Blood

Revised 2025-08-30 3:16:09 AM EDT

Blood’s functions can be grouped into three categories:

Definitions

Transport

Gases (oxygen, carbon dioxice), nutrients, wastes, hormones, drugs, heat.

Regulation

pH, temperature, fluid/electrolyte balance, osmotic pressure.

Protection

Against infection (white blood cells, antibodies), against blood loss (clotting via platelets), and against abnormal cells (cancers).

Its functions are tied to the specific components of blood, described below.

Blood as a tissue

Blood makes up about eight percent of body weight and is slightly more viscous (slower flowing) than water. It consists of a watery ground substance (plasma) and the cells that float in the plasma: red blood cells, white blood cells, and platelets (which are collectively called “formed elements”). This combination of a ground substance plus living cells plus the functional proteins that are dissolved in plasma means that, structurally, blood is a connective tissue even though it looks very different from other connective tissues like fascia, bone, and cartilage. Blood’s role in transport makes it a connector for all body systems, which means it also fits our functional definition of connective tissue.

Red blood cells

Blood contains red blood cells (“RBCs”, or “erythrocytes”), which are the primary vehicle moving oxygen around the body. They are so efficient at carrying oxygen that they carry about 98–99% of all oxygen in the body, with only a small fraction of the body’s oxygen dissolved directly in plasma. The protein that makes them so good at carrying oxygen is called hemoglobin (Hb). Hemoglobin makes up about one-third of the weight of each red blood cell – roughly 95–97% of its dry content. Iron is an essential component of each hemoglobin molecule, which is why dietary sources of iron are essential to enable the blood’s oxygen-carrying capabilities.

Each RBC is shaped like a jelly doughnut that had the jelly squished out (a “biconcave disc”). RBCs also lack a nucleus, which means they cannot repair themselves. Each RBC lives about 90 to 120 days before being broken down in the liver and spleen where components such as iron and amino acids are recycled.

Author note: TK: link to anemia in the pathology section or mention it here.

White blood cells

White blood cells (“WBCs”, or “leukocytes”), serve as the body’s gardeners, constantly tending the internal environment and surveying it for threats. Just as gardeners protect a landscape from weeds and pests while pruning unhealthy growth, WBCs defend against foreign invaders such as bacteria and viruses, while also identifying and responding to abnormal cell growths that could become harmful (e.g., tumors).

Definitions

Emigration

To reach the tissues where they are needed, WBCs leave the bloodstream in a process called emigration, in which they move from the vessels into surrounding tissues.

Diapedesis

A key step in this is diapedesis, the ability of WBCs to squeeze through tiny gaps in capillary walls without damaging them.

Phagocytosis

Once at the site of trouble, many WBCs neutralize threats through phagocytosis, the process of engulfing and digesting harmful microbes or debris.

Author note: TK link to additional information in immune system section.

Platelets

Platelets (also called thrombocytes), are cell fragments that coordinate blood clotting by interacting with a plasma protein called fibrinogen to form a plug at injury sites. Once activated, platelets release chemicals that trigger the coagulation cascade, ensuring the clotting process begins promptly. They also secrete signals that attract additional platelets to the damaged area, building a stronger plug, as well as factors that stimulate blood vessel repair by encouraging surrounding cells to grow and heal. To reinforce stability, platelets release substances that stabilize the clot, preventing it from breaking down too soon. Among the contents of each platelet is actin and myosin: contractile proteins that allow the platelet plug to shrink and tighten, reducing the wound’s size. These are the same actin and myosin proteins that allows muscles to contract and relax.

Author note: TK link to pathology section on clotting (“Of course clotting at the wrong time or place can be unhelpful…”)

Plasma

Plasma is the pale yellow liquid portion of blood that accounts for about 55% of blood’s total volume. It is the medium in which red blood cells, white blood cells, and platelets are suspended. Plasma is roughly 90% water, and the other 10% of it is the variety of dissolved substances it is transporting throughout the body. These dissolved substances include plasma proteins and other substances.

Dissolved substances are transported by plasma

Just as red blood cells are the primary vehicles for moving oxygen around the body, plasma is a transit system for other substances that need to be moved among the body’s tissues. These substances include:

• dissolved gases such as carbon dioxide and a minority of the body’s oxygen
• macronutrients like amino acids, glucose, and fatty acids
• micronutrients including vitamins and minerals (for example, iron and calcium)
• electrolytes such as sodium and potassium ions, which are critical for fluid balance, muscle contraction, and nerve signaling
• enzymes that regulate biochemical reactions
• hormones that coordinate communication between organs
• metabolic wastes like ammonia and urea
• drugs and environmental toxins for processing or elimination

Functional plasma proteins

Among the most important dissolved substances in plasma are the plasma proteins (sometimes called functional proteins), which make up about 7–8% of plasma by weight and perform several essential jobs in blood itself:

Definitions

Albumin

Albumin helps maintain osmotic pressure. This is a technical way of saying it controls the rate at which fluids move back and forth between capillaries and tissues.

Globulins

Globulins have multiple roles, with some acting as immunoglobulins (antibodies) that defend against pathogens, and others serving as transport proteins for lipids, hormones, and metal ions.

Fibrinogen

Fibrinogen, though normally inactive, becomes vital during clot formation, as it converts from its dissolved form into insoluble fibrin strands that stabilize blood clots.

The proteins are also what lead us to classify blood as a connective tissue (recalling that all connective tissues are made of 1) living cells, 2) a ground substance, and 3) intrinsic proteins).

Plasma is the source of other fluids

Many of the body’s other fluids are derived from plasma, as it serves as the foundation for the body’s extracellular environment. When plasma leaves the blood vessels through the walls of capillaries, it forms interstitial fluid, which bathes and nourishes the body’s cells. In some regions of the body specialized tissues collect or further modify the interstitial fluid to create substances with unique functions:

Definitions

Lymph

Lymph is interstitial fluid collected by lymphatic vessels and returned to the bloodstream after passing through lymph nodes, where immune surveillance occurs.

Synovial fluid

Synovial fluid in joints originates from plasma but is enriched with hyaluronic acid and lubricating proteins, allowing it to reduce friction and support smooth movement.

Cerebrospinal fluid (CSF)

Cerebrospinal fluid (CSF) is produced by filtration and secretion of plasma at the choroid plexus of the brain’s ventricles, where it cushions the central nervous system, circulates nutrients, and removes waste.

Serous fluids

Serous fluids in body cavities (pleural, pericardial, and peritoneal) are specialized plasma filtrates designed for protection and lubrication.

Plasma and RBCs balance the body’s acid/base chemistry

Blood helps regulate pH (normally 7.35–7.45) through a combination of buffer systems, respiratory control, and kidney function. The primary buffer is the bicarbonate system, where carbonic acid and bicarbonate ions neutralize excess acids or bases, while proteins such as hemoglobin also help by binding or releasing hydrogen ions. The majority of that bicarbonate buffering happens in red blood cells, plasma, and interstitial fluid. The respiratory system provides faster regulation by adjusting breathing rate: rapid breathing removes carbon dioxide to raise pH, while slower breathing retains carbon dioxide to lower pH. For long-term balance, the kidneys excrete hydrogen ions into urine and reabsorb bicarbonate into the blood. Together, these mechanisms allow the body to maintain a very narrow pH range, ensuring proper enzyme activity and overall metabolic stability.

Author note: TK add a note about how much MBLEx authors care

Blood helps regulate body temperature

Blood plays a key role in regulating the body’s temperature (“thermoregulation”). Warm blood carries metabolic heat from active tissues to the rest of the body, helping maintain an even internal temperature. When the body overheats, vasodilation of skin vessels increases blood flow near the surface, allowing heat to radiate outward and aiding cooling through sweat. In cold conditions, vasoconstriction reduces skin blood flow, conserving core heat. In the extremities, countercurrent exchange between arteries and veins helps stabilize temperature by transferring heat from warm arterial blood to cooler venous blood.