Blood is a connective tissue, and like all connective tissues, it is composed of cells suspended in an extracellular matrix.

The cellular components, known collectively as formed elements, include:

• red blood cells (RBCs or erythrocytes)

• white blood cells (WBCs or leukocytes)

• platelets (thrombocytes), which are cell fragments. 

The extracellular matrix, called plasma, is a fluid that suspends these formed elements, allowing blood to circulate continuously throughout the cardiovascular system. Plasma is primarily water but also contains proteins, nutrients, hormones, gases, and waste products.

Functions of Blood

Blood performs several critical functions beyond simply transporting oxygen and nutrients. These functions can be grouped into transportation, defense, and homeostasis.

• Transportation

Blood is the primary transport system of the body. It delivers oxygen from the lungs and nutrients from the digestive tract to body cells. Nutrients are often first processed in the liver before being distributed to tissues. Blood also carries hormones from endocrine glands to target organs and removes wastes, such as carbon dioxide (to the lungs) and metabolic byproducts (to the kidneys and liver) for excretion.

• Defense

White blood cells protect the body from external threats, such as bacteria and viruses, and internal threats, such as cancerous or virus-infected cells. Platelets and plasma proteins participate in hemostasis, forming clots to prevent excessive blood loss following vascular injury.

• Maintenance of Homeostasis

Blood contributes to temperature regulation by redistributing heat. For example, during exercise or exposure to heat, blood flow increases to the skin to dissipate heat; in cold environments, blood flow is restricted to conserve heat in the core. Blood also maintains chemical homeostasis, acting as a buffer to regulate pH and helping to balance water content in tissues and cells.

Characteristics of Blood

One of the most recognizable characteristics of blood is its color. 

• Blood that has recently picked up oxygen in the lungs is bright red

• Blood that has delivered oxygen to body tissues appears darker or dusky red. 

This difference in color occurs because hemoglobin, the oxygen-carrying pigment within red blood cells, changes color depending on its level of oxygen saturation.

Blood is also viscous, meaning it is thicker and more resistant to flow than water. In fact, blood has a viscosity approximately five times greater than that of water. This viscosity results from the presence of plasma proteins and formed elements suspended in the plasma. Viscosity plays a major role in determining blood pressure and blood flow. 

Just as honey flows more slowly than water due to its greater thickness, blood’s higher viscosity increases resistance within blood vessels and influences how easily it circulates through the body.

The temperature of blood is slightly higher than the average internal body temperature. While normal body temperature is approximately 37 °C (98.6 °F), blood temperature averages around 38 °C (100.4 °F), with normal daily variations of about 0.5 °C. As blood flows through vessels, it encounters friction and resistance, particularly as vessels age and lose elasticity. This friction generates heat, contributing to blood’s slightly elevated temperature.

Blood also maintains a relatively narrow pH range, averaging 7.4, with normal values between 7.35 and 7.45 in healthy individuals. This means blood is slightly alkaline compared to pure water, which has a pH of 7.0. The stability of blood pH is critical for normal cellular function and is maintained by various buffering systems present in the blood.

In terms of volume, blood makes up approximately 8 percent of an adult’s body weight. Adult males typically have 5–6 liters of blood, while adult females generally have 4–5 liters, reflecting differences in body size and composition.

Blood Plasma

Blood plasma is the liquid portion of blood. Like most fluids in the body, it is made mostly of water, about 92 percent. This water serves as a transport medium that allows substances to move easily throughout the body.

Dissolved or suspended in the plasma are many different substances, especially proteins. In fact, plasma contains hundreds of different molecules, including nutrients, hormones, waste products, electrolytes, and gases. While some of these substances are present in large amounts, many are found only in very small concentrations, yet they still play important roles in maintaining normal body function.

Blood Plasma

Blood plasma is the liquid portion of blood. Like most fluids in the body, it is made mostly of water, about 92 percent. This water serves as a transport medium that allows substances to move easily throughout the body.

Dissolved or suspended in the plasma are many different substances, especially proteins. In fact, plasma contains hundreds of different molecules, including nutrients, hormones, waste products, electrolytes, and gases. While some of these substances are present in large amounts, many are found only in very small concentrations, yet they still play important roles in maintaining normal body function.

Plasma Proteins

Proteins make up about 7 percent of blood plasma, accounting for nearly all of the plasma components that are not water. Most of these proteins are plasma-specific proteins, along with smaller amounts of regulatory proteins such as enzymes and some hormones. Together, these proteins play major roles in transport, immunity, blood clotting, and maintaining blood pressure.

There are three main groups of plasma proteins: albumins, globulins, and fibrinogen.

Albumin

Albumin is the most abundant plasma protein and is produced by the liver. Its primary role is to act as a transport protein, carrying substances such as fatty acids and steroid hormones through the bloodstream. Because lipids do not dissolve well in water, binding to albumin allows them to be transported in plasma.

Albumin also plays a critical role in maintaining osmotic pressure, which helps keep water inside the blood vessels. By drawing water into the bloodstream from surrounding tissues, albumin helps regulate blood volume and blood pressure. Albumin makes up about 54 percent of total plasma proteins, with normal blood levels ranging from 3.5 to 5.0 g/dL.

Globulins

Globulins are the second most abundant group of plasma proteins and include three subtypes: alpha, beta, and gamma globulins.

• Alpha and beta globulins help transport iron, lipids, and fat-soluble vitamins (A, D, E, and K) and also contribute to maintaining osmotic pressure.

• Gamma globulins, also called immunoglobulins or antibodies, are involved in the body’s immune response. Unlike most plasma proteins, immunoglobulins are produced by specialized white blood cells called plasma cells, rather than by the liver.

Globulins make up about 38 percent of plasma proteins, with typical blood levels of 1.0–1.5 g/dL.

Fibrinogen

Fibrinogen is the third major group of plasma proteins and is produced by the liver. It plays an essential role in blood clotting, helping to form fibrin threads that stop bleeding when blood vessels are damaged. Fibrinogen accounts for about 7 percent of total plasma proteins, with normal levels ranging from 0.2 to 0.45 g/dL.

Other Plasma Solutes

In addition to proteins, blood plasma contains many other dissolved substances that are essential for normal body function. These substances are present in smaller amounts but play important roles in maintaining homeostasis.

Plasma contains electrolytes, such as sodium, potassium, and calcium ions, which are critical for nerve signaling, muscle contraction, and fluid balance. It also carries dissolved gases, including oxygen, carbon dioxide, and nitrogen, which are involved in respiration and cellular metabolism.

Plasma also transports nutrients such as glucose, amino acids, lipids, and vitamins, which provide energy and materials for growth and repair. In addition, plasma carries metabolic waste products, which are transported to organs like the kidneys, lungs, and liver for elimination from the body.

Together, these nonprotein solutes make up only about 1 percent of plasma volume, but they are vital for sustaining life.

References

J Gordon Betts, Desaix, P., Johnson, E., Johnson, J. E., Korol, O., Kruse, D., Poe, B., Wise, J., Womble, M. D., & Young, K. A. (2013). Anatomy & physiology. Openstax College, Rice University. https://openstax.org/details/books/anatomy-and-physiology

Based on OpenStax, Anatomy and Physiology (2013), licensed under CC BY 4.0.

Access for free at https://openstax.org/books/anatomy-and-physiology/pages/1-introduction.

Content paraphrased; adaptations were made.