Blood can be referred to as a red river of life. Both blood and a river are liquid. In an organism, blood flows throughout the body transporting nutrients, oxygen and carrying away wastes. It is a tissue composed mainly of water with cells suspended in it. Blood follows a specific pathway through the body in the same way a river flows on its journey towards the sea along a specific pathway. There are arteries, veins and capillaries in the body which allow blood to flow to all parts of the body. This can be compared to all the tributaries, streams, etc. in an area that make up the transport system of a river.
A river is made of water with dissolved nutrients, oxygen and other substances just like blood which maintains the life of the organisms that live within in. When the river meets the sea, it deposits salts along with other nutrients to the ocean which help to replenish the supply of inorganic and organic substances therein.
If part of a river system becomes blocked, the pathway of the water on its journey to the sea can be harmed. This can have devastating effects on the ecosystem. If a person's artery becomes blocked, an area of the body may not be supplied with oxygen and they can suffer a heart attack or stroke. Both the pH of the blood and of the river must be at optimal levels or the health of organisms that rely on them may be at risk.
In conclusion, blood can be compared to a river because both are essential to life.