18.1 An Overview of Blood

Blood Composition and Functions

Blood is a connective tissue that circulates through the cardiovascular system, delivering substances to cells and carrying waste away. Its composition and physical properties are tightly regulated, making it central to homeostasis throughout the body.

Functions of Blood

Blood performs three broad categories of functions: transport, defense, and homeostasis.

Transport

• Carries oxygen from the lungs to tissues via red blood cells (erythrocytes)
• Moves carbon dioxide from tissues back to the lungs for exhalation
• Delivers nutrients from the digestive system to cells throughout the body
• Transports metabolic waste products from cells to excretory organs (kidneys, liver) for removal
• Distributes hormones from endocrine glands to their target tissues

Defense

• White blood cells (leukocytes) protect against pathogens and foreign substances. Some leukocytes engulf bacteria and cellular debris through phagocytosis, while others produce antibodies that target specific antigens.
• Platelets (thrombocytes) and clotting factors prevent blood loss through hemostasis, the process of forming clots at injury sites.

Homeostasis

• pH regulation: Buffer systems in the blood maintain pH between 7.35 and 7.45, which is the narrow range needed for proper cellular function.
• Temperature regulation: Blood distributes heat generated by metabolically active tissues (like skeletal muscle) throughout the body. When you're overheating, increased blood flow to the skin allows heat to dissipate.
• Osmotic balance: Plasma proteins and electrolytes maintain the osmotic pressure of blood, preventing cells from shrinking or swelling.

Components and Proportions of Blood

If you spin a blood sample in a centrifuge, it separates into two main layers:

• Plasma (~55% of blood volume): the liquid matrix containing water, proteins, electrolytes, nutrients, and dissolved gases.
• Formed elements (~45% of blood volume): the cells and cell fragments suspended in plasma.

The formed elements break down further:

• Erythrocytes (red blood cells) make up about 99% of formed elements. They contain hemoglobin, the protein responsible for binding and transporting oxygen.
• Leukocytes (white blood cells) account for less than 1% of formed elements but are critical for immune defense. The five types are neutrophils, lymphocytes, monocytes, eosinophils, and basophils, each with distinct roles.
• Thrombocytes (platelets) are cell fragments derived from large bone marrow cells called megakaryocytes. They're essential for blood clotting and hemostasis.

Hematocrit is the percentage of total blood volume occupied by red blood cells. A normal hematocrit is roughly 42% for females and 47% for males (often approximated as ~45% for a healthy adult). This value is clinically useful because abnormal hematocrit can indicate conditions like anemia (low) or polycythemia (high).

Unique Properties of Blood

These physical properties come up frequently on exams, so know the values and what they mean.

• Viscosity: Blood is about 3–5 times more viscous than water because of its suspended formed elements and dissolved plasma proteins. Higher viscosity means greater resistance to flow, which directly affects blood pressure.
• Specific gravity: Blood has a specific gravity of approximately 1.060, compared to water at 1.000. The dissolved solutes and cells make it denser than water.
• Osmolarity: Normal blood osmolarity is 280–300 mOsm/L. This is maintained by plasma proteins, electrolytes, and hormonal regulation (particularly antidiuretic hormone and aldosterone).
• pH: Normal blood pH is 7.35–7.45, making it slightly alkaline. This range is maintained by chemical buffer systems, the respiratory system (adjusting $CO_2$ levels), and the kidneys (adjusting $HCO_3^-$ levels). A pH below 7.35 is acidosis; above 7.45 is alkalosis.

Key Elements in Blood Plasma

Plasma is about 91–92% water, which serves as the solvent for everything else. The remaining 8–9% consists of proteins, electrolytes, nutrients, waste products, and dissolved gases.

Plasma Proteins (7–8% of plasma)

• Albumin is the most abundant plasma protein. It maintains colloid osmotic pressure (pulling water back into capillaries) and acts as a transport molecule for hormones, fatty acids, and certain drugs.
• Globulins come in three subtypes: alpha, beta, and gamma. Alpha and beta globulins transport lipids and fat-soluble vitamins (A, D, E, K). Gamma globulins are immunoglobulins (antibodies) produced by plasma cells for immune defense.
• Fibrinogen is converted to fibrin during the clotting cascade. Fibrin strands form the mesh-like structural framework of a blood clot.

Electrolytes

Key electrolytes include sodium ($Na^+$), potassium ($K^+$), calcium ($Ca^{2+}$), magnesium ($Mg^{2+}$), chloride ($Cl^-$), and bicarbonate ($HCO_3^-$). These maintain osmotic balance and support nerve conduction, muscle contraction, and pH buffering.

Nutrients, Waste Products, and Dissolved Gases

Plasma transports glucose, amino acids, and lipids (cholesterol, triglycerides) to cells. It also carries waste products like urea and creatinine to the kidneys, and dissolved gases including $O_2$ and $CO_2$.

Blood Formation and Regulation

Hematopoiesis is the process of blood cell production. In adults, it occurs primarily in the red bone marrow of flat bones (such as the sternum, pelvis, and vertebrae) and the epiphyses of some long bones.

All blood cell types originate from hematopoietic stem cells (hemocytoblasts), which are multipotent. "Multipotent" means these stem cells can differentiate into multiple cell types but are restricted to the blood cell lineage. From a single stem cell type, the body produces red blood cells, all five types of white blood cells, and platelets.

Clinical connections:

• Blood transfusion is the transfer of blood or blood components from a donor to a recipient. It's used to treat significant blood loss, severe anemia, and clotting disorders, among other conditions. Matching blood types is critical to avoid transfusion reactions.
• Blood doping is the practice of artificially increasing red blood cell count (through transfusion or erythropoietin injections) to boost oxygen-carrying capacity and athletic performance. It's banned in competitive sports and carries serious health risks, including increased blood viscosity that can lead to stroke or heart failure.

Blood Analysis

A hemocytometer is a specialized glass counting chamber used under a microscope to determine the concentration of blood cells in a sample. A small volume of diluted blood is loaded into the chamber's grid, and cells are counted manually. This provides a direct measurement of cells per unit volume and is a foundational technique in hematology labs.