What's in blood and where is it made?

What's in blood and where is it made?

Blood is a complex, dynamic mixture of cells, proteins, ions, sugars, signalling molecules, nutrients and gases suspended or dissolved in water. Blood also re-distributes heat. The composition of blood constantly varies in response to our diet, exercise status, hydration, time of the day, injury and challenges from pathogens. As well as its fundamental role in mammals, blood products have important ex-vivo applications. For example, serum and albumin are important reagents that enable cell culture.  What are the components of blood? Where do these components of blood come from?

As blood travels around the body, various organs and tissues along its route contribute to the production of its components. Some of these tissues and organs, such as bone marrow,  are primarily focused on producing blood components, whereas others, such as the liver, have many other important functions.  As well as carrying nutrition, oxygen and cells of the immune system, the blood carries signalling molecules around the body such as hormones, cytokines and extracellular vesicles. The blood also carries waste products from tissues to organs of the body, such as the liver, lungs and kidneys that filter and secret them.  All these components have different lifespans and mechanisms for removal from the blood. The table below focuses on their function and origin. Components are listed in approximate order of their size. 


See also: Sera and Plasma: Do you know the difference?



Component Function Source
Oxygen Required for metabolism Air via the lungs
Carbon Dioxide A waste product of metabolism All cells and air via the lungs
Nitrogen Component of other molecules Lungs
Nitrous oxide A vasodilator All cells
Water  A fluid carrier for other components of the blood and a source of hydration for cells in the body.     Ingested and absorbed through the digestive system. 
Small Molecules
Salts Maintaining osmolarity, source of ions for cellular function. When overall salt levels are too high, this is sensed by the brain making us thirsty Ingested and absorbed through the digestive system. 
Metabolites Waste products from cells are transported in the blood for removal, principally by the kidneys and the liver.  All cells
Sugars Source of energy Ingested and absorbed through the digestive system. 
Vitamins and nutrients Required for cellular function and metabolism  Ingested and absorbed through the digestive system. Skin (vitamin D)
Signalling Molecules
Cytokines and growth factors.  Small protein messenger molecules. Please see this article for further details.   They are generated by all cells.
Hormones Messenger molecules can be proteins, such as insulin, or chemicals, such as thyroxin.  Examples of hormones include: Secreted from a variety of sources
Oestrogen The development of female sex organs regulates the menstrual cycle.  Ovaries
Testosterone Development of male sex organs, musculature, and facial hair.  Made mainly in testes but other origins (e.g. ovaries)
Insulin Regulates blood sure levels.  Pancreas
Glucagon Regulates blood sure levels.  Pancreas
Renin Assists in blood pressure control.  Kidneys
Thyroxine A precursor to triiodothyronine.  Thyroid
Triiodothyronine  Controls metabolic rate. Liver, kidneys and others
Endorphins Block pain.  Anterior pituitary
Anti-diuretic hormone   Stimulates water retention by the kidneys.  Made in the hypothalamus, secreted by the pituitary
Major Blood Proteins
Albumin Serum Albumin is the major protein in plasma. Albumin binds to and transports hormones, fatty acids, and other compounds. Albumin buffers pH, and maintains oncotic pressure, among other functions.  Made in the liver

Produced in millions of different forms to recognize foreign proteins. 

Produced by B cells (see below) with help from T cells).

An insoluble protein arranged in long chains. Enables clotting. 

Formed by the action of thrombin on fibrinogen.

A glycoprotein complex that is converted to fibrin during injury. 

Made in the liver 
Haemoglobin Haemoglobin binds to oxygen to transport it from the lungs to other tissues in the body. Haemoglobin also transports about 75% of the carbon dioxide in the blood for exhalation. Haemoglobin is packaged in red blood cells to prevent it from wreaking oxidative havoc in the vasculature and exposed tissues.  Red bone marrow
Lipids (Fats)
Cholesterol Important for building cell walls, making hormones, bile acids and vitamin D. Dangerous in excess contributing to heart disease.  Can be combined with proteins into Lipoproteins (See below) Made in the liver and also absorbed from foods via the digestive system
Triglycerides Provide a stored source of energy between meals. Can be combined with proteins into Lipoproteins (See below) Provided by foods, fat cells and made in the liver. 
   Lipoproteins (complexes of lipids and proteins)                   
Lipoproteins  Cholesterol and triglycerides are found in Lipoproteins – these “round parcels” also contain proteins. There are four main groups of lipoproteins:  
Chylomicrons – the largest lipoprotein  Carry triglycerides from the intestine. Foods
Very low-density lipoproteins (VLDL) Transport triglycerides made in the liver. Liver

Low-density lipoproteins (LDL)
transport cholesterol made in the liver. This cholesterol is LDL-cholesterol. It is generally considered to be bad cholesterol. Liver
High-density lipoprotein (HDL) Removes excess cholesterol away from artery walls and cells back to the liver for recycling and disposal. Generally considered good. Artery walls
Extracelular Vescicles
Extracellular vesicles (EVs) Small hollow spheres of varying size with lipid walls containing protein and RNA cargo. Includes exosomes (50-200nm), microparticles (200 nm to 1 µm) generated by viable cells and apoptotic bodies generated by dying cells. Exosomes provide messenger function and are present at about 1x1012 per ml.  Generated by all cells.
Platelets  Small, disc-shaped fragments that help to form clots in injured tissue. These are formed from the cytoplasm of megakaryocytes. They are found at a ratio of about 1 per 20 red blood cells and have a lifespan of 7-10 days. Megakaryocyte cells found in bone marrow and lung
Red Blood Cells
Red blood cells Make up about 40% of total blood volume. Their primary role is to provide a package for haemoglobin. Red bone marrow
White Blood cells
White Blood cells (leukocytes) Present at a ratio of 1 white blood cell for every 600-700 red blood cells. Fight of infection and help to remodel damaged tissue. White blood cells are also called leukocytes Bone marrow, thymus
Lymphocytes A type of white blood cell, further divided into individual cell types  Bone marrow, thymus
T cells. Fight infection and diseases such as cancer.  Thymus
Natural Killer cells. Part of the innate immune system provides a first response to infection by attacking virus-infected cells.  Bone marrow
B cells.  Adaptive immune cells produce antibodies against pathogens.  Bone marrow
Neutrophils Most prevalent lymphocytes account for about 50% of the total. Phagocytic cells that ingest pathogens. Bone marrow
Monocytes Phagocytic cells that ingest pathogens. Move from blood into inflamed tissue where they differentiate into macrophages which can orchestrate immune response and tissue remodelling.  Bone marrow
Eosinophil Kill parasites, destroy cancer cells, and are involved in allergic responses.  Bone marrow
Basophil  Participate in allergic responses Bone marrow
Macrophages Probably wrong to classify them as white blood cells as they are not primarily found in the blood. Macrophages are phagocytic cells that can originate in the tissues where they are found (tissue-resident macrophages) and also differentiate from monocytes after they have extravasated from the blood through the blood capillary walls into tissues. Bone marrow and other tissues


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