The circulatory, respiratory, digestive, and excretory systems are involved in
the exchange of materials across cell membranes. All of these systems
have special features that provide large surface areas to allow for necessary
exchanges.
The circulatory system consists of a pump, the heart, and blood vessels
that distribute the blood to all parts of the body. The blood is a carrier
fluid that transports molecules and heat. The exchange of materials between
the blood and body cells takes place through the walls of the capillaries.
Hemoglobin in red blood cells is very important in the transport of
oxygen.
The skin is the outer covering of the body and performs several
functions, including protecting underlying tissues, preventing water loss,
and regulating temperature.
The respiratory system consists of the lungs and associated tubes that
allow air to enter and leave the lungs. The diaphragm and muscles of the
chest wall are important in the process of breathing. In the lungs, tiny sacs
called alveoli provide a large surface area in association with capillaries,
which allows for rapid exchange of oxygen and carbon dioxide.
The digestive system is involved in disassembling food molecules.
This involves several processes: grinding by the teeth and stomach, emulsification of fats by bile from the liver, addition of water to dissolve molecules, and enzymatic action to break complex molecules into simpler
molecules for absorption. The intestine provides a large surface area for
the absorption of nutrients because it is long and its wall contains many
tiny projections that increase surface area.
To maintain good health, people must receive nutrient molecules
that can enter the cells and function in the metabolic processes. The
proper quantity and quality of nutrients are essential to good health. An
important measure of the amount of energy required to sustain a human
at rest is the basal metabolic rate. To meet this and all additional requirements,
the United States has established recommendations for each nutrient.
Should there be metabolic or psychological problems associated
with a person's normal metabolism, a variety of disorders may occur,
including obesity, anorexia nervosa, bulimia, kwashiorkor, and vitamin deficiency
diseases.
The excretory system is a filtering system of the body. The kidneys
consist of nephrons into which the circulatory system filters fluid. Most of
this fluid is useful and is reclaimed by the cells that make up the walls of
these tubules. Materials that are present in excess or those that are harmful
are allowed to escape.
A nerve impulse is caused by sodium ions entering the cell as a result
of a change in the permeability of the cell membrane. Thus, a wave of depolarization
passes down the length of a neuron to the synapse. The axon
of a neuron secretes a neurotransmitter, such as acetylcholine, into the
synapse, where these molecules bind to the dendrite of the next cell in the
chain, resulting in an impulse in it as well. The acetylcholinesterase present
in the synapse destroys acetylcholine so that it does not repeatedly stimulate the dendrite.
The body's various systems must be integrated in such a way that the
internal environment stays relatively constant. This concept is called
homeostasis.
Several kinds of sensory inputs are possible. Many kinds of chemicals
can bind to cell surfaces and be recognized. This is probably how
the sense of taste and the sense of smell function. Light energy can be
detected because light causes certain molecules in the retina of the eye
to decompose and stimulate neurons. Sound can be detected because
fluid in the cochlea of the ear is caused to vibrate, and special cells detect
this movement and stimulate neurons. The sense of touch consists
of a variety of receptors that respond to pressure, cell damage, and
temperature.
There are two major kinds of organs that respond to nervous and
hormonal stimulation: muscles and glands. There are three kinds of muscle:
skeletal muscle, which moves parts of the skeleton; cardiac muscle—
the heart; and smooth muscle, which makes up the muscular walls of
internal organs.Glands are of two types: exocrine glands, which secrete through
ducts into the cavity of an organ or to the surface of the skin, and endocrine
glands, which release their secretions into the circulatory system.
It is becoming clear that the endocrine system and the nervous system are
interrelated. Actions of the endocrine system can change how the nervous
system functions, and the reverse is also true.
