Understanding the Composition of Blood

Blood is far more complex than the red liquid we see when we get a cut. As nursing professionals, understanding what makes up blood helps us interpret lab results, recognize abnormalities, and provide better patient care. Blood is a specialized connective tissue that flows through our circulatory system, delivering essential nutrients and oxygen while removing waste products. Let’s break down exactly what blood is made of and why each component matters.

The two main fractions of blood 🔗

Blood consists of approximately 55% plasma and 45% cellular components. When you spin a blood sample in a centrifuge, these two fractions separate naturally due to their different densities. The heavier cellular components sink to the bottom while the lighter, yellowish plasma rises to the top. This simple separation technique is fundamental to many laboratory tests and helps us analyze each component independently.

An average adult carries about 5 liters of blood, which makes up roughly 7-8% of total body weight. This means a person weighing 70 kilograms has approximately 5 liters of blood circulating through their vessels at any given moment.

The cellular fraction: Blood’s working cells 🔗

The cellular fraction contains three distinct types of cells, each with specialized functions that keep our bodies functioning properly.

Erythrocytes (red blood cells) 🔗

Red blood cells account for 40-45% of blood volume and are responsible for the characteristic red color of blood. These cells have a distinctive biconcave disk shape, resembling donuts with flattened centers rather than holes. This unique shape isn’t accidental-it increases the surface area for gas exchange and allows the cells to squeeze through narrow capillaries.

Red blood cells contain hemoglobin, an iron-rich protein that binds oxygen in the lungs and releases it to tissues throughout the body. Each red blood cell lives approximately 120 days before being removed by the spleen. The bone marrow constantly produces new red blood cells under the influence of erythropoietin, a hormone produced primarily by the kidneys.

Leukocytes (white blood cells) 🔗

White blood cells are far less numerous than red blood cells, making up only about 1% of blood volume. Despite their smaller numbers, they play crucial roles in defending against infections and foreign invaders. There are several types of white blood cells, including neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Neutrophils serve as the first line of defense and comprise 55-70% of total white blood cells, while lymphocytes include T cells and B cells that coordinate immune responses and produce antibodies.

Thrombocytes (platelets) 🔗

Platelets aren’t actually complete cells but rather small cell fragments derived from megakaryocytes in the bone marrow. These tiny fragments circulate for 9-10 days and are essential for blood clotting. When a blood vessel is injured, platelets quickly gather at the site, stick to the damaged area, and form a plug that helps stop bleeding. They also release chemical signals that activate the coagulation cascade, leading to the formation of a stable fibrin clot.

Plasma: The liquid foundation 🔗

Plasma is composed of approximately 92% water, with the remaining 8% consisting of dissolved solutes. This straw-colored liquid serves as the transportation medium for blood cells, nutrients, waste products, hormones, and proteins. Plasma constituents are classified into two main categories based on their ability to pass through semipermeable membranes.

Non-diffusible plasma constituents 🔗

Non-diffusible constituents are large molecules that cannot easily cross capillary walls or cell membranes. These are primarily proteins that serve vital functions throughout the body.

Albumin 🔗

Albumin is the most abundant plasma protein, accounting for approximately 54% of total plasma protein content. Synthesized by the liver, albumin plays multiple critical roles. It maintains colloid osmotic pressure at around 25 mmHg, which prevents fluid from leaking out of blood vessels into surrounding tissues. When albumin levels drop due to liver disease, kidney disease, or malnutrition, patients often develop edema as fluid accumulates in the interstitial spaces. Albumin also serves as a transport vehicle, carrying fatty acids, steroid hormones, thyroid hormones, and various drugs through the bloodstream.

Globulins 🔗

Globulins make up approximately 38% of total plasma proteins and include several subgroups. Alpha and beta globulins transport iron, lipids, and fat-soluble vitamins. Gamma globulins, also known as immunoglobulins or antibodies, are produced by B lymphocytes and provide immunity against pathogens. These antibodies recognize specific antigens on bacteria, viruses, and other foreign materials, marking them for destruction by other immune cells.

Fibrinogen 🔗

Fibrinogen is the least abundant of the major plasma proteins but plays an essential role in hemostasis. When activated by the clotting cascade, fibrinogen converts to fibrin, forming threadlike strands that weave together to create a stable blood clot. Fibrinogen accounts for about 7% of total plasma protein and is produced by the liver.

Enzymes and lipoproteins 🔗

Plasma contains numerous enzymes that catalyze biochemical reactions and serve as markers for organ function. For example, elevated levels of certain liver enzymes indicate hepatocellular damage. Lipoproteins such as HDL (high-density lipoprotein) and LDL (low-density lipoprotein) transport cholesterol and other lipids through the bloodstream, playing important roles in cardiovascular health.

Diffusible plasma constituents 🔗

Diffusible constituents are smaller molecules that can easily cross capillary walls and cell membranes. These substances are constantly exchanged between plasma and interstitial fluid.

Electrolytes 🔗

Major electrolytes in plasma include sodium, potassium, chloride, bicarbonate, calcium, magnesium, and phosphate. These ions are essential for maintaining fluid balance, transmitting nerve impulses, contracting muscles, and regulating pH. Even slight imbalances in electrolyte concentrations can have serious consequences. For instance, potassium levels outside the normal range can cause cardiac arrhythmias and muscle weakness.

Anabolic constituents 🔗

Anabolic constituents are substances used to build and maintain tissues. Glucose serves as the primary energy source for cells and is tightly regulated by insulin and other hormones. Amino acids are the building blocks of proteins and are absorbed from the digestive tract or released during protein breakdown. Plasma normally contains small amounts of various amino acids that cells take up for protein synthesis.

Catabolic products 🔗

Urea is the primary nitrogenous waste product from protein metabolism, produced in the liver when amino acids are broken down. The kidneys filter urea from blood and excrete it in urine. Normal blood urea nitrogen levels range from 10-20 mg/dL, but elevated levels indicate impaired kidney function or increased protein breakdown. Creatinine is produced from muscle creatine metabolism and is also filtered by the kidneys. Because creatinine production is relatively constant, it serves as a reliable marker of kidney function. Normal serum creatinine ranges from 0.6-1.2 mg/dL but varies based on muscle mass, age, and sex.

Hormones 🔗

Plasma transports hormones from endocrine glands to their target organs. These chemical messengers regulate metabolism, growth, reproduction, and stress responses. Hormones such as insulin, cortisol, thyroid hormones, and sex hormones travel through the bloodstream, mostly bound to carrier proteins, until they reach cells with appropriate receptors.

Vitamins 🔗

Both water-soluble vitamins (such as vitamin C and B vitamins) and fat-soluble vitamins (A, D, E, and K) circulate in plasma. Water-soluble vitamins dissolve directly in plasma, while fat-soluble vitamins are transported bound to lipoproteins or specific carrier proteins.

Clinical significance 🔗

Understanding blood composition is fundamental to interpreting common laboratory tests. A complete blood count assesses the cellular components, while a basic metabolic panel evaluates electrolytes, glucose, and kidney function markers. Abnormalities in any blood component can indicate underlying disease. For example, low hemoglobin indicates anemia, elevated white blood cells suggest infection or inflammation, and abnormal electrolyte levels point to kidney disease or hormonal imbalances. Decreased albumin levels may signal liver disease, kidney disease, or malnutrition, while elevated fibrinogen can indicate inflammation or cardiovascular risk.

As nurses, we regularly encounter patients whose blood composition is altered by disease, medication, or treatment. Chemotherapy affects rapidly dividing cells, often lowering white blood cell and platelet counts. Kidney disease leads to accumulation of urea and creatinine while disrupting electrolyte balance. Liver disease impairs protein synthesis, reducing albumin and clotting factor production.

What do you think? How might understanding the specific roles of different blood components help you better explain lab results to patients? When you see abnormal values in a complete blood count or metabolic panel, which blood components would you investigate first based on the patient’s symptoms and medical history?

References

1. https://www.hematology.org/education/patients/blood-basics
2. https://www.redcrossblood.org/donate-blood/how-to-donate/types-of-blood-donations/blood-components.html
3. https://bio.libretexts.org/Courses/Lumen_Learning/Anatomy_and_Physiology_II_(Lumen)/04:_Module_2-_The_Cardiovascular_System-_Blood/4.02:_An_Overview_of_Blood
4. https://my.clevelandclinic.org/health/diagnostics/22358-electrolyte-panel
5. https://www.ncbi.nlm.nih.gov/books/NBK305/