Blood, a specialized fluid making up about 7% to 8% of total body weight, plays a central role in maintaining overall health by transporting essential substances throughout the body. This fluid constantly circulates through veins, arteries, and capillaries, delivering oxygen and nutrients while removing waste products. Analyzing blood components offers valuable insights into a person’s health status, making blood tests a routine diagnostic tool. These examinations can help detect various conditions and monitor bodily functions before symptoms appear.
Plasma: The Liquid Foundation
Plasma constitutes the liquid portion of blood, accounting for approximately 55% of its total volume, appearing as a light yellowish fluid. It primarily consists of about 90% water, serving as the main medium for transporting blood cells, nutrients, hormones, and waste products. Plasma also contains dissolved substances, including proteins, salts, glucose, fats, amino acids, and antibodies.
Plasma proteins are important, with albumin helping maintain fluid balance, preventing leakage into other tissues. Fibrinogen is a key protein involved in blood clotting, while globulins contribute to immune responses and transport various substances. Plasma also carries dissolved gases like oxygen and carbon dioxide, as well as waste compounds such as urea, which are transported to the kidneys for excretion.
Red Blood Cells
Red blood cells, or erythrocytes, are the most abundant cells in the blood, comprising about 40% to 45% of its volume and giving blood its red color. These cells are crucial for oxygen transport, carrying oxygen from the lungs to tissues and returning carbon dioxide to the lungs for exhalation. Their biconcave disc shape and lack of a nucleus allow them to be flexible and navigate through narrow blood vessels efficiently.
Each red blood cell contains hemoglobin, an iron-rich protein that binds to oxygen molecules. Hemoglobin’s ability to bind oxygen increases as more oxygen molecules attach, facilitating efficient delivery. Blood tests measure red blood cell count and hemoglobin levels, indicating oxygen-carrying capacity. Hematocrit, the volume percentage of red blood cells, helps assess conditions like anemia (low levels) or polycythemia (high levels).
White Blood Cells
White blood cells, or leukocytes, are components of the body’s immune system, defending against infections and foreign invaders. Unlike red blood cells, they are diverse and categorized into several types, each with specialized functions. Neutrophils are phagocytes that engulf and digest bacteria, playing a primary role in fighting bacterial infections. Lymphocytes, including T and B cells, are central to adaptive immunity, recognizing and targeting specific pathogens or abnormal cells.
Monocytes transform into macrophages in tissues, where they clean up cellular debris and pathogens. Eosinophils are involved in allergic reactions and combating parasitic infections, while basophils release histamine and other chemicals that mediate inflammatory responses. A total white blood cell count measures the overall number of these cells, while a differential count determines the percentage of each type, providing clues about infections, inflammation, allergic reactions, or other underlying conditions.
Platelets
Platelets, also known as thrombocytes, are small, irregularly shaped cell fragments essential for hemostasis, the process of stopping bleeding. These fragments lack a cell nucleus and originate from larger cells in the bone marrow. When a blood vessel is damaged, platelets quickly gather at the injury site, adhering to the vessel wall and forming a plug.
This plug triggers a coagulation cascade, forming a fibrin mesh that reinforces it, creating a stable blood clot. A platelet count measures the number of these fragments in the blood; a normal range is between 150,000 and 450,000 platelets per microliter of blood. Low platelet levels (thrombocytopenia) can increase the risk of excessive bleeding, while high levels (thrombocytosis) may elevate the risk of blood clots.
Dissolved Substances and Biomarkers
Beyond cellular components, blood plasma carries dissolved substances and biomarkers that offer diagnostic information. Nutrients such as glucose, cholesterol, and triglycerides are routinely measured to assess metabolic health and risk factors for conditions like diabetes or cardiovascular disease. Glucose levels indicate blood sugar regulation, while cholesterol and triglyceride levels reflect fat metabolism.
Electrolytes, including sodium, potassium, chloride, calcium, and magnesium, are minerals with electrical charges crucial for maintaining fluid balance, nerve and muscle function, and heart rhythm. Imbalances can signal issues with kidney function, hydration, or other bodily systems. Levels of specific enzymes, such as alanine transaminase (ALT) and aspartate transaminase (AST) from the liver, troponin from the heart, and amylase and lipase from the pancreas, can indicate organ damage or inflammation.
Hormones like thyroid hormones, insulin, cortisol, and sex hormones are also examined to evaluate endocrine system function. Waste products, notably urea and creatinine, are measured to assess kidney function, as their accumulation can indicate impaired filtration. Other important biomarkers include C-reactive protein (CRP), produced by the liver in response to inflammation and infection, and specific antibodies, which can identify ongoing infections or autoimmune conditions.