The Circulatory System: The Blood
Blood is the fluid in the body that is pumped from the heart and circulated around the body via
blood vessels. It is the means by which important substances are transported throughout the
body. The average person has ​5 liters​ of blood.
Composition​:
● Plasma: 55% This is the liquid portion of blood, which allows cells and platelets to float
and easily move through blood vessels. Additionally, it transports ​antibodies ​(from
immune cells), proteins (​albumin and ​fibrinogen​) and clotting factors. Waste from cells
are also transported to be removed from the body via the kidneys.
○ Blood plasma is made up mostly of:
■ Water: ~ 90%
■ dissolved substances:
● Food:​ ​glucose​, amino acids, fatty acids and glycerol
● Carbon dioxide (in the form of bicarbonate ion)
● Nitrogenous wastes (urea)
● Hormones
● Mineral salts: Na+, K+, Cl+
● Blood Cells: 45%
○ White Blood Cells (WBC): also known as ​leukocytes ​(immune system cells that
protects the body). ​All the white blood cells are able to move like an amoeba, and
can move out of blood vessels into the surrounding tissues anywhere in the body.
■ The five types: ​Neutrophils, Basophils, Eosinophils, Monocytes and
Lymphocytes;
● Protect the human body by:
○ Phagocytosis: ​engulfs disease causing organisms at the
sites of infection.
○ Secretion of antibodies: lymphocytes ​specifically produce
antibodies that kill ​pathogens​.
○ Red Blood Cells: also known as ​erythrocytes.
■ 1mm​3​ ​of blood contains​ 5million ​RBCs.​ ​7 - 8μm, in size.
■ Biconcave​ disc shape: thinnest in the center;
● Increases surface area for diffusion of oxygen in and out of the
cell; no nucleus allows the cell to hold a large amount of oxygen,
so these cells only live for 3-4mths.
■ Contains the red pigmented protein ​haemoglobin (contains ​iron​) which
attaches to O​2​.
■ Transports a large amount of oxygen (O​2​) and a small amount of carbon
dioxide (CO​2​). Oxygen is needed for respiration, which is the process that
produces energy.
○ Platelets:
■ These are small pieces of cells that contain no nucleus; formed in the bone
marrow. They are disc-shaped and small, about 2 - 4μm in diameter.
■ Helps the blood to clot to prevent significant blood loss and to aid in
wound healing.
Image from: Khan Academy
_Image modified from ​Components of the blood: Figure 4​, by OpenStax College, Biology (​CC BY 4.0
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