Human Physiology, 14th edition (2016)

Blood, Heart, and Circulation 405

13.1 FUNCTIONS AND

COMPONENTS OF THE

CIRCULATORY SYSTEM

Blood serves numerous functions, including the trans-

port of respiratory gases, nutritive molecules, metabolic

wastes, and hormones. Blood travels through the body in a

system of vessels leading from and returning to the heart.

urinary, digestive, endocrine, and integumentary systems in

maintaining homeostasis.

Functions of the Circulatory System

The functions of the circulatory system can be divided into

three broad areas: transportation, regulation, and protection.

1. Transportation. All of the substances essential for cellu-

lar metabolism are transported by the circulatory system.

These substances can be categorized as follows:

a. Respiratory. Red blood cells, or erythrocytes, transport

oxygen to the cells. In the lungs, oxygen from the inhaled

air attaches to hemoglobin molecules within the erythro-

cytes and is transported to the cells for aerobic respiration.

Carbon dioxide produced by cell respiration is carried by

the blood to the lungs for elimination in the exhaled air.

b. Nutritive. The digestive system is responsible for the

mechanical and chemical breakdown of food so that

it can be absorbed through the intestinal wall into the

blood and lymphatic vessels. The blood then carries

these absorbed products of digestion through the liver

to the cells of the body.

c. Excretory. Metabolic wastes (such as urea), excess water

and ions, and other molecules not needed by the body are

carried by the blood to the kidneys and excreted in the urine.

2. Regulation. The circulatory system contributes to both

hormonal and temperature regulation.

a. Hormonal. The blood carries hormones from their site

of origin to distant target tissues where they perform a

variety of regulatory functions.

b. Temperature. Temperature regulation is aided by the

diversion of blood from deeper to more superficial cuta-

neous vessels or vice versa. When the ambient tempera-

ture is high, diversion of blood from deep to superficial

vessels helps cool the body; when the ambient tempera-

ture is low, the diversion of blood from superficial to

deeper vessels helps keep the body warm.

3. Protection. The circulatory system protects against blood

loss from injury and against pathogens, including foreign

microbes and toxins introduced into the body.

a. Clotting. The clotting mechanism protects against blood

loss when vessels are damaged.

b. Immune. The immune function of the blood is per-

formed by the leukocytes (white blood cells) that pro-

tect against many disease-causing agents (pathogens).

Major Components of the Circulatory System

The circulatory system consists of two subdivisions: the cardio-

vascular system and the lymphatic system. The cardiovascular

system consists of the heart and blood vessels, and the lymphatic

system, which includes lymphatic vessels and lymphoid tissues

within the spleen, thymus, tonsils, and lymph nodes.

The heart is a four-chambered double pump. Its pumping

action creates the pressure head needed to push blood through

Jessica went to her physician complaining of fatigue and

mentioned that she had been experiencing heavier men-

struations over the past several months. He mentioned

that she had mitral valve prolapse, but didn’t think that

was the cause of her fatigue and advised her take more

iron in her diet while they waited for the blood test results.

However, a subsequent ECG revealed that she had atrial

fibrillation, which he said might also explain her fatigue.

The physician prescribed a drug called rivaroxaban,

and told Jessica that she should perhaps exercise more

moderately and that she should definitely stop smoking.

Some of the new terms and concepts you will encoun-

ter include:

• Anemia, blood clotting factors, heart valves and
  heart sounds


• Electrocardiogram and heart arrhythmias


• Atherosclerosis, thrombosis, and cardiovascular
  diseases

Clinical Investigation

LEARNING OUTCOMES

After studying this section, you should be able to:

1. Identify the functions and components of the
   circulatory system.


2. Describe the relationship between interstitial fluid,
   plasma, and lymph.

A unicellular organism can provide for its own maintenance and
continuity by performing the wide variety of functions needed for
life. By contrast, the complex human body is composed of spe-
cialized cells that depend on one another. Because most are firmly
implanted in tissues, their oxygen and nutrients must be brought to
them, and their waste products removed. Therefore, a highly effec-
tive means of transporting materials within the body is needed.
The blood serves this transportation function. An estimated
60,000 miles of vessels throughout the body of an adult ensure
that continued sustenance reaches each of the trillions of living
cells. But the blood can also transport disease-causing viruses,
bacteria, and their toxins. To guard against this, the circulatory
system has protective mechanisms—the white blood cells and
the lymphatic system. In order to perform its various functions,
the circulatory system works together with the respiratory,