red blood cells are produced to compensate for lower oxygen, or the intake of certain hormones.

Plasma: Composition and Proteins

Plasma acts as the interstitial fluid of the blood, representing the ECF (extracellular fluid) surrounding the white blood cells and erythrocytes.

• Basic Composition:
  • Water: Approximately $90\%$ of plasma is water.
  • Proteins: The next most significant component.
  • Solutes: Includes nutrients (glucose, amino acids), waste (urea, creatinine, uric acid), gases ($CO_{2}$, $O_{2}$), and electrolytes (sodium, potassium).
• General Protein Facts:
  • Most plasma proteins are synthesized in the liver.
  • Proteins contribute to the viscosity (thickness) and osmolarity of the blood.
  • Serum is defined as plasma that does not contain clotting proteins.

Major Classes of Plasma Proteins

Plasma proteins are categorized into three major functional groups:

• Albumins:
  • The most numerous protein class, making up $60\%$ of blood proteins.
  • Physically the smallest protein type.
  • Provides viscosity and contributes to colloid osmotic pressure (COP). COP is essential for the exchange of materials at capillary beds.
  • Assists in the transport of hydrophobic substances and helps maintain blood pH.
• Globulins: These are divided into three sub-types:
  • Alpha (\alpha) Globulins: Includes prothrombin (an enzyme used in clotting) and lipoproteins (used to transport hydrophobic materials).
  • Beta (\beta) Globulins: Includes transferrin (which carries iron in the blood), complement proteins (involved in immune response), and lipoproteins (transporting hydrophobic materials).
  • Gamma (\gamma) Globulins: These are the antibodies involved in the immune response. Unlike most plasma proteins, these are not made in the liver; they are produced by plasma cells (activated B cells).
• Fibrinogen:
  • Fibrinogen is the inactive form of the protein.
  • Its active form is fibrin, a sticky protein essential for the blood clotting process.

White Blood Cells (Leukocytes) Distribution

The formed elements include five distinct types of white blood cells, each with a standard percentage distribution:

• Neutrophils: The most numerous white blood cell, making up approximately $70\%$ of total leukocytes. They function as small phagocytes.
• Lymphocytes: Account for about $20\%$ of white blood cells. There are three major types of lymphocytes, including T-type cells.
• Monocytes: Make up approximately $6\%$ of the white blood cell population.
• Eosinophils: Account for about $3\%$ of the white blood cell population.
• Basophils: The least numerous, making up $1\%$ or less of the white blood cell population.