Tissues: Blood (part 1)

A quick bit on beginner's blood:

The average adult should contain around 5-6 litres of blood which transports a range of substances around the body.

Around 55% of blood volume is plasma, a water-based fluid within which various substances such as proteins, nutrients and hormones are dissolved or suspended. Also suspended in plasma are the blood cells of which there are 3 types:

• Erythrocytes - red blood cells responsible for transporting oxygen and carbon dioxide between the lungs and the rest of the body. Erythrocytes are the most numerous of the blood cells with 2 million dying and being replaced every second.
  
• Leukocytes - white blood cells responsible for protecting the body from damage by microbes and other dangerous substances. They come in various forms with different functions. They're less numerous than erythrocytes, but much larger.
  
• Thrombocytes - platelets. Tiny cell fragments which are central to the blood clotting process.

The blood is the primary transport system of the body ensuring every cell receives the oxygen, nutrients and waste removal it needs in order to survive. A network of blood vessels allow blood to be pumped around the body by the heart.

Arteries transport blood away from the heart whilst veins transport blood back towards the heart. Small networks of capillaries are found throughout and these allow for the exchange of substances between blood and body tissues.

At rest the heart should contract between 65 and 75 times per minute pumping blood either to the lungs to be oxygenated or to the rest of the body to transport that oxygen. In this time all blood should have passed through the heart once.