The average human adult has about 8-10 pints of blood in the body, which is 8% of total body weight. A pint of blood is equivalent to 473.18 mL.
Blood is connective tissue fluid that flows through arteries and veins, providing body tissues with oxygen and nutrition necessary for their survival. It filters waste products and eliminates them from the body.
Blood is mostly liquid and thicker than pure water, containing cells, proteins, nutrients, minerals suspended in a fluid called plasma. Clotting of blood inside the blood vessels is prevented by their smooth inner surface and finely tuned balance of clotting factors.
What is the composition of blood?
Blood plasma is an intravascular liquid component of blood obtained after removal of blood cells. Plasma holds proteins and other components of the blood in suspension. It makes up about 55% of the body's total blood volume.
Plasma is 95% water and contains important dissolved proteins, such as serum albumins, globulins, and fibrinogen), glucose, clotting factors, electrolytes (Na+, Ca2+, Mg2+, HCO3-, Cl-, etc.), hormones, carbon dioxide, and oxygen (which makes up about 6%-8%). It has an intravascular osmotic effect that maintains the balance of electrolytes and protects the body from infection and other blood disorders.
About 50% of blood is composed of three major groups of blood cells: red blood cells, white blood cells, and platelets. Most blood cells are produced by the single type of unspecialized cell present inside the soft fatty tissue of the bone cavity.
- Red blood cells contain hemoglobin, which delivers oxygen to tissues and helps them continue functioning properly.
- White blood cells are part of the immune system and are of five different types: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. These help protect against infections and have a role in inflammation and allergic reactions.
- Platelets are smaller cells that play an important role in blood clotting and seals of micro punctures that occur in the body.
The rate of blood cell production is controlled by demand. The typical lifespan of blood cells ranges from a few hours to days for white blood cells, about 10 days for platelets, and about 120 days for red blood cells.
Plasma proteins serve various important functions, including the transport of lipids, hormones, vitamins, and minerals throughout. Some proteins act as enzymes, complement components, protease inhibitors, or kinin precursors. The three main plasma proteins include albumin, globulin, and fibrinogen.
- Serum albumin makes up about 55% of blood proteins. It helps maintain the oncotic pressure of plasma (prevents body swelling and keeps fluids in the vascular compartment) and acts as a carrier molecule for the transport of lipids and steroid hormones.
- Globulins make up about 38% of blood proteins. They help transport ions, hormones, and lipids and play an important part in the body's defense system.
- Fibrinogen makes about 7$ of blood proteins, and its conversion to insoluble fibrin is essential for blood clotting.
Remaining plasma proteins (1%) are regulatory proteins, such as enzymes, proenzymes, and hormones. Blood proteins are mostly synthesized in the liver.
Nutrients and hormones
Blood plays a role in the functions of the endocrine system. Nutrients including glucose, amino acids, vitamins, minerals, and fatty acids are absorbed in the bloodstream through capillaries in intestinal villi. These nutrients are transported along with blood to various organs of the body. Blood also acts as a transport medium for hormones secreted by the endocrine system’s glands and helps them reach their target site so that they can carry out their actions.
How much blood can be safely donated at a time?
About one pint (or a little more) of blood is collected during a session of blood donation. For most individuals, the donation process is safe and usually painless. The body replenishes the lost blood within the next few weeks.
Healthy people can safely donate blood once every 12 weeks.
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Medline Plus. Blood. https://medlineplus.gov/blood.html
Schwartz RS. Blood. Encyclopædia Britannica. https://www.britannica.com/science/blood-biochemistry
American Society of Hematology. Blood Basics. https://www.hematology.org/education/patients/blood-basics