Human Physiology, 14th edition (2016)

Interactions Between Cells and the Extracellular Environment 131

of a tissue, the cells of different tissues within an organ, and the

cells of different organs interact with each other through chem-

ical regulators secreted into the extracellular environment.

Body Fluids

The water content of the body is divided into two compart-

ments. Approximately 67% of the total body water is contained

within cells, in the intracellular compartment. The remain-

ing 33% of the total body water is found in the extracellular

compartment. About 20% of this extracellular fluid is con-

tained within the vessels of the cardiovascular system, where

it constitutes the fluid portion of the blood, or blood plasma.

The blood transports oxygen from the lungs to the body

cells, and carbon dioxide from the body cells to the lungs. It

also transports nutrients derived from food in the intestine to

the body cells; other nutrients between organs (such as glucose

from the liver to the brain, or lactic acid from muscles to the

liver); metabolic wastes from the body cells to the liver and

kidneys for elimination in the bile and urine, respectively; and

regulatory molecules called hormones from endocrine glands

to the cells of their target organs.

The remaining 80% of the extracellular fluid is located

outside of the vascular system, and makes up the tissue fluid,

also called interstitial fluid. This fluid is contained in the

gel-like extracellular matrix, as described in the next section.

Body fluid distribution is illustrated in chapter 14, figure 14.8,

in conjunction with a discussion of the cardiovascular system.

This is because the interstitial fluid is formed continuously

from blood plasma, and it continuously returns to the blood

plasma through mechanisms described in chapter 14 (see

fig. 14.9). Oxygen, nutrients, and regulatory molecules travel-

ing in the blood must first pass into the interstitial fluid before

reaching the body cells; waste products and hormone secre-

tions from the cells must first pass into the interstitial fluid

before reaching the blood plasma ( fig. 6.1 ).

6.1 Extracellular Environment

The extracellular environment surrounding cells consists of

a fluid compartment in which molecules are dissolved, and

a matrix of polysaccharides and proteins that give form to

the tissues. Interactions between the intracellular and extra-

cellular environment occur across the plasma membrane.

Jessica, a physiology student who takes a diuretic drug

for high blood pressure, says she drinks water constantly

and yet is constantly thirsty. During a lab exercise, she

discovers that her urine contains glucose. Alarmed, she

goes to her physician, who tests her blood and urine and

obtains an electrocardiogram (ECG).

Some of the new terms and concepts you will

encounter include:

• Hyperglycemia, glycosuria, and hypokalemia


• Osmolality and osmotic pressure

Clinical Investigation

LEARNING OUTCOMES

After studying this section, you should be able to:

1. Describe the intracellular and extracellular
   compartments of the body.


2. Identify the components of passive transport, and
   distinguish passive from active transport.

Figure 6.1 The extracellular environment. The extracellular environment contains interstitial tissue fluid within a matrix
of glycoproteins and proteoglycans. Interstitial fluid is derived from blood plasma that filters through the pores (not shown) between
the cells of the capillary walls, and delivers nutrients and regulatory molecules to the tissue cells. The extracellular environment is
supported by collagen and elastin protein fibers, which also form the basal lamina below epithelial membranes.

Glycoproteins and

proteoglycans of

extracellular matrix

Interstitial

fluid

Blood Elastin protein fibers

Blood capillary

Collagenous

protein fibers

Basal lamina

(basement membrane)

Epithelial membrane

The extracellular environment includes all constituents of the
body located outside of the cells. The cells of our body must
receive nourishment from, and release their waste products
into, the extracellular environment. Further, the different cells