What Students Need to Know About Composition And Function Of Blood

Blood facts

  • Approximately 8% of an adult’s body weight is made up of blood.
  • Females have around 4-5 liters, while males have around 5-6 liters. This difference is mainly due to the differences in body size between men and women.
  • Its mean temperature is 38 degrees Celsius.
  • Red blood cells’ primary function is to transport oxygen between the lungs and tissues of the body.
  • White blood cells, the cells of the immune system, provide defense against pathogens.
  • Platelets are involved in clot formation during wound healing.
  • Blood is an extracellular matrix tissue in which various blood cells are suspended in the plasma matrix.
  • Blood is vital for normal metabolic function due to the transfer of oxygen, carbon dioxide, and glucose to and from the body’s tissues. It also transports a number of other cells and molecules throughout the body.
  • It has a pH of 7.35-7.45, making it slightly basic (less than 7 is considered acidic).
  • Whole blood is about 4.5-5.5 times as viscous as water, indicating that it is more resistant to flow than water. This viscosity is vital to the function of blood because if blood flows too easily or with too much resistance, it can strain the heart and lead to severe cardiovascular problems.
  • Blood in the arteries is a brighter red than blood in the veins because of the higher levels of oxygen found in the arteries.
  • An artificial substitute for human blood has not been found.

Blood, by definition, is a fluid that moves through the vessels of a circulatory system. In humans, it includes plasma (the liquid portion), blood cells (which come in both red and white varieties), and cell fragments called platelets.

  • Plasma is the main component of blood and consists mostly of water, with proteins, ions, nutrients, and wastes mixed in.
  • Red blood cells are responsible for carrying oxygen and carbon dioxide.
  • Platelets are responsible for blood clotting.
  • White blood cells are part of the immune system and function in immune response.

Cells and platelets make up about 45 percent of human blood, while plasma makes up the other 55 percent. The diagram below shows red blood cells, white blood cells of different types (large, purple cells), and platelets. Blood cells develop from hematopoietic stem cells and are formed in the bone marrow through the highly regulated process of hematopoiesis. Hematopoietic stem cells are capable of transforming into red blood cells, white blood cells, and platelets. These stem cells can be found circulating in the blood and bone marrow in people of all ages, as well as in the umbilical cords of newborn babies. Stem cells from all three sources may be used to treat a variety of diseases, including leukemia, lymphoma, bone marrow failure, and various immune disorders.

Formed Elements

About 45% of our blood is composed of what we refer to as formed elements. You likely call them blood cells. There are different types of blood cells found in our blood. Let’s look at those now.

The first type of blood cells are the erythrocytes, or as they are commonly called, red blood cells or RBCs. These are the most numerous of the three types of blood cells. RBCs have the job of transporting oxygen and carbon dioxide. There are approximately 4.2 to 6.2 million RBCs per cubic mm of blood at any given point in time.

The next type of blood cell is the leukocyte, also known as white blood cells or WBCs. These are members of our body’s defense team since they protect us from invading bacteria and other pathogens. There are anywhere from 5,000 to 9,000 per cubic mm of blood.

Not all leukocytes are the same. There are 5 different WBCs. The majority of our WBCs are neutrophils, making up around 65% of the WBCs. Lymphocytes make up 25% of the WBCs, and monocytes make up about 5%. Small amounts of eosinophils and basophils are also found in the blood, making up 4% and 1%, respectively.

The last of the 3 blood cell types are the thrombocytes, commonly referred to as platelets. These are also the tiniest of the blood cells. Platelets work in the body to help stop the bleeding whenever a blood vessel is damaged. We have 140,000 to 340,000 platelets per cubic mm of blood.

Plasma

The other 55% of our blood is composed of plasma. Plasma is the liquid portion of blood. Since this portion is liquid, you probably have figured out that the main component in plasma is water. Water makes up about 90% of the plasma. So, what is in the other 10% of the plasma?

Well, proteins make about 8% of plasma. There are 4 different types of proteins in the plasma. The most abundant of the plasma proteins at 57% are albumins. It is responsible for helping to maintain blood volume. Globulins are another plasma protein. They make up 38% of the proteins in the plasma, and they work with the WBCs. Fibrinogen at 4% and prothrombin at 1% are the final 2 plasma proteins, and they help in the clotting process.

The smallest portion of the blood plasma is made up of an assortment of different solids. These make up the remaining 3% of the plasma. Some solids you’ll find in the plasma are ions, or electrolytes, such as potassium, sodium, and calcium. Various nutrients needed by the body – like glucose, amino acids, and lipids – and waste products from metabolism – like urea, uric acid, and creatinine – are also found in the plasma. Oxygen and carbon dioxide are the blood gases which are found in the plasma as well. The last of the solids are hormones. There is a wide assortment of hormones released by various glands in the body that are transported in the plasma of the blood.

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Red Blood Cells (also called erythrocytes or RBCs)

Known for their bright red color, red cells are the most abundant cell in the blood, accounting for about 40 to 45 percent of its volume. The shape of a red blood cell is a biconcave disk with a flattened center – in other words, both faces of the disc have shallow bowl-like indentations (a red blood cell looks like a donut).

Production of red blood cells is controlled by erythropoietin, a hormone produced primarily by the kidneys. Red blood cells start as immature cells in the bone marrow and after approximately seven days of maturation are released into the bloodstream. Unlike many other cells, red blood cells have no nucleus and can easily change shape, helping them fit through the various blood vessels in your body. However, while the lack of a nucleus makes a red blood cell more flexible, it also limits the life of the cell as it travels through the smallest blood vessels, damaging the cell’s membranes and depleting its energy supplies. The red blood cell survives on average only 120 days.

Red cells contain a special protein called hemoglobin, which helps carry oxygen from the lungs to the rest of the body and then returns carbon dioxide from the body to the lungs so it can be exhaled. Blood appears red because of the large number of red blood cells, which get their color from the hemoglobin. The percentage of whole blood volume that is made up of red blood cells is called the hematocrit and is a common measure of red blood cell levels.

White Blood Cells (also called leukocytes)

White blood cells protect the body from infection. They are much fewer in number than red blood cells, accounting for about 1 percent of your blood.

The most common type of white blood cell is the neutrophil, which is the “immediate response” cell and accounts for 55 to 70 percent of the total white blood cell count. Each neutrophil lives less than a day, so your bone marrow must constantly make new neutrophils to maintain protection against infection. Transfusion of neutrophils is generally not effective since they do not remain in the body for very long.

The other major type of white blood cell is a lymphocyte. There are two main populations of these cells. T lymphocytes help regulate the function of other immune cells and directly attack various infected cells and tumors. B lymphocytes make antibodies, which are proteins that specifically target bacteria, viruses, and other foreign materials.

Different types of leucocytes can be found in the blood:

  • neutrophil granulocytes (banded and segmented)
  • eosinophil granulocytes
  • basophil granulocytes
  • lymphocytes
  • monocytes

Platelets (also called thrombocytes)

Unlike red and white blood cells, platelets are not actually cells but rather small fragments of cells. Platelets help the blood clotting process (or coagulation) by gathering at the site of an injury, sticking to the lining of the injured blood vessel, and forming a platform on which blood coagulation can occur. This results in the formation of a fibrin clot, which covers the wound and prevents blood from leaking out. Fibrin also forms the initial scaffolding upon which new tissue forms, thus promoting healing.

A higher than normal number of platelets can cause unnecessary clotting, which can lead to strokes and heart attacks; however, thanks to advances made in antiplatelet therapies, there are treatments available to help prevent these potentially fatal events. Conversely, lower than normal counts can lead to extensive bleeding.

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Oxygen and Glucose Transport

Blood’s primary function is to transport molecules around the body to support critical metabolic processes. All cells require oxygen and glucose to undergo cellular respiration. Tissues cannot survive very long without these two molecules. Disruption of this process is most dangerous to the brain, which can survive only about two minutes without oxygen and glucose. These terms are used to describe oxygen or blood deficiency to tissues in the body:

  • Hypoxia: a state in which the tissues do not receive adequate oxygen supply, generally due to decreased tissue perfusion or decreased oxygen intake.
  • Ischemia: a reversible condition in which a tissue does not receive adequate blood supply, usually from an obstructed or ruptured blood vessel.
  • Infarction: a usually irreversible condition in which tissues die as a result of prolonged oxygen or blood supply.

Most tissues can survive in a hypoxic or ischemic state for a few hours before infarction sets in. Heart infarction, which often occurs during a heart attack, will cause infarction in other tissues as blood is no longer pumped.

In addition to oxygen and glucose, the blood transports several other important molecules. Carbon dioxide, which travels through the blood mostly as bicarbonate, is transported from tissues as a waste product of cellular respiration to the lungs during gas exchange. Many hormones (chemical messengers) also travel through the blood as a form of communication between interrelated organs, which are often involved in homeostatic control.

Homeostatic Functions

Blood is involved in maintaining homeostasis through several ways. Temperature regulation occurs in part as a result of the dilation and constriction of vessels in the blood. Blood pH is a regulated variable of the respiratory system, because the pH of blood is directly proportional to the amount of carbon dioxide dissolved in blood. This makes blood pH an indicator of respiratory homeostasis. Blood glucose levels are regulated by insulin and glucagon secretion. Blood volume and blood pressure are directly proportional regulated variables that are tied to the activity of the heart and the fluid retention of the kidney. If any of these variables are too high or too low, severe problems can occur. For that reason, a number of complex negative feedback mechanisms exist to keep all variables within homeostatic range, despite influences from the internal and external environments.

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