SafekipediaBlood is a special kind of body fluid found in the circulatory system of humans and other animals with backbones. It carries important things like nutrients and oxygen to every cell in the body, and it also takes away waste products that the cells don’t need anymore. This helps keep the body healthy and working properly.
Blood is made up of tiny cells floating in a liquid called plasma. Plasma is mostly water and also contains proteins, glucose, minerals, and hormones. The three main types of cells in blood are red blood cells, which carry oxygen using a special material called hemoglobin; white blood cells, which help fight infections; and platelets, which help blood to clot when there is a cut or injury.
The heart pumps blood through a network of blood vessels to reach all parts of the body. In animals with lungs, blood that has picked up oxygen from the lungs is bright red, while blood that has dropped off oxygen and picked up a waste product called carbon dioxide is darker red. Blood plays a very important role in keeping every part of the body alive and healthy.
Functions
Blood has many important jobs in your body. It carries oxygen from your lungs to all your tissues using a special protein called hemoglobin in red blood cells. It also brings nutrients like glucose, amino acids, and fatty acids to your cells so they can grow and stay healthy.
Blood helps remove waste products such as carbon dioxide, urea, and lactic acid. It plays a key role in your immune system by moving white blood cells and antibodies around your body. When you get a cut, blood can clot to stop the bleeding. It also carries hormones and helps regulate your body temperature.
Constituents
See also: Reference ranges for common blood tests
Blood makes up about 7% of a person's body weight and is a bit heavier than water. An adult usually has around 5 liters of blood, which is made up of liquid called plasma and tiny cells floating inside it. The main cells are red blood cells that carry oxygen, white blood cells that help fight germs, and small pieces called platelets that help blood clot.
Most of blood—about 55%—is plasma, a watery liquid that carries important things like sugar and oxygen to the body’s cells and takes away waste. Plasma is mostly water and also contains proteins and tiny particles that help with clotting and fighting infections. It keeps the blood at just the right balance so that it can work properly.
| Parameter | Value |
|---|---|
| Hematocrit | 45 ± 7 (38–52%) for males 42 ± 5 (37–47%) for females |
| pH | 7.35–7.45 |
| base excess | −3 to +3 |
| PO2 | 10–13 kPa (80–100 mm Hg) |
| PCO2 | 4.8–5.8 kPa (35–45 mm Hg) |
| HCO3− | 21–27 mM |
| Oxygen saturation | Oxygenated: 98–99% Deoxygenated: 75% |
Physiology
Main article: Circulatory system
Main article: Lymphatic system
Blood moves around the body through a network of blood vessels powered by the heart. It carries oxygen from the lungs to all the body’s cells and brings back waste products for removal. The heart pumps blood through arteries to tissues and then through veins back to the heart.
Blood cells are made in the bone marrow, with the liver producing many of its proteins. Oxygen is mainly carried by a substance called hemoglobin in red blood cells, which helps keep our body’s tissues healthy. Blood also helps control body temperature by moving heat to where it’s needed.
Color
Main article: Hemoglobin
Blood gets its color mainly from a substance called hemoglobin. When blood has oxygen, like in arteries, it looks bright red. When it doesn’t have much oxygen, like in veins, it looks a darker red. This is why blood looks different when it’s taken for a test.
Some special conditions can change the color of blood. For example, in carbon monoxide poisoning, blood can look very bright red. Certain animals, like some skinks, even have green blood because of a waste product called biliverdin.
Disorders
General medical
Injury can cause blood loss through bleeding. A healthy adult can lose almost 20% of blood volume before feeling restless, and 40% before experiencing shock. Thrombocytes are important for blood clotting and can help stop bleeding. Trauma to internal organs or bones can cause internal bleeding, which can sometimes be severe. Dehydration can reduce blood volume by lowering the water content of the blood. This might cause fainting but rarely results in shock except in very severe cases.
Shock happens when tissues don't get enough blood flow, and can be caused by blood loss, infection, or poor heart function. Atherosclerosis narrows arteries, reducing blood flow, and can be worsened by smoking, high blood pressure, high lipids, or diabetes. Blood clots can block blood vessels, leading to tissue damage or death.
Hematological
Anemia occurs when there isn't enough red blood cells or hemoglobin, often due to bleeding, blood disorders, or nutritional deficiencies. It can be confirmed with a blood test and may require blood transfusions, which need compatible blood types. Sickle-cell anemia is a type of anemia where red blood cells change shape.
Leukemia is a cancer of the blood-forming tissues. Overproduction of red cells or platelets can be a sign of pre-cancer. Myelodysplastic syndromes involve poor production of blood cells. Hemophilia is a genetic disorder that impairs clotting, leading to excessive bleeding. Insufficient platelets can cause bleeding disorders, while defects in clotting regulation can lead to dangerous blood clots.
Blood can carry infections such as HIV, hepatitis B and C, bacteremia, sepsis, malaria, and trypanosomiasis. Because of these risks, bloodstained objects are treated as biohazards.
Carbon monoxide poisoning
Main article: Carbon monoxide poisoning
Carbon monoxide can bind to hemoglobin in the blood, preventing it from carrying oxygen. This can be life-threatening, especially in poorly ventilated spaces with fires or when smoking tobacco.
Treatments
Blood for transfusion comes from people who donate it, and it is stored in special places called blood banks. There are different blood types, like the ABO and Rhesus systems, and it is important to match these correctly to avoid serious problems. Other blood products such as platelets and plasma can also be given to patients through a vein.
Many medicines, from antibiotics to chemotherapy, are given into a vein because they cannot be properly absorbed by the digestive system. In cases of severe blood loss, special liquid solutions can be given to help save lives until proper treatment is available. These solutions are often better quick fixes than giving blood right away.
Today, bloodletting is only used for a few rare diseases. In the past, it was a common practice for many illnesses, but we now know it was not always helpful.
Etymology
The word "blood" comes from Old English blod, which is related to words in other Germanic languages like German Blut and Swedish blod. These words all share similar meanings. However, there is no widely accepted explanation for where this word comes from in older Indo-European languages.
Main article: Old English
History
Ancient Greek thinkers observed that when blood is left in a glass container, it separates into four layers. They believed this showed that the body contained four different fluids, each linked to a different temperament. They also thought that blood was made from food. For example, Plato believed that fire in our bellies helped turn food into blood.
In modern times, scientists discovered the ABO blood group system in 1900. This system classifies blood into four types: A, B, AB, and O. The first successful blood transfusion using this system happened in 1914, and later the Rhesus factor was discovered in 1937.
Culture and religion
Blood holds deep meaning in many cultures and religions due to its vital role in life. It is often seen as a symbol of family ties, with phrases like "blood is thicker than water" reflecting strong family bonds. In several major religions, blood carries special significance. For example, in Jewish, Christian, and Islamic traditions, blood is treated with great respect. These faiths often avoid consuming blood, based on ancient teachings that link blood to the essence of life.
In many traditions, blood plays a role in rituals and ceremonies. For instance, some indigenous Australian cultures use red ochre, which resembles blood, in sacred dances and rituals. In historic European practices, blood was sometimes used in ceremonies to connect people to gods or to mark important moments. These uses show how blood has been viewed as a powerful symbol across different cultures and beliefs.
Invertebrates
Main article: Hemolymph
In animals without a backbone, there is a special fluid called hemolymph that works similarly to blood. Unlike blood, hemolymph is not kept inside a closed system of blood vessels. Some sea creatures like crustaceans and mollusks use a protein called hemocyanin to help carry oxygen instead of the protein hemoglobin that humans use. Most insects do not need special molecules in their hemolymph to carry oxygen because their bodies are so small that a system called the tracheal system can easily provide the oxygen they need.
Other uses
Blood can be important for solving crimes. Experts look at blood left at a scene to find clues about what happened and who might have been involved. They study how blood is spread to learn more about the event.
Artists sometimes use blood in their work. Some performances and photos include blood to create strong visual effects. In family history, words like "bloodline" refer to a person's ancestry and ethnic background.
Images
This article is a child-friendly adaptation of the Wikipedia article on Blood, available under CC BY-SA 4.0.
Images from Wikimedia Commons. Tap any image to view credits and license.