The Circulatory System

Do you know what blood is made of?

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What is the blood made of?

Blood is the substance that transports materials around animals, this is pumped by the heart and delivered via the Circulatory System.

The blood is made of 4 main parts. These each serves a specific function which is outlined below:

  1. Red blood cells (Erythrocytes) are the most numerous of the four parts and are responsible for transporting oxygen. Red blood cells have lots of haemoglobin that oxygen binds to and then is transported to where it is needed, e.g. the muscles for aerobic respiration
  2. White blood cells are tasked with removing or destroying foreign cells e.g. pathogens which make their way inside our bodies. There are many types of white blood cells but two of the main ones are lymphocytes and phagocytes. Lymphocytes produce antibodies and phagocytes engulf cells to digest them. 
  3. Platelets are cell fragments that do not have nuclei. These fragments collect where an injury occurs that breaks the skin (internally or externally). Platelets produce substances which help to clot the blood and close up wounds. 
  4. Plasma is a liquid which carries the other three parts of the blood, transporting it via the Circulatory System. Plasma also transports dissolved substances around the body, including the things our bodies need (glucose) and waste products (carbon dioxide and urea). 
Component Function
Red Blood Cells (Erythrocytes) Transport of oxygen and carbon dioxide
White Blood Cells (Phagocytes and Lymphocytes) Protection against harmful microorganisms and foreign substances
Plasma Transportation and exchange of hormones, nutrients, and waste products
Platelets Blood clotting and formation of new blood vessels during wound healing

Adaptations of red blood cells

Red blood cells, also known as erythrocytes, have several adaptations that allow them to efficiently transport oxygen and carbon dioxide throughout the body. Here are 4 main adaptations of red blood cells:

  1. Biconcave shape: Red blood cells have a unique biconcave shape, which allows them to have a large surface area-to-volume ratio. This allows for increased gas exchange and makes it easier for the cells to enter capillaries, even the smallest ones.
  2. Lack of a nucleus: Red blood cells do not have a nucleus, which allows for more space for haemoglobin, the protein that binds to oxygen and carbon dioxide. This increases the oxygen-carrying capacity of the cells.
  3. High concentration of haemoglobin: Hemoglobin is the protein found in red blood cells that bind to oxygen and carbon dioxide. Red blood cells have a high concentration of haemoglobin, which increases the oxygen-carrying capacity of the cells.
  4. Flexibility: Red blood cells are very flexible and can change shape easily, allowing them to pass through the small capillaries and narrow vessels that supply the body’s tissues.

Blood Vessel Structure and Function

The blood vessels are the pipes that carry blood through the body.

Arteries are blood vessels that carry blood away from the heart and have thick walls to withstand the pressure of the blood. This is important as the blood has just been pumped from the heart.

Veins are blood vessels that carry blood back to the heart and have thinner walls and a wide lumen (diameter), this is to allow the blood that is flowing at a low pressure to flow back easily and without resistance. Veins also contain valves to prevent blood from flowing backwards.

Capillaries are small blood vessels that have a branching structure and allow for the exchange of substances between the blood and the body’s cells. For example, the muscles need glucose for respiration and need to remove carbon dioxide from respiration. The branching structure increases the rate of diffusion by increasing the surface area.

Vessel Type Characteristic Function
Arteries Thick walls, narrow diameter Carry blood away from heart
Veins Thin walls, wide diameter Carry blood back to heart
Capillaries Branching structure, one cell thick Allow for exchange of substances between blood and body's cells