Blood is a circulating tissue composed of fluid plasma and cells (red blood cells, white blood cells, platelets). Medical terms related to blood often begin in hemo- or hemato- (BE: haemo- and haemato-) from the Greek word "haima" for "blood". Therefore, hemophobia is characterized by the abnormal fear of blood.
The main function of blood is to supply nutrients (oxygen, glucose) and constitutional elements to tissues and to remove waste products (such as carbon dioxide and lactic acid). Blood also enables cells (leukocytes, abnormal tumor cells) and different substances (amino acids, lipids, hormones) to be transported between tissues and organs. Problems with blood composition or circulation can lead to downstream tissue dysfunction.
The blood is circulated around the lungs and body by the pumping action of the heart.
Anatomy of blood
Blood is composed of several kinds of corpuscles; these formed elements of the blood constitute about 40% of whole blood. The other 60% is blood plasma, a fluid that is the blood's liquid medium, apearing yellow in color. The normal pH of human arterial blood is approximately 7.40 (normal range is 7.35-7.45). Blood that has a pH below 7.35 is acidic, while blood pH above 7.45 is alkaline. Blood pH along with arterial carbon dioxide tension (PaCO2) and HCO3 readings are helpful in determining the acid-base balance of the body. Blood is about 7% of the human body weight , so the average adult has a blood volume of about 5 liters, of which 2.7-3 liters is plasma. The combined surface area of all the erythrocytes in the human anatomy would be roughly 2,000 times as great as the body's exterior surface.
The corpuscles are:
Red blood cells or erythrocytes (96%). In mammals, mature red blood cells lack a nucleus and organelles. They contain the blood's hemoglobin and distribute oxygen.
The red blood cells (together with endothelial vessel cells and some other cells) are also marked by proteins that define different blood types.
White blood cells or leukocytes (3.0%), are part of the immune system; they destroy infectious agents.
Platelets or thrombocytes (1.0%) are responsible for blood clotting (coagulation)
Blood plasma is essentially an aqueous solution containing 96% water, 4% blood plasma proteins, and trace amounts of other materials. Some components are:
blood clotting factors
various other proteins
various electrolytes (mainly sodium and chlorine)
Together, plasma and corpuscles form a non-Newtonian fluid whose flow properties are uniquely adapted to the architecture of the blood vessels.
Physiology of blood
Production and degradation
Blood cells are produced in the bone marrow; the process is termed hematopoiesis. The proteinaceous component is produced overwhelmingly in the liver, while hormones are produced by the endocrine glands and the watery fraction maintained by the gut and the kidney.
Blood cells are degraded by the spleen and the Kupffer cells in the liver. The liver also clears proteins and amino acids. The kidney secretes many small proteins into the urine. Erythrocytes usually live up to 120 days before they are systematically replaced by new erythrocytes created by the process of hematopoiesis.
Transport of oxygen
Blood oxygenation is measured in several ways, but the most important measure is the hemoglobin saturation percentage. This is a non-linear (sigmoidal) function of the partial pressure of oxygen. About 98.5% of the oxygen in a sample of arterial blood in a healthy human breathing air at normal pressure is chemically combined with the Hb. Only 1.5% is physically dissolved in the other blood liquids and not connected to Hb. The hemoglobin molecule is the primary transporter of oxygen in mammals and many other species
With the exception of pulmonary and umbilical arteries and their corresponding veins, arteries carry oxygenated blood away from the heart and deliver it to the body via arterioles and capillaries, where the oxygen is consumed; afterwards, venules and veins carry deoxygenated blood back to the heart.
Differences in infrared absorption between oxygenated and deoxygenated blood form the basis for realtime oxygen saturation measurement in hospitals and ambulances.
Under normal conditions in humans, hemoglobin in blood leaving the lungs is about 96-97% saturated with oxygen; 'deoxygenated' blood returning to the lungs is still approximately 75% saturated. A fetus, receiving oxygen via the placenta, is exposed to much lower oxygen pressures (about 20% of the level found in an adult's lungs) and so fetuses produce another form of hemoglobin with a much higher affinity for oxygen (hemoglobin F) in order to extract as much oxygen as possible from this sparse supply.
In insects, the blood (more properly called hemolymph) is not involved in the transport of oxygen. (Openings called tracheae allow oxygen from the air to diffuse directly to the tissues). Insect blood moves nutrients to the tissues and removes waste products.
In some small invertebrates like insects, oxygen is simply dissolved in the plasma. Larger animals use respiratory proteins to increase the oxygen carrying capacity. Hemoglobin is the most common respiratory protein found in nature. Hemocyanin (blue) contains copper and is found in crustaceans and mollusks. It is thought that tunicates (sea squirts) might use vanabins (proteins containing vanadium) for respiratory pigment (bright green, blue, or orange).
In many invertebrates, these oxygen-carrying proteins are freely soluble in the blood; in vertebrates they are contained in specialized red blood cells, allowing for a higher concentration of respiratory pigments without increasing viscosity or damaging blood filtering organs like the kidneys.
Transport of carbon dioxide
When systemic arterial blood flows through capillaries, carbon dioxide diffuses from the tissues into the blood. Some carbon dioxide is dissolved in the blood. Some carbon dioxide reacts with hemoglobin to form carbamino hemoglobin. The remaining carbon dioxide is converted to bicarbonate and hydrogen ions. Most carbon dioxide is transported through the blood in the form of bicarbonate ions.nileesh
Transport of hydrogen ions
Some oxyhemoglobin loses oxygen and becomes deoxyhemoglobin. Deoxyhemoglobin has a much greater affinity for H+ than does oxyhemoglobin so it binds most of the hydrogen ions.
In humans and other hemoglobin-using creatures, oxygenated blood is a bright red in its color. Deoxygenated blood is a darker shade of red, which can be seen during blood donation and when venous blood samples are taken. However, due to an optical effect caused by the way in which light penetrates through the skin, veins typically appear blue in color. This has led to a common misconception that before venous blood is exposed to air it is blue.
Health and disease
Hippocratic medicine considered blood one of the four humors (together with phlegm, yellow bile and black bile). As many diseases were thought to be due to an excess of blood, bloodletting and leeching were a common intervention until the 19th century (it is still used for some rare blood disorders).
In classical Greek medicine, blood was associated with air, springtime, and with a merry and gluttonous (sanguine) personality. It was also believed to be produced exclusively by the liver.
Blood pressure and blood tests are amongst the most commonly performed diagnostic investigations that directly concern the blood.
Blood transfusion is the most direct therapeutic use of blood. It is obtained from human donors by blood donation. As there are different blood types, and transfusion of the incorrect blood may cause severe complications, crossmatching is done to ascertain the correct type is transfused.
Other blood products administered intravenously are platelets, blood plasma, cryoprecipitate and specific coagulation factor concentrates.
Many forms of medication (from antibiotics to chemotherapy) are administered intravenously, as they are not readily or adequately absorbed by the digestive tract.
As stated above, some diseases are still treated by removing blood from the circulation.
It is the fluid part of the blood that saves lives where severe blood loss occurs, other preparations can be given such as ringers atopical plasma volume expander as a non-blood alternative, and these alternatives where used are rivalling blood use where used.
Mythology and religion
Due to its importance to life, blood is associated with a large number of beliefs. One of the most basic is the use of blood as a symbol for family relationships; to be "related by blood" is to be related by ancestry or descendance, rather than marriage. This bears closely to bloodlines, and sayings such as "blood is thicker than water" and "bad blood", as well as "Blood brother".
Among the Germanic tribes (such as the Anglo-Saxons and the Vikings), blood was used during the sacrifices, the Blóts. The blood was considered to have the power of its originator and after the butchering the blood was sprinkled on the walls, on the statues of the gods and on the participants themselves. This act of sprinkling blood was called bleodsian in Old English and the terminology was borrowed by the Roman Catholic Church becoming to bless and blessing. The Hittite word for blood, ishar was a cognate to words for "oath" and "bond"
In Judaism, blood cannot be consumed even in the smallest quantity (Leviticus 3:17 and elsewhere); this is reflected in the dietary laws. Blood is purged from meat by salting and pickling.
Other rituals involving blood are the covering of the blood of fowl and game after slaughtering (Leviticus 17:13); the reason given by the Torah is: "Because the life of every animal is [in] his blood" (ibid 17:14), although from its context in Leviticus 3:17 it would appear that blood cannot be consumed because it is to be used in the sacrificial service (known as the korbanot), in the Temple in Jerusalem.
Ironically, Judaism has historically been the religion to be most affected by blood libels.
Christians believe that the Eucharist wine is, or represents, the blood of Jesus. This belief is rooted in the Last Supper as written in the four gospels of the Bible, in which Jesus stated to his disciples that the bread which they ate represented his body, and the wine represented his blood. "This cup is the new testament in my blood, which is shed for you." (Luke 22:20). The accepted Christian belief is that Jesus' blood atoned for the sins of the people.
Jehovah's Witnesses are prohibited from eating blood and accepting tranfusions of whole blood or any of red cells, white cells, platelets or plasma. They are permitted to accept fractions, and the acute normovolemic hemodilution (ANH) and autologous blood salvage (cell saver) procedures.
Vampires are fictional beings thought to cheat death by drinking the blood of the living.
Chinese and Japanese culture
In Chinese culture, it is often said that if a man's nose produces a small flow of blood, this signifies that he is experiencing sexual desire. This often appears in Chinese-language and Hong Kong films. This is also evident in Japanese culture and is parodied in anime and manga. Male characters will often be shown with a nosebleed if they have just seen a female nude or in little clothing, or if they have had an erotic thought or fantasy.
Blood donation is a process by which a blood donor voluntarily has blood drawn for storage in a blood bank or for subsequent use in a blood transfusion.
To understand the importance of the availability of a sustainable donated blood supply, the catch-cry of the Australian Red Cross Blood Service is, "80% of Australians will need blood in their lifetime, but less than 3% of Australians give blood each year." In the United States, the odds of knowing someone who will need blood are "97%", according to that branch of the Red Cross. According to polling conducted by [Canadian Blood Services], 52% of Canadians have needed blood or know someone who has.
Blood donations may be scheduled at local centres, or at times a "blood drive" will occur. These are events where a blood bank or other blood collecting organization will set up in a convenient location—such as a shopping centre, large employer, university, or a local church—for people to stop in without appointment during their daily routine to donate blood.
The process of giving blood involves screening the donor, the actual donation, and a brief recovery period.
This applies to both whole blood donations and plasmapheresis (donating only one's plasma)
Before arriving at the donation site
In the days prior to donating blood, it is important that donors prepare themselves for a process that can temporarily weaken the body. Perhaps the most important thing is to eat and drink well. By drinking plenty of fluids the day before and of the donation, the donor can help prevent hypovolemia. Eating well is also important, partly because it can also help with hydration, but mostly because high nutrient levels give the body the raw materials it needs to respond quickly to the loss of blood. Eating regular, well-balanced meals in the days before the donation is important, but it is especially critical to have a good, low-fat meal before the donation. Eating foods high in iron (which has many dietary sources) is also a good idea for those giving whole blood or red blood cells. Finally, activities that promote general physical fitness (such as exercise, sex, and sleep) are all especially wise in the several days before a donation. However, donors should check their local blood bank's guidelines; some countries require donors to fast before donation to avoid interfering with screening.
Western countries typically impose screening for blood donor candidates. In the past, it was the practice in America and other countries to separate blood donations on the basis of race, ethnicity, or religion, or to exclude certain groups from the donor pool on those bases. Currently, in the United States, these practices have been eliminated, and donor attributes are considered only in terms of their likelihood to affect the probability of transmission of disease. All blood products in the U.S. are labeled as coming from either "paid" or "volunteer" donors, with paid units being more likely to transmit infection. (Several other countries avoid paid donations altogether for this reason.)
Other donor characteristics are also taken into account: starting in 1985, the American Red Cross and Food and Drug Administration policies prohibit accepting blood donations from gay/bisexual men, specifically from any "male who has had sex with another male since 1977, even once,"  or from IV drug users or recent immigrants from certain nations with high rates of HIV infection. While the inclusion of homosexual/bisexual men on the prohibited list has created some controversy, the FDA & Red Cross cite the public policy need to protect the blood supply from HIV & similar diseases as justification for the ban. Policies vary in other countries; for instance, Australia formerly had a similar ban, but now only prohibits donating blood within one year after male-male sex (longer than the typical window period for HIV tests performed on donated blood).
Similarly important donor eligibility requirement in the US is related to concerns about variant Creutzfeld-Jacob Disease (vCJD): persons who have spent long periods of time in countries where "mad cow disease" is found , are not eligible to donate. As part of the screening interview, blood donors are questioned about past residency in countries on the exclusion list, tracing back as far as 1980. The list of countries of residence that may disqualify a potential donor, includes most of Western Europe (with stronger restrictions on those with past residence in the United Kingdom), Turkey, and all of Eastern Europe except Russia, Ukraine, Belarus, and the Baltic countries (those formerly part of the Soviet Union). From 1980 through 1996, if a person visited or resided in country on the exclusion list for a time that adds up to five years or more, such person is not eligible to donate; if a person spent time that adds up to three months or more in any country within the United Kingdom, they are indefinitely deferred.
When a donor arrives at a donation site, they typically fill out a consent form as well as answer an extensive questionnaire to help determine their eligibility. Questions include the donors' age, weight, most recent donation, current health, and various risk factors such as tattooing, drug use (recreational or performance enhancing), residency abroad, recent international travel, and sexual history. Answers are associated with the donor's blood, but not name, to provide anonymity.
Often the blood hemoglobin concentration will be checked next, typically performed by a phlebotomist. While various tests exist, the most common ones are:
hematocrit: done in some places. requires a centrifuge. A measurement of no less than 38% allows donation to continue.
colorimetric hemoglobin test using a hemoglobin photometer: a machine-read result from a chemical reaction on a testing strip.
copper sulfate screening test (aka "float test"): measures the specific gravity of the donor's blood by placing a drop into a copper sulfate solution. The solution is calibrated so that a hemoglobin concentration of in >12.5 g/dl (the cut-off in the U.S. for donation) sinks.
Additionally, the donor may indicate to not use their blood but still go through the blood drawing process, again to protect the individual's medical privacy. All blood is later tested for diseases including STDs. If a disease is found, the donor will be notified and their blood discarded. It is discouraged for individuals to use blood donation for the purpose of anonymous STD screening.
The blood donation itself happens next. The donor lies supine on a cushioned bench and extends an arm. The inner elbow is disinfected, and a cannula is inserted into a vein. The donor often has a tourniquet wrapped around his or her arm, or may be prompted to squeeze a ball repeatedly, to help speed the process. Blood flows from the vein, through the needle and a tube, and into a special collection bag which is placed on a small scale to measure the amount of blood withdrawn. Typically, around 500 millilitres, about a pint, is drawn during the blood donation process.
Rather than donating whole blood, a donor sometimes has the option to donate only some blood components while retaining others. This process is known as apheresis, and is more involved, time consuming, and requires more specialized equipment. The benefit is that more of the desired components can be concentrated and removed, and the donor is usually able to donate significantly more frequently than if whole blood had been removed. In some cases, the usefulness of the removed components is not as sensitive to blood type considerations.
The typical method of apheresis is to draw whole blood from the donor, then centrifuge the blood to separate its components (see apheresis for more information). The desired components (e.g. platelets, plasma) are removed and then the remaining components are returned to the donor.
Once the donation is complete, the donor is given a bandage or gauze to stop further blood flow and is normally allowed to leave. Beverages and snacks may be provided to restore blood volume and blood sugar, and to settle the stomach. In some countries, stickers are worn by donors to show the donor's pride and to encourage others to give. The entire process, from screening through recovery, takes about one hour. The actual blood donation takes between four and fifteen minutes. Donors are discouraged from heavy exercise or lifting until the next day. Plasma volumes will return to normal in around 24 hours, while red blood cells are replaced by bone marrow into the circulatory system within about 3-5 weeks, and lost iron replaced over 6-8 weeks. This recovery process can be speeded by eating foods high in complex carbohydrates, iron, and other trace minerals. Due to the time frame required for iron replacement, donors are eligible to donate whole blood approximately eight to twelve weeks after the previous donation (the exact period varying by country); in the USA, the waiting period for the two red cell units apheresis donors is 112 days.
Cryopreservation of red blood cells is done to store special, rare red blood cells for up to 10 years. The cells are first incubated in a 40% glycerol solution which acts as a cryoprotectant ("antifreeze") within the cells. The units are then placed in special sterile containers in a deep freezer at less than -60°C.
Donating blood may reduce risk for heart disease, and stimulates the generation of red blood cells. In patients prone to iron overload (e.g. due to hemochromatosis), blood donation prevents the accumulation of iron.
Anecdotally, elderly people in good health have reported feeling invigorated by giving blood on a regular basis.
The psychological benifits of donating blood is the sense the donor has that he/she has made a contribution and may have helped save a person's life.
A person who anticipates the need for a blood transfusion at a later date (usually because of scheduled surgery) may make an autologous donation, in which their blood is stored and later transfused back into its original donor. Besides ensuring the availability of compatible blood (especially important for patients with rare blood types), this procedure also eliminates the risk of disease transmission from infected donors.