Blood

The “other” connective tissue

Though it may seem counterintuitive, blood is by definition a connective tissue: it's made up of cells and cellular constituents blended into an extracellular matrix, or ground substance, called plasma. Because plasma is a liquid, blood is sometimes referred to as a "fluid connective tissue". The "formed" elements of blood — red blood cells (RBCs), white blood cells or (WBCs) and platelets (cell fragments) — all circulate freely throughout the circulatory system within this fluid medium.

Blood’s extracellular matrix

Blood plasma can be thought of as an aqueous protein solution consisting of 92 percent water and seven percent dissolved or suspended proteins. The other one percent is made up of hundreds of other substances, many found only in trace amounts.

Plasma proteins fall into three major groups, listed here in order of abundance:

• Albumin – A binding protein produced by the liver; when bound to lipids such as fatty acids or steroid hormones, allows transport of these hydrophobic biomolecules within the aqueous plasma environment; also, an osmotic regulator in the blood, helping maintain both blood pressure and blood volume
• Globulins – Subtyped into alpha, beta and gamma globulins
  • Alpha and beta globulins are produced in the liver and provide cells with transported iron, lipids and fat-soluble vitamins (A, D, E and K)
  • Gamma globulins — also called antibodies or immunoglobulins — are produced by specialized leukocytes known as plasma cells and are involved in the body's immune response
• Fibrinogen – A plasma protein produced by the liver and an essential material needed for blood clotting

Aside from the most prevalent water and protein components, many other substances are found in blood plasma, including:

• Electrolytes (e.g., sodium, calcium, potassium and others)
• Dissolved gases (e.g., oxygen, nitrogen, carbon dioxide)
• Organic nutrients (e.g., vitamins, lipids, glucose, amino acids and waste substances)

Multiple roles

Blood is the vehicle for delivering oxygen and nutrients to tissues throughout the body, as well as a means of removing metabolic waste products. Along with this primary function, however, blood plays other essential roles.

• Maintaining homeostasis – Blood contains proteins and other compounds that act as buffers, helping keep consistent the body's pH and chemical balance; water content within body cells is in part regulated by albumin and other proteins carried by blood
• Heat distribution – Blood is the means for the body’s ongoing homeostatic temperature regulation: when the body is overheated, blood flow increases from the core to the extremities, dissipating heat and returning to cool the core; in a cold environment, blood flows from the extremities inward, conserving heat at the body’s core
• Defense – Part of a complex immunological response, white blood cells (or leukocytes) in the blood provide protection against threats both from the outside (such as pathogenic bacteria introduced by injury) and the inside (such as cancerous body cells, or body cells already infected with viral particles); after injury to blood vessels, blood-borne platelets and proteins interact to seal damaged areas and minimize blood loss