Blood is categorized as connective tissue. It is in fact highly specialized and is created by formed elements. These formed elements include erythrocytes, leukocytes, and platelets which are also known as thrombocytes. These elements are carried in the plasma, the liquid form of blood. These elements are responsible for the duties of transport, immunity, and clotting.

Blood comprises approximately 8% of an average adult’s body weight and the average adult body carries approximately 5 liters of blood within their system at all times. Arterial blood is blood which is departing from the heart. Except for the arterial blood that is entering the lungs, arterial blood tends to be bright red. This coloration signifies the high amounts of oxygen and hemoglobin, also known as oxyhemoglobin, that is within the erythrocytes.


Blood that is on its way back to the heart is called venous blood. Venous blood is devoid of the oxygen rich cells and this is reflected in the darker red color. This does not apply to venous blood from the lungs, which is well oxygenated.

Water has a viscosity of 1.0. Blood is thicker than water and has a viscosity of 4.5 to 5.5. The Ph level of blood ranges anywhere between 7.35 and 7.45. Blood temperatures within the core of the body, the thorax, is about 100.4 degrees Fahrenheit, or 38 degrees Celsius.


Image: Blood

A “unit” of blood, such as that which is donated or that which is received by patients in emergency situations, measures ½ of a liter which is 1/10 the total volume of blood in the human body.

Blood is segregated into two basic divisions, the cellular components and the plasma, which is the fluid division of blood. When blood is taken from the human body medically speaking, the blood is then centrifuged, which by default leaves the cellular components on the bottom and the plasma on the top. This makes it possible to see the different elements of blood with the naked eye. 45% of the total blood volume of the human body is made up the cellular elements. This portion is known as the hematocrit. The other 55% is plasma. Hemotacrit can give an approximation of the blood’s ability to carry oxygenated cells throughout the body based on any base volume of blood.


The elements of blood, the red blood cells, the white blood cells, and the platelets are each respectively medically termed the erythrocytes, the leukocytes, and the thrombocytes. Of these the red blood cells far out number the other formed elements. The red blood cells are more plentiful in males than in females. A cubic millimeter of blood contains anywhere from 4.3 million to 5.2 million in the female body and 5.1 million to 5.8 million in the body of an adult male. A cubic milliliter of blood only contains between 5,000 and 10,000 white blood cells and 250,000 to 450,000 platelets.


Red blood cells, or erythrocytes, have the appearance of biconcave disks. They take on the shape of a flattened circle with a depression in the middle. The shape of a red blood cell, although unique, is designed for the diffusion of gas which provides a viable surface for transporting oxygen to the various areas of the body. Red blood cells which have reached maturity do not have either a nucleus or mitochondria. Instead, anaerobic aspiration is their source of energy. A red blood cell lives for approximately 120 days, and after that time frame they are destroyed by cells in the liver and the spleen known as phagocytes.


Image: Erythrocytes


Leukocytes are white blood cells. White blood cells are best known for their ability to help fight off infection and illness. When the body creates additional white blood cells, it runs a fever.

Anatomically speaking, white blood cells vary physically from their red blood cell brothers. White blood cells are larger and have both a nucleus and mitochondria. While red blood cells can not move independently, white blood cells propel themselves in an amebic fashion. White blood cells are able to seek out an infection site outside of the blood stream thanks to their ability to move independently. They are able to make their way through the pores within the walls of the capillaries in order to fight infections that are not within the blood stream. This action of movement through the walls of the capillaries is known as diapedesis.

When medical or research professionals view the white blood cells under the microscope, the only way these cells become visible is through the process of staining. White blood cells are placed in the appropriate category based on the appearance they have when stained. There are some white blood cells that have granules within their cytoplasm. These are known as granular leukocytes. Agranular leukocytes are the white blood cells with granules so tiny that they can barely be seen under a high powered microscope, or not seen at all. Granular leukocytes are oddly shaped and can often be recognized in this fashion. Their shape comes from the offset condition of the nuclei which are distorted into lobes attached by slight strands. Thus granular leukocytes are additionally classified as polymorphonuclear leukocytes.


Image: Leukocytes

Leukocytes of either type are identified through the staining process. The stains used in this process are either pink to red or blue to purple. The pink to red stain is known as eosin and hemotoxylin (blue to purple) is known as the basic stain. The granule of the cell is stained and is either reflected via a pink or a purple hue. The pink granules are eosinophils and the purple granules are the basophils. Some cells do not seem to respond to either stain and are therefore known as neutriphils. The adult human body contains anywhere from 54% to 62% neutrophils, making it the most abundant white blood cell in the system.

Agranular white blood cells are classified as either monocytes or lymphocytes. Monocytes and lymphocytes are relatively easy to distinguish from each other. The monocyte can be identified via the large nuclei with a variable shape. This is also the largest cell found in the blood stream. The lymphocytes can be branded via the thin layer of cytoplasm that encompasses their nuclei, which is also relatively large.


Each individual element of the blood is considered a formed element. Of these, the platelets are the smallest. They are produced by the remains of the larger cells known as megakaryocytes. The megakaryocytes are found in the red bone marrow. This is in fact the reason why blood cells are considered formed elements instead of cells. The platelets lack nuclei, and when they enter the blood stream they are similar to the leukocytes in their ability to produce their own movement. In one cubic millimeter of blood there are approximately 250,000 to 450,000 platelets and they survive in the blood stream for anywhere from 5 to 9 days. After that they are destroyed in the filtering process of the either the spleen or the liver.


Image: Platelets

Platelets are vital to the blood clotting process. For a clot to form, the platelets make up nearly the entire blood clot. The platelet plug is reinforced by the threads of fibrin which is activated by the phospholipids in the cell. These phospholipids activate the plasma’s blood clotting factors, creating what is commonly seen on the outside of the body as a scab. The action of the platelets, which can be viewed as the string of platelets joining together to form the plug, release serotonin into the blood stream. The serotonin encourages the body to restrict the flow of blood through the blood vessels. This constraint of the blood vessels reduces blood flow at the injury site, allowing the platelets to clot, form the plug, and ceases the flow of blood to outside the site of injury.


The process of forming new blood cells is called hemopoiesis. Erythrocytes are formed through the process known as erythropoiesis and leukocytes are formed through the process known as leukopoiesis. Two various distinctions of tissue structure is responsible for these two processes. The red bone marrow is located in some bones of the skull, the humeri, ribs, femora, pelvis, and sternum. This bone marrow contains myeloid tissue. The myeloid tissue creates platelets, erythrocytes, and granular leukocytes. Agranular leukocytes are created from the tissue in the spleen, lymph nodes, tonsils, and thymus. This is known as lymphoid tissue. In a developing fetus, the homepoietic centers are found in the yolk sac, spleen, and liver. Once a fetus becomes an independent human being, the spleen and the liver are no longer sites for cell creation and become the sites for blood cell destruction.

The liver and the spleen destroy the blood cells which have been in the system for their life span and need to be replaced by new blood cells. This process, the erythropoiesis, is highly active. It has been determined that the body must create 2.5 million erythrocytes every second to replace those which are being destroyed. Erythrocytes only live in the blood stream for approximately 120 days. When erythrocytes are destroyed by the liver and the spleen, the common mineral iron is taken from the destroyed formed element and returned to the red bone marrow for recycling in the body’s production of more erythrocytes.


Image: Hemopoiesis

Agranular leukocytes are effectively useful in the human body for anywhere from 100 to 300 days. Granular leukocytes are only functional in the body for 12 hours to 3 days maximum. Regardless of whether the body is producing cells within the myeloid tissues and lymphoid tissues the process of development is identical. When the body produces the undifferentiated cells known as mesenchymal cells develop into the stem cells which are then known as the hemocytoblasts. These stems cells are capable of rapid division. Some stem cells known as daughter cells are then left behind to become new stem cells. This ensures that the stem cells never become exhausted. The daughter cells which are not left behind then are determined to perform other tasks of cell creation. In some cases the hemocytoblasts become proerythroblasts which will in time develop into erythrocytes. Myeloblasts then form granular leukocytes. Lymphocytes are created by lymphoblasts, monocytes are created by monoblasts, and platelets are developed from megakaryoblasts.


The fluid matrix of the blood is called blood plasma. When the formed elements of the blood are removed, the blood plasma can be clearly seen under a high powered microscope. Approximately 55% of any given volume of blood is made up of the blood plasma, and 90% of the blood plasma is made up of water. Visually, it appears to be the color of straw, a tan yellow. The blood plasma which is not created by water is made up of hormones, vitamins, amino acids, lipids, proteins, inorganic salts, and carbohydrates.

Blood plasma is responsible for the transportation of nutrients in the blood stream, as well as the transportation of gases and vitamins. Blood plasma also regulates electrolyte and fluid balances. The blood Ph should remain between 7.35 and 7.45 in a healthy human body. Between 7% and 9% of the blood plasma is created by plasma proteins. These proteins are within the blood and remain in the blood as well as the interstitial fluids in order to contribute to homeostasis. These proteins are divided into three basic types which include the albumins, globulins, and the fibrinogen. The smallest of these would be the albumins which create just over 60% of the blood plasma proteins. These albumins are vital to maintaining blood pressure via their contribution to blood viscosity as well as providing the blood cells with molecules which can carry and transport. These vital albumins are created by the liver.


Blood Plasma
Image: Blood Plasma

36% of the blood plasma proteins are created by the globulin. The human body contains three basic divisions of globulins. The alpha and beta globulins are responsible for the transportation of lipids as well as transporting fat soluble vitamins. The liver creates these globulins. Lymphoid tissue creates the gamma globulins. These globulins are the basic antibodies required for immunity. Without the gamma globulins, an individual human body would suffer from about 90% more general and deadly illnesses, including those the human body can be immunized against.

About 4% of the blood plasma’s proteins are created by fibrinogen. The removal of this protein from the blood creates serum and these are created by the liver. Their basic responsibility to unite with the platelets in creating blood clotting factors within the blood stream.

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