Body Fluid Compartments

Tissues, organs, and systems can all be divided into two major parts, or compartments. The intracellular compartment is that part inside the cells; the extracellular compartment is that part outside the cells. Both compartments consist primarily of water—

Table 1.4

Organ Systems of the Body


Major Organs

Primary Functions


Skin, hair, nails

Protection, thermoregulation


Brain, spinal cord, nerves

Regulation of other body



Hormone-secreting glands,

Secretion of regulatory

such as the pituitary,

molecules called hormones

thyroid, and adrenals


Bones, cartilages

Movement and support


Skeletal muscles

Movements of the skeleton


Heart, blood vessels,

Movement of blood and

lymphatic vessels



Bone marrow, lymphoid

Defense of the body against


invading pathogens


Lungs, airways

Gas exchange


Kidneys, ureters, urethra

Regulation of blood volume

and composition


Mouth, stomach, intestine,

Breakdown of food into

liver, gallbladder, pancreas

molecules that enter the



Gonads, external genitalia,

Continuation of the human

associated glands and ducts


they are said to be aqueous. The two compartments are separated by the cell membrane surrounding each cell (see chapter 3).

The extracellular compartment is subdivided into two parts. One part is the blood plasma, the fluid portion of the blood. The other is the fluid that bathes the cells within the organs of the body. This is called tissue fluid, or interstitial fluid. In most parts of the body, blood plasma and tissue fluid communicate freely through blood capillaries. The kidneys regulate the volume and composition of the blood plasma, and thus, indirectly, the fluid volume and composition of the entire extracellular compartment.

There is also selective communication between the intra-cellular and extracellular compartments through the movement of molecules and ions through the cell membrane, as described in chapter 6. This is how cells obtain the molecules they need for life and how they eliminate waste products.

Test Yourself Before You Continue

1. State the location of each type of primary tissue in the skin.

2. Describe the functions of nervous, muscle, and connective tissue in the skin.

3. Describe the functions of the epidermis and explain why this tissue is called "dynamic."

4. Distinguish between the intracellular and extracellular compartments and explain their significance.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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