Composition of Blood

Blood is an essential medium for the transport of materials around the body. It carries a variety of substances, including gases, metabolites, nutrients and hormones, acting as an exchange system for tissues within the body. One of its most recognised functions is the exchange of O2 and CO2 from the lungs to the rest of the body. But it also transports hormones from the glands to their target receptors. Waste products to the liver and kidneys. And nutrients from the digestive tract to the tissues. As well as transportation blood serves as a protector and regulator with roles in inflammation and balancing pH and water levels.

Blood has 2 main components, Plasma and formed elements, these include Erythrocytes, Leukocytes and Platelets.

Plasma is identified as the clear extracellular fluid that holds the cellular components. It is a mixture of proteins, enzymes, nutrients, hormones and gases. The most abundant component are proteins such as albumin, globulin and fibrinogen, each having its own specific function. In general the key roles for plasma proteins are:

  • Nutrition, serve as a reserve supply of amino acids when broken down by macrophages, which can then be used by cells to synthesise new products.
  • Carrier, small molecules including lipids may bind to specific plasma proteins where they are transported to other tissues for use.
  • Stabilise pH, binding to excess H+ to keep the blood slightly basic.
  • Coagulation,interacting with other proteins to initiate formation of blood clots. Helping to protect against blood loss and invasion of foreign microorganisms.
  • Osmotic pressure, helping to regulate the distribution of water between the blood and tissue.

Albumin, is the smallest in size but most abundant, accounting for 55% of the total plasma protein concentration. It is synthesised in the liver as a single polypeptide chain (610 aa) with a molecular weight of 69 kDa.  The size of the protein and it being highly charged (both positive and negative) aids in its role as a transport protein, carrying various biomolecules such as fatty acids, trace elements and drugs. It also has a key role in maintaining the osmotic pressure, with reductions in its concentration linked to the loss of fluid from the blood and the gain of fluid in the interstitial space. 

Globulins, can be divided into 3 classes, alpha, beta and gamma, with the alpha class containing 2 subtypes, (alpha 1 and alpha 2). They are divided into these classes based on their molecular weight from smallest to largest.  Alpha globulins include HDL and LDL which are involved the in the regulation and transport of cholesterol within the body. HDL considered the good cholesterol removes excess found in the circulation, preventing build up in the arterial walls. LDL considered the bad cholesterol favours the deposition instigating cardiovascular disease. A2 globulins also contain alpha 2 glycoproteins, plasminogen, prothrombin and hepatoglobulin. Beta globulins include beta lipoproteins which are very lipid rich an transferrin, an iron transport protein. Gamma globulins are immunoglobulins classified as either IgG, IgA and IgM, which have antibody activity.

Fibrinogen, is a fibrous protein with a molecular weight of 34kDa. It is converted to a sticky protein called fibrin by the enzyme thrombin, where it plays an essential role in blood coagulation.

In addition to the main plasma proteins enzymes such as acid phosphatase and alkaline phosphatase are found in the plasma which are a useful diagnostic marker for a number of cancers and bone turnover. Alongside the plasma proteins amino acids, nitrogenous waste, nutrients, gases and electrolytes may be found in circulation.

Cells within the blood,

Erythrocytes, red blood cells can be identified by their round biconcave shape. The outer surface displays glycoproteins and glycolipids which determine a persons blood type. 2 proteins in particular, spectrin and actin provide the cell with flexibility and strength, enabling it to bend and squeeze through small blood vessels and bouncing back to its original shape in larger vessels. The main role of RBCs include

1. Transporting oxygen from the lungs to other tissues in the body.

2. Transporting carbon dioxide from tissues to the lungs for excretion.

RBCs are well adulated to this role as during maturation they lose much of their inner cellular components, including the mitochondria and nucleus. This increases the cells surface area ratio for quicker transportation. In addition without these organelles aerobic respiration is not necessary, meaning O2 and CO2 is not used up by the cell itself when carrying out its role. The downside to no nucleus however is that the mature cell is unable to repair itself  with the typical lifespan around 120 days. The production of new blood cells instead takes place in the bone marrow in response to the hormone EPO, released by the kidney. They develop from pluripotent stem cells and once an immature RBC has produced the maximum amount of haemoglobin protein this initiates the breakdown of the nucleus and mitochondria where it may then progress to a mature RBC. This process is very rapid with around 7 trillion new cells thought to form each month.

Leukocytes, white blood cells are divided into 2 types granulocytes (including neutrophils, eosinophils and basophils) and agranulocytes (including lymphocytes and monocytes) .

Granulocytes are recognised under the microscope by the appearance of their organelles which look like granules.


  1. Neutrophils, are cells involved in our innate immune system, they are first to migrate towards the site of infection and initiate an immune response. They make up about 60% of immune cells and contain a high concentration of antimicrobial proteins. They also have a sort life span which means they can use dramatic mechanisms to control microbes and halt infection.
  2. Eosinophils, like neutrophils help to protect the body from bacteria as well as parasites. They are found within the blood but also tissues ensuring they are closer to where the site of infection may occur. They help to fight off microbes by first engulfing, excreting and then exhibiting. 
  3. Basophils, are found predominantly in the skin and mucosal tissue representing only 1% of white blood cells in the body. Basophils fight infection by releasing histamine which initiates the inflammatory response. Histamine causes the blood vessels to dilate and become leaky, increasing the flow of blood to the area and the transport of other inflammatory mediators. Basophils also secrete heparin which prevent coagulation and enable the WBCs to circulate freely.


  1. Lymphocytes, include T cells, B cells, and natural killer cells. T cells are developed in the thymus gland and display a specialised receptor on their surface which is able to recognise and bind to antigens. B cells are developed in the bone marrow and like T cells display a specialised receptor which can recognise and bind antigens. Natural killer cells are able to recognise infected cells and cause their destruction, before any significant damage can be caused.
  2. Monocytes, are the largest type of WBC. They are developed in the bone marrow and are released into our blood and tissues. They have the ability to develop into macrophages which enables them to ingest harmful pathogens and break them down. Following this they can display part of the pathogen on their outer surface (antigen), acting as a flag for other immune cells to migrate to the infected area. In addition to this monocytes may ingest other dead cells from the site of infection helping with wound repairing.

Platelets, are tiny cell fragments found within the blood. They are produced in the bone marrow as a by product of larger cells, this gives them an uneven rough shape. Their main role is within blood coagulation where they help produce blood clots to seal a wound and prevent blood loss. When a wound is opened signals are sent and released from platelets, activating them to change shape and become more sticky. Alongside this fibrinogen (which becomes fibrin) develops a fibrous web over the wound which the platelets become caught in. Producing a blood clot over the area.

  • Secrete vasoconstrictors, causing vascular spasms in broken blood vessels,
  • Form platelet plugs,
  • Dissolve unwanted blood clots, 
  • Digest and break down bacteria,
  • Secrete cytokines, attracting neutrophils and monocytes to the site of infection,
  • Secrete growth factors to maintain the lining of blood vessels.

As we age the composition of the blood is understood to alter. These changes may help to explain the increase in clot formations and atherosclerosis as we get older. Some of these changes include increased fibrinogen, increased rigidity in the red blood cells structure, increased formation of fibrin degradation products, earlier activation of the coagulation system and a rise in blood/ plasma viscosity. Blood pH may also decrease with more H+ ions. This can cause RBC's to swell and become more turgid, coupled with a general decrease in total body water as we get older this can affect blood flow significantly.