In this chapter... Circulatory system function depends on several key aspects of cellular biology. The nature of water, pH and buffers, for example. Osmosis and water balance are also central to the circulatory system. We will also examine diseases that affect blood, like malaria. Lastly, we’ll explore inheritance of different blood types.

The circulatory system is a lot like a country’s system of roads, rivers and railways. These are easy to take for granted, but when something goes wrong – a bridge collapse or heart attack – the effects are crippling. Our circulatory system constantly adjusts to our changing needs. Even a simple action like standing up would cause us to faint unless our arteries adjusted for this change by squeezing more blood up to the brain. The living blood transported around the body supports the life of every cell. No cell can live long without it.

Why this is important…You might say the circulatory system is at the heart of the body (pun intended). Without the ability to move materials around the body quickly, most animal life would not be possible. And since blood reflects the chemistry of all the cells in the body, it is a window to human health.

Foundations

The anatomy of the circulatory system is essentially two loops or circuits – one for the lungs and one for the rest of the body (Figure 6-1). The heart pumps the blood out through arteries and it returns through veins.

Human circulatory system.
Major arteries and veins shown in human body. Second view shows two major circuits of blood from heart - one to the lungs and one to the body.

FIGURE 6-1. Anatomy of the circulatory system. Left: heart, major arteries (red) and veins (blue) throughout the body. Right: conceptual diagram of two major circuits of blood flow from the heart. ("Circulatory System" by LadyofHats is in the Public Domain, CC0 || "Blood flow through the heart" by OpenStax College is licensed under CC BY 3.0)

Key terms

Artery – a specialized tube (blood vessel) that carries blood away from the heart

Vein – a specialized tube (blood vessel) that carries blood toward the heart

Details

Of the two circuits shown in Figure 6-1, we will focus on the systemic circuit that flows to the body cells. The key function of this circuit is to carry materials to the cells of the body, such as:

Oxygen from the lungs
Nutrients from the digestive tract – glucose, amino acids, lipids and nucleic acids
Hormones from endocrine glands
White blood cells to tissues to defend from microorganisms

Blood also carries materials away from cells, such as:

Nitrogen wastes which go to the kidneys for elimination
Carbon dioxide to the lungs to exhale

Besides transporting materials, blood also regulates major aspects of the entire body, such as:

Fluid levels in body by hydrating cells or carrying away excess fluids
pH of the body (in coordination with the respiratory system)
Body temperature (especially blood vessels in the skin)

Historical note: The Greek philosopher, Galen, established many truths about how the body works, but he didn’t get everything right. He believed that blood was produced by the liver and diffused into the rest of the body. This view dominated western medicine for about 1400 years, until 1618 when William Harvey found that the circulatory system was a closed system of arteries and veins. Harvey proposed that the heart pumped blood around in a circuit. To make these discoveries, he dissected live fish to watch its blood flow.

Learning objectives

Name and identify the major organs of the circulatory system
Describe the basic functions of the circulatory system