The Circulatory System: Blood Composition and Essential Functions
The circulatory system is made up of a pump, the heart, a liquid, blood, and a series of ducts, blood vessels, through which blood is pumped throughout the body.
The Circulatory System: Components and Function
What is Blood? Composition and Volume
Blood is composed of a liquid part (plasma) and different types of blood cells. If you separate the two components, the volume is typically 55% plasma and 45% blood cells.
Blood Plasma: The Liquid Component
The liquid of blood plasma is fundamentally formed by yellowish water (91%) in which a variety of dissolved substances are present.
Blood Cells: Formed Elements
Blood cells are formed in the red bone marrow that fills the inside of some bones. They are of three types:
Red Blood Cells (Erythrocytes)
These are the most abundant cells in the blood. There are around 5 million per mm³ (microliter) of blood. They are cells that have lost their nucleus and have become mere bags filled with red pigment, called hemoglobin, which is responsible for their color.
White Blood Cells (Leukocytes)
Their number varies, averaging about 8,000 per mm³ of blood. There are several types of white blood cells, and all possess a nucleus.
Platelets (Thrombocytes)
These are not truly cells, but cell fragments. There are about 250,000 platelets per mm³ of blood. They are usually found forming small groups. [Note on shapes: White Blood Cells = Sun/Ball; Red Blood Cells = Donut; Platelets = Pieces]
Primary Functions of Blood Transport
The blood, with its high water content, functions as an efficient transport system:
Transporting Solids (Nutrients and Waste)
It transports solid substances dissolved in plasma, such as nutrients and wastes.
Transporting Respiratory Gases (Oxygen and CO2)
Carbon dioxide dissolves well in water and is transported by blood plasma. However, oxygen is poorly soluble in water. Oxygen is primarily transported by hemoglobin located in the red blood cells.
Other Vital Functions of Blood
Thermoregulation (Heat Distribution)
Blood distributes heat throughout the body. During intense physical activity, the blood is heated as it passes through the muscles. This same blood is cooled as it passes through the skin, which helps normalize body temperature.
Immune Defense
White blood cells defend the body against infections. Some engulf foreign bodies such as bacteria; others manufacture substances that aid in their destruction.
Hemostasis (Control of Bleeding)
Platelets collaborate in the coagulation of blood and help stop bleeding caused by ruptured blood vessels.