Blood

Blood 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. It also takes away waste that the cells don’t need. This helps keep the body healthy.

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.

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 is darker red. Blood helps keep every part of the body alive and healthy.

Functions

Hemoglobin, a globular proteingreen = haem (or heme) groupsred & blue = protein subunits

Blood has many important jobs in your body. It carries oxygen from your lungs to all your tissues. It does this with a special protein called hemoglobin in red blood cells. Blood also brings nutrients like glucose to your cells so they can grow and stay healthy.

Blood helps remove waste such as carbon dioxide, urea, and lactic acid. It is important for your immune system because it moves 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 keep your body temperature just right.

Constituents

A scanning electron microscope (SEM) image of a normal red blood cell (left), a platelet (middle), and a white blood cell (right)

Blood is about 7% of a person's body weight and is a little heavier than water. An adult has about 5 liters of blood. Blood is made 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. Plasma is 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 has proteins and tiny particles that help with clotting and fighting infections. It helps keep the blood balanced so it can work properly.

Constitution of normal blood
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 for the body to get rid of. 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 helping to make many of its parts. 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 is mostly colored by something called hemoglobin. When blood carries oxygen, like in your arteries, it looks bright red. When it doesn’t have much oxygen, like in your veins, it looks a darker red. That’s why blood can look different when it’s taken for a test.

Some special conditions can change the color of blood. For example, in carbon monoxide poisoning, blood might look very bright red. Some animals, like certain skinks, even have green blood because of something called biliverdin.

Disorders

General medical

Injury can cause blood loss through bleeding. A healthy adult can lose some blood before feeling tired, and more before feeling very weak. Small particles in the blood, called thrombocytes, help stop bleeding. Injury to organs inside the body or to bones can cause internal bleeding, which can sometimes be serious. Dehydration can lower the amount of water in blood, which might cause fainting.

Shock happens when parts of the body don’t get enough blood. This can be caused by blood loss, infection, or heart problems. Atherosclerosis can make blood vessels narrower, reducing blood flow. This can be made worse by smoking, high blood pressure, high lipids, or diabetes. Blood clots can block blood vessels and harm tissues.

Hematological

Anemia happens when there aren’t enough red blood cells or hemoglobin. This can be due to bleeding, blood disorders, or not enough nutrients. It can be found with a blood test and may need blood from another person, which must match. Sickle-cell anemia is a type of anemia where red blood cells change shape.

Leukemia is a disease that affects blood-making tissues. Some conditions can be signs that disease may develop. Hemophilia is a genetic disorder that makes it hard for blood to clot, causing too much bleeding. Not enough of the small blood particles can cause bleeding problems, while problems with clotting 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, objects with blood on them are handled carefully.

Carbon monoxide poisoning

Main article: Carbon monoxide poisoning

Carbon monoxide can stick to the blood and stop it from carrying oxygen. This can be very dangerous, especially in places with poor air, like during fires or when smoking tobacco.

Treatments

Blood for transfusion comes from people who donate it. It is stored in special places called blood banks. There are different blood types, like the ABO and Rhesus systems. It is important to match these correctly.

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.

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. It is related to words in other languages, like German Blut and Swedish blod. All these words mean something similar. But we do not really know where the word came from before Old English.

Main article: Old English

History

Ancient Greek thinkers noticed that when blood sits in a glass, it splits into four layers. They thought this meant the body had four different fluids, each linked to a different personality. They also believed that blood came from food. For example, Plato thought that fire in our stomachs helped change food into blood.

In modern times, scientists found the ABO blood group system in 1900. This system groups 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 found in 1937.

Culture and religion

Blood is important in many cultures and religions because it is essential for life. It is often a symbol of family ties, like the phrase "blood is thicker than water" which means families are very close. In Jewish, Christian, and Islamic traditions, blood is treated with respect. These religions usually avoid eating blood, based on old teachings that connect blood to life.

In many traditions, blood has a role in rituals and ceremonies. For example, some indigenous Australian cultures use red ochre, which looks like blood, in special dances and rituals. In old European practices, blood was sometimes used in ceremonies to connect people to gods or to mark important events. These examples show how blood is seen as a powerful symbol in many 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. Most insects do not need special molecules in their hemolymph to carry oxygen because their bodies are small enough for a system called the tracheal system to easily provide the oxygen they need.