Beth Talks Blood: Part 3

The blood circulatory system (cardiovascular system) delivers oxygen and nutrients to all the cells in the body. It is comprised of the heart and blood vessels that run through the entire body. Blood is circulated around the body through blood vessels by the pumping action of the heart [1].

In animals with lungs, arterial blood carries oxygen from inhaled air to the tissues of the body and venous blood carries carbon dioxide, a waste product of metabolism by cells, from the tissues to the lungs to be exhaled.

Functions of blood & circulation [2, 3]:

  • Supply of oxygen & nutrients to tissues 
  • Removal of waste such as carbon dioxide, urea and lactic acid
  • Helps to maintain body temperature
  • Messenger functions – transport of hormones and signalling of tissue damage
  • sends antibodies to fight infection 
  • Contains clotting factors to help blood to clot and the body tissues to heal

Blood vessels [2, 3]:

Arteries and arterioles:

  • Carry oxygenated blood away from the heart
  • Thick hollow tubes 
  • Highly elastic which allows them to dilate (widen) and constrict (narrow) as blood is forced through them by the pumping heart 
  • Branch and re-branch becoming smaller until they become all arterioles which are even more elastic 


  • Distribute oxygen and nutrients to the body’s tissues
  • Remove deoxygenated blood and waste 
  • Extremely thin, only one cell thick 
  • Connect the arterioles to the venules 

Veins and Venules:

  • Very small veins
  • Merge into larger veins 
  • Carry blood back to the heart 
  • Vessel walls are similar to arteries but are slightly thinner and less elastic 
  • Carry deoxygenated blood towards the lungs to receive oxygen 

There isn’t only one blood circulatory system, it’s made up of two systems that are connected [1, 3]

1. The systemic circulation provides organs, tissues and cells with blood so that it can get oxygen and other vital substances. 

2. The pulmonary circulation is where the fresh oxygen we breath enters the blood and at the same time carbon dioxide is released from the blood. 

The circulation of blood starts when the heart relaxes between beats. The blood flows from both the atria into the ventricles which then expand, the following phase is called the ejection period which is where both ventricles pump the blood into large arteries

In the systemic circulation, the left ventricle pumps oxygen rich blood into the main artery (aorta). The blood then travels from the main artery to larger and smaller arteries and into the capillary network. The blood drops off the oxygen, nutrients and other important substances and picks up the carbon dioxide and waste. The blood is now low in oxygen and is collected by veins to travel to the right atrium and into the right ventricle, this is where the pulmonary system begins – the right ventricle pumps low oxygen blood into the pulmonary artery (branches off into smaller arteries and capillaries). Capillaries form a fine network around the pulmonary vesicles (grape like air sacs at the end of the airways). Carbon dioxide is released from the blood into the air inside the pulmonary vesicles and fresh oxygen enters the blood stream. When we breathe out, the carbon dioxide leaves our body [1, 3].

Thanks so much for reading, I hope you enjoyed this post!

If you have any questions, please feel free to drop them in the comments.

See you in the next one,

Beth x





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