Blood

Hemoglobin,aglobular protein
green = haem (or heme) groups
red & blue = protein subunits

Ascanning electron microscope(SEM) image of a normalred blood cell(left), aplatelet(middle), and awhite blood cell(right)

Bloodis aliquidinhumansand manyanimals.There are about 5 litres of blood in the human body.Blood is pushed through theorganismby theheart.It bringsnutrientsandoxygento thetissuesof the body. It also takes awaywasteandcarbon dioxidefrom tissues.

Blood is made up ofblood plasmaand variouscells.These includered blood cells,white blood cellsandplatelets.Platelets helpblood to clot.Hemoglobinis in red blood cells. White blood cells help fightinfectionsand healwounds.

Plasma

Blood plasma is the yellow liquid in which blood cells float. Plasma is made up of nutrients,electrolytes(salts),gases,non-protein hormones, waste,lipids,and proteins. These proteins arealbumin,antibodies(also calledimmunoglobulins), clotting factors, and proteinhormones.Plasma that does not have the proteinfibrinogenis called serum and cannot clot. Adults have about 3 liters of plasma. Plasma is a liquid, mostly water (90%). Plasma takes up 55% of blood volume.

Red blood cells

Red blood cells

Another name for red blood cell iserythrocyte.'Erythro' means red; 'cyte' means cell.RBCis anacronymforredbloodcell.

RBCs carryoxygenandcarbon dioxidearound the body.Cellsin the body need oxygen to live. Cells also make carbon dioxide as a waste.

RBCs are filled withhemoglobin.This is a protein. It is made to carry a large amount of oxygen. Hemoglobin hasironin it. The iron and oxygen gives hemoglobin its red color.Erythropoietinpromotes the creation of RBCs.Blood typeantigensare carried on the surface of red cells.

RBCs also help the blood stay at a normalpH.The blood needs to be at a pH of 7.4. If it is much more or less than 7.4, a person can get very sick or die. RBCs stops changes in blood pH. The proteins and the carbon dioxide in the RBC do this.

White blood cells

White blood cellsare a big part of theimmune system.They attack things that do not belong in the body. They kill germs such asbacteriaandviruses.They killcancercells. White blood cells also help to fight other toxic substances.

White blood cells find where the germs are and start to destroy them. WBCs arrive in the blood. They also go out of the blood in places where there is infection. WBCs do this to fight the germs that make the infection. If they go out of the blood to fight aninfection,they may return in thelymphatic system.So, WBCs are inlymph nodes.

Another name for white blood cell isleukocyte.Leuko means white. -cyte means cell.WBCis anacronymforwhitebloodcell. There are three main kinds of WBCs. They arelymphocytes,granulocytesandmonocytes.Some of the WBCs mature into cells which do similar work in the tissues.

The different WBCs work in different ways. Some WBCs kill and eat germs and cancer cells. Some WBCs makeantibodies.These are proteins that stick to a cell and tell other WBCs to kill it. Some WBCs make chemicals. They release these chemicals to fight things that do not belong in the body. These chemicals causeinflammationin a part of the body. When a germ makes someone sick, the body shows it. If abacteriagets under someone's skin and causes aninfection,the skin gets red, hot, and painful. This redness, heat, and pain are signs ofinflammation.This shows that WBCs are fighting the infection and killing the bacteria.

Platelets

Platelets help make blood clot.A clot is when theliquidblood becomessolid.The body makes blood clot when the skin is cut. This stops blood from going out of the skin too much.

For blood to be able to clot is essential. But rarely, some blood clots are bad. If a blood clot happens in ablood vesselgoing to the brain, it can cause astroke.If it happens in a blood vessel going to the heart, it can cause aheart attack.This does not usually happen to young, healthy people with the exception of clotting conditions.

Platelets are not the only things that make clots. There are proteins in the blood that help make clots. Both platelets andclotting proteinsare needed to make good clots.

Gold in human blood

On average, the human body has about 0.2 milligrams of gold, mostly found in the blood.

Where blood comes from

Illustration of bone marrow cells fromGray's Anatomy

Blood cells are made in thebone marrow.The spleen (a smaller, vital organ situated inside the ribcage) manages the amount of white blood cells that flow throughout your blood, it also can manage many functions of the liver. The bone marrow is the soft material in the middle of bones. Special cells in the bone marrow make most of the blood cells in the body.

Plasma proteins are made mostly by theliver.The water and electrolytes in plasma come from the food and water that a person digests.

Although blood is afluid,in some respects it is a kind ofconnective tissue.Its cells originate inbone marrowand thespleen,and in the blood there are potential molecular fibres in the form offibrinogen.These are activated when ablood clotforms.