Human Circulatory System and Blood Composition
The Human Circulatory System
The circulatory system handles transport within the body. It is built by blood vessels—a network of numerous conduits reaching all cells—and the heart, an organ that pushes blood through them.
The Internal Environment and Interstitial Plasma
Cells of any living organism are immersed in a medium from which they get all the nutrients they need and into which they discharge excretion substances originating from their metabolic reactions. In multicellular organisms like humans, this is called the internal environment. It consists of interstitial plasma, a liquid medium located in the space between cells.
The interstitial plasma must be continually renewed to prevent nutrient exhaustion and the accumulation of toxic excretion products.
Composition and Functions of Blood
Blood is a viscous, salty liquid that runs inside the circulatory system's vessels. It is estimated that every human being has approximately 5.5 liters of blood.
Primary Functions of Blood
- Transport: Carries nutrients and oxygen (via plasma and red cells).
- Waste Collection: Collects waste products for excretion.
- Defense: Involved in defending the body against infection (via white cells).
- Hormone Transport: Carries hormones through the plasma.
- Regulation: Participates in temperature regulation.
Blood is made up of a liquid part, plasma, in which red blood cells, white blood cells, and platelets float.
Blood Plasma and Its Components
Blood plasma is yellowish in color and is comprised of water containing various dissolved molecules: salts, nutrients, waste products, proteins, and hormones.
Types of Blood Cells
Red Blood Cells (Erythrocytes)
Red blood cells are the most numerous; men have about 5 million per cubic millimeter (mm³), and women have slightly less. They are disc-shaped and have no nucleus. They contain hemoglobin, an iron-based pigment responsible for the red color of blood. Hemoglobin carries oxygen from the lung alveoli to every cell in the body where it will be used.
White Blood Cells (Leukocytes)
White blood cells are less numerous than red cells, with a count of 6,000 to 7,000 per mm³. They are involved in defending the body against microbes that enter the system.
Platelets (Thrombocytes)
Platelets are cell fragments (200,000 to 300,000 per mm³) containing substances that allow blood coagulation. This process prevents blood from escaping quickly and thus prevents bleeding.