Friday, 3 July 2015

All About ......The Horse's Circulatory System

Following on from last week's All About .. The Horse'sRespiratory System blog today I am covering a bit about the circulatory system!  This system is based on the heart and uses blood to carry out  a number of functions:
  • to carry oxygen from the lungs to all the body cells, 
  • to carry carbon dioxide from the body cells to the lungs, 
  • to carry water and nutrients from the gut to the body cells, 
  • to carry waste from the body cells to the kidneys, 
  • to take messages (hormones) from the endocrine glands to other organs, 
  • to take the bodies' defence forces to the sites of attack, 
  • to take heat from the centre of the body to the surface when needed.


Blood is made up of Plasma, Blood Cells and Platelets.  Blood tests are a useful way to tell if a horse has a disease or problem which has no other clear symptoms (see my blood test blog)  Blood is a fluid but when it is exposed to air it should clot and solidify rapidly.

Plasma - is a straw coloured serum  and is the main transport medium in the body.  It is made up of 90% water but also contains; proteins, glucose, lipids, amino acids, salts, enzymes, hormones, antigens, antibodies and urea.  These are the nutrients, messages, defence forces and waste that the Circulatory System is transporting around the body!  The serum bathes the cells of the body.  The amount of other substances (mentioned above) is influenced by the food and water consumed, the requirements of the body and the function of the kidneys.  The plasma also contains fibrinogen which aids the clotting of blood.  

Blood cells -are either red or white. 
  • The Red Blood Cells (RBC) originate in the bone marrow and contain haemoglobin.  This substance combines with oxygen to carry it around the body, or with carbon dioxide to take it out of the body. Packed Cell Volume (PCV) on a blood test refers to the % of RBC in the blood.  RBC's are also known as erythrocytes.  
  • White Blood Cells (WBC) defend against disease.  They do this by attacking and destroying germs.  WBC's are also known as leucocytes.  They are divided into 2 categories granulocytes (or polymorph's) and agranulocytes which are subdivided into lymphocytes and monocytes.  These agranulocytes originate in the lymphatic system (see next week's blog).  Polymorphs originate in the bone marrow like the RBC's and can be subdivided into neutrophils, eosinophils and basophils.  The neotrophils engulf bacteria to form pus.  The eosinophils detoxify foreign proteins and the basophils help control inflammation.
Platelets - are fragments of cells broken off from large cells in the bone marrow.  They aid in the clotting of blood. 

The Heart
The heart is a huge muscle which is divided into 4 chambers.  The top 2 chambers are called the atria and the bottom ones ventricles.  The atria collect the blood whilst the ventricles pump the blood.  Between each top and corresponding bottom chamber there are non-return valves.  

The System
Arteries carry the blood away from the heart and veins carry blood to the heart.   Arteries are strong and thick as the oxygenated blood is being transported through them using a pumping action.  They are also capable of expanding and contracting.  The aorta is the main artery and this connects to the heart at the left ventricle, the aorta then divides into smaller arteries and even smaller arterioles as the blood is taken from the heart.  When these arterioles reach their destination (ie: part of the body) they branch again into capillaries, these are thin walled and allow the fluids and gases to pass into and from the body tissues.  These arterial capillaries then become venous capillaries.

The venous capillaries then carry the deoxygenated blood  (also containing the waste products), which passed from the body tissues, away. As these become wider they are called venules, again these join together to become veins which eventually join together to become the vena cava (the main vein) which carries the blood back to the heart at the right atrium.  Veins have valves which ensures the blood flows in the correct direction as they have no pumping action.  This part of the  system is called the Systemic Circulatory System.

The Pulmonary Circulatory System is the part of the circulatory system which involves taking the blood from the heart to the lungs and back again.  The deoxygenated blood is returned to the right atrium of the heart (through the vena cava).  It is then pumped into the right ventricle which then pumps the blood along the pulmonary artery into the lungs.  Here, as mentioned in last week's blog the carbon dioxide and water pass out into the air in the alveolar sacs and oxygen passes from the air into the blood.  This, now oxygenated, blood is now transported in the pulmonary vein back to the left atrium of the heart to be pumped around the body.

This explains why if your horse cuts an artery the vet should be called IMMEDIATELY and First Aid administered.  The pumping action in arteries mean that the blood will be forced in  a rhythm as the heart pumps, it will be pushed out and with it all oxygen and nutrients for the body.  Although a horse can lose quite a lot of blood and still recover it is IMPERATIVE that they are attended to quickly to prevent the loss of excessive amounts.

Look out next week for more about the other circulation system - the Lymphatic System.

Take a look at Wednesday's video 'How to  .... Put on a Tail Bandage'  
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Until next time!

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