CLINICAL

systems of life

Homeostasis Part 1: anatomy and physiology

Authors Brendan Doherty, MSc, PGCE, RN, is Artery Plasma Extracellular
nurse patient access manager, Prince of Wales fluid
Hospital, Sydney, Australia; Colette Foudy, RN,
GradDip, is clinical care coordinator, intensive care
unit, St George Private Hospital, Sydney, Australia.

This article, the first in a series of four, looks at the

anatomy and physiology of homeostasis.
Homeostasis comprises the dynamic processes that
enable optimum conditions to be maintained for
cells, in spite of continual changes taking place Arteriole Cell
internally and externally (Clancy and McVicar, 1995).
All the systems of the human body are involved,
with particular contributions by the endocrine, Venule Capillary
nervous, respiratory and renal systems. Whenever an
imbalance occurs, regulatory systems become active
Capillary
to restore optimum conditions, usually by a process
Fig 1. extracellular
known as negative feedback in which a deviation fluid and blood vessels
from the normal level is detected and initiates Vein
changes that bring the level back to where it should
be (Clancy and McVicar, 1995). These systems have to Brain Ear Pineal gland Hypothalamus
endure for survival and adapt to modifications of the
environment so must therefore evolve. Eyes

Anatomy and physiology Taste and smell Spinal cord Thyroid Pituitary
Many factors affect the suitability of body fluids to sensors
sustain life. These will be explored later in this series.
Thymus
They include:
l Oxygen (O2) and carbon dioxide (CO2) Andrenals
concentrations;
l The pH of the internal environment; Pancreas
l Concentrations of nutrients and waste products;
l Concentration of salt and other electrolytes
(osmoregulation);
l Volume and pressure of extracellular fluid; Ovaries
l Body temperature;
l Blood glucose level.
As these properties affect the chemical reactions that
sustain life, there are built-in physiological mechanisms
to maintain them at desirable levels. The body needs Skin Testes
homeostasis to maintain stability and survive by sensors
ensuring that the internal environment remains
relatively constant (Tortora and Anagnostakos, 2003).
To enable cells to survive, the composition of the Nerves
intracellular and extracellular fluids must be
accurately maintained at all times. Intracellular fluid
accounts for two-thirds of total water content (Tortora
and Anagnostakos, 2003). Extracellular fluid includes
gases, nutrients, plasma (in blood vessels) and ions, Nervous system endocrine system
Johnny Zygo

all of which are necessary for maintaining life

(Tortora and Anagnostakos, 2003) (Fig 1). Fig 2. The endocrine and central nervous systems
Extracellular fluid circulates constantly within

26 NT 4 April 2006 Vol 102 No 14 www.nursingtimes.net

 keywords n Anatomy and physiology n Homeostatic control

the blood and lymphatic system and is known as Negative feedback References
the bodys internal environment (Tortora and Homeostasis is constantly disturbed by external
Anagnostakos, 2003). factors, which can be described as a form of stress Bodyguide (2005) Endocrine
The body is said to be in homeostasis when its on the internal environment (Tortora and System. www.besthealth.com/
internal environment contains: Anagnostakos, 2003). These stresses can be: besthealth/bodyguide/reftext/html/
l Optimum levels of gases, ions, water and nutrients; l Internal: occurring within the body, for example in endo_sys_fin.html#homeostatic.
l Is at optimal temperature; pain, or as a result of high or low blood pressure;
l Has optimal pressure for the health of cells. l External: occurring outside the body, for example as Clancy, J., McVicar, A. (1995)
A disturbance in these optimum conditions causes a result of heat, cold or loud noises. Physiology and Anatomy:
failure of the organs and may lead to death (Tortora Since these stresses affect the chemical reactions A Homeostatic Approach.
London: Edward Arnold Publishers.
and Anagnostakos, 2003). sustaining life, there are built-in physiological
mechanisms to maintain or return them to
Docherty, B. (2005) The
Homeostatic control desirable levels.
arteriovenous system: part one, the
The endocrine and central nervous systems are Each organ or structure has its own intrinsic way
anatomy. Nursing Times; 101: 34,
the major control systems for regulating homeostasis of keeping the internal environment within normal 2829.
(Tortora and Anagnostakos, 2003) (Fig 2). The limits. When homeostasis is altered there are two
endocrine system consists of a series of glands possible responses: Docherty, B. (2002)
that secrete chemical regulators (hormones). The l In negative feedback the system responds Cardiorespiratory physical
nervous system can detect deviation from the to reverse the direction of change. As this tends assessment for the acutely ill: part
bodys normal equilibrium (state of homeostasis) to keep things constant, it allows the maintenance one. British Journal of Nursing;
and sends messages to the affected organ to of homeostasis. For example, if there is a fall 11: 11, 750758.
counteract this disturbance. Over a relatively in calcium in the blood, the parathyroid glands
short time it restores the required balance. Both sense the decrease and secrete more parathyroid Sherwood, L. (1997) Human
systems act mostly automatically but there is some hormone, thereby increasing calcium release from Physiology: From Cells to Systems.
voluntary control over the nervous system the bones; Pacific Grove, CA: West Publishing.
(Sherwood, 1997). l Positive feedback increases the variable in the
Tortora, G.J., Anagnostakos, N.P.
The control and maintenance of blood sugar levels same direction, a destabilising effect that does
(2003) Principles of Anatomy and
is an example of homeostatic regulation by the not result in homeostasis. Positive feedback is used
Physiology. New York, NY: John
endocrine system. Blood sugar is maintained by two when rapid change is needed. For example in
Wiley and Sons.
hormones secreted by the pancreas: insulin and childbirth the hormone oxytocin is produced to
glucagon. Blood sugar rises after digestion of food. In stimulate and enhance labour contractions
response, pancreas cells are stimulated to secrete (Bodyguide, 2005).
insulin, which enables sugar uptake by cells and the
storage of sugar in the liver and muscles. In effect, Components of homeostasis
insulin decreases blood sugar levels to normal A system requires three components for homeostasis:
(Tortora and Anagnostakos, 2003). l A receptor;
The respiratory system provides an example l A control centre;
of homeostatic regulation by the nervous system. l An effector.
In normal breathing there is a state of homeostasis. These components do specific jobs that allow
During exercise the respiratory system must work regulation of the internal environment.
faster to keep the O2 in the extracellular fluid and A receptor detects external changes that could
in the cells within normal limits, preventing excessive influence the internal environment. For example,
build-up of CO2 and disturbance to the blood pH the following are involved in the regulation of
through the accumulation of acid (Tortora and blood pressure:
Anagnostakos, 2003). Because muscles require l Receptors are in the baroreceptor system;
more O2 during exercise, more CO2 is released and l The control centre is the medulla oblongata;
therefore also needs to be excreted (Tortora and l The effector is the cardiovascular system.
Anagnostakos, 2003). When there is an increase in heart rate more blood
These chemical changes are detected in certain is pumped into the arteries resulting in an increase in
nerve cells, which send this message to the cardio- blood pressure (Docherty, 2005). This is detected by
respiratory centre in the medulla oblongata in the baroreceptors, which are located in the walls of
the base of the brain (Docherty, 2002). The brain certain arteries.
sends a message to the heart to increase its pumping These receptors send impulses to the control
action (heart rate) to take on more O2 and enable centre (the medulla oblongata), which interprets This article has been double-blind
the blood to give up excess CO2. Respiratory muscles the message and sends impulses to the effectors peer-reviewed.
also receive instructions from the brain to contract (the cardiovascular system). These slow the For related articles on this subject
faster, enabling a rise in both O2 delivery and pulse, decreasing blood pressure (Tortora and and links to relevant websites see
CO2 exhalation. Anagnostakos, 2003). n www.nursingtimes.net

NT 4 April 2006 Vol 102 No 14 www.nursingtimes.net 27