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The Cardiovascular System: Blood

Blood is made up of a number of different cells, fragments of cells, various nutrients and hormones and liquid.  An adult will have about 9 pints of blood within their body.

Blood has three main functions:

  • Transportation – blood is vital for the transport of oxygen, nutrients, hormones and waste products.
  • Maintenance – blood is vital for the maintenance of normal fluid and electrolyte balance.  Blood also helps to regulate body temperature and therefore is vital to the maintenance of homeostasis within our bodies.
  • Protection – Blood contains immune cells which protect us against infection as well as clotting agents that seal wounds reducing fluid loss and protect us from the entry of pathogens into the wound.


Plasma makes up between 50 and 60% of our blood.  It consists of around 90% water and 10% proteins, nutrients, salts, gases and waste products.  Plasma acts as a medium for the transport of suspended components around the body.  It is responsible for the transport of important nutrients such as carbohydrates, fats, proteins, vitamins and minerals.  Plasma is also acts as a medium for the transport of hormones, waste products such as urea, creatinine and bilirubin, and small quantities of oxygen and carbon dioxide are also transported within plasma.

Red Blood Cells (Erythrocytes)

These are responsible for the transport of oxygen around the body but also have a role in the transport of carbon dioxide away from working muscles. Erythrocytes have a large surface area that allows gases to diffuse rapidly across their surface, making them ideal for the transport of oxygen. 

Erythrocytes contain a red pigment, called haemoglobin, which binds with oxygen within the erythrocyte.  One important constituent of haemoglobin is iron which plays a vital role in the normal functioning of haemoglobin.  When haemoglobin comes into contact with oxygen the they quickly associate to form oxyhaemoglobin. Haemoglobin that is not associated with oxygen is called deoxyhaemoglobin.

Oxyhaemoglobin is bright red whilst deoxyhaemoglobin is a darker red.  A normal healthy adult will have about 25 trillion erythrocytes.  Erythrocytes are responsible for around 98.5% of total oxygen transport within blood, with the other 1.5% being transported within the plasma.  

Red blood cells are also responsible for the transport of some of the carbon dioxide that is released from tissues.  The carbon dioxide combines with haemoglobin to form carbaminohemoglobin

Erythrocytes also transport nitric oxide which plays an important role in the relaxation of blood vessels and therefore helping to regulate blood pressure.

White Blood Cells (Leukocytes)

These play a vital role in protecting the body against bacteria as well as removing dead cells and debris from the body.  Leukocytes are able to leave the circulation and enter tissues.  They are attracted to invading pathogens, foreign materials or dead cells through a process of chemotaxis.  Leukocytes destroy bacteria by a process of phagocytosis – they literally surround bacteria and then release chemicals to destroy the bacteria. The same process is used when they encounter dirt, dead cells, foreign bodies etc.  This process leads to the death of the leukocyte.  It is the accumulation of dead leukocytes, bacteria, fluid and cell debris that is seen as pus at the site of infection of a wound or abrasion.  There are five main types of leukocyte:

  • Neutrophils – most common leukocyte.  They secrete enzymes called lyosymes to destroy bacteria.
  • Eosinophils – most common in tissues where there is an allergic response.  They reduce inflammation by releasing chemicals to destroy histamine.
  • Basophils – the least common leukocyte.  They release histamine to increase inflammation.  May also release heparinto inhibit blood clotting.
  • Lymphocytes – the smallest leukocytes, they are mainly found in lymphatic tissue.  There are two main types: 1) B cells that release antibodies to attack bacteria, and 2) T cells that attack cells in which viruses are present.  They also attack tumour cells.
  • Monocytes – the largest of the leukocytes.  They phagocytize bacteria, dead cells and cell fragments.

Platelets (Thrombocytes)

Platelets are tiny fragments of cells produced within the bone marrow.  The surface of platelets has special compounds called glycoproteins along with other proteins.  These allow platelets to attach to collagen.  Plays play a very important role in preventing and reducing the amount of blood loss through wounds.  Platelets are able to seal holes in small blood vessels by forming a plug.  Where there is a larger wound they will form a clot.