HLTAP301A BASIC ANATOMY & PHYSIOLOGY
CHAPTER 1 – INTRODUCTION TO THE HUMAN BODY
What is anatomy and physiology? Anatomy is the study of the structure and shape of
the body, its parts and the inter-relationships that exist within these parts. Physiology is
the study of the function of those both parts. Structure defines function, yet anatomy
and physiology cannot be separated any more than a person can be separated into
mind, body and spirit.
Gross Anatomy studies structures that are visible to the human eye, whereas
microscopic anatomy includes Histology, the study of tissues and Cytology, the study of
cells. Systemic Anatomy studies the body systems individually and Regional Anatomy
studies a specific area of the body, ie. the knee. Developmental Anatomy is the study of
how anatomy changes over the life cycle. A sub-category of this is embryology, which
is the study of developmental changes within the body before birth.
Levels of Organisation of the Body
The human body had many levels of organisation and the six major levels that we will
learn about are:
Chemical level
Cellular level
Tissue level
Organ levels
Organ system level
Organism
The simplest level of the structural hierarchy is the chemical level. An atom is a basic
unit of matter – it consists of a central nucleus surrounded by negatively charged
electrons.
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The name atom comes from Greek atomos ‘un’ temno ‘to cut’ which means ‘uncuttable’
or something that cannot be divided further. It was once thought that an atom could not
be divided, however, we now know that is not true.
Atoms combine to form simple molecules, such as protein, sugar or water. These
molecules then combine to form organelles, which are the basic components of a cell.
The cellular level is comprised of many different types of cells – muscle cells, blood
cells, nerve cells.
The next level in the structure is the tissue level, which occurs when groups of cells
work together to perform a specific function. There are four basic types of tissues –
epithelial tissue, connective tissue, muscle tissue and nervous tissue.
When two or more different types of tissues join together, they form an organ. Organs
have very specific functions within the body. Organs may be recognised by their shape
– for example – the stomach.
The stomach is comprised of several tissues – the
outside covering is a serous membrane (epithelial tissue and connective tissue) that
protects the stomach. Underneath are the muscle tissue layers that contract to help
churn the food and push it into the small intestine. The inner lining of the stomach is an
epithelial tissue layer, which contributes fluid and chemicals to aid the digestive process.
The next level of structural organisation is the organ system level. A system consists of
related organs that have a common function. The digestive system is a good example
of this – it is comprised of organs such as the pharynx, oesophagus, stomach, small and
large intestines, rectum, liver, gallbladder and pancreas. Sometimes an organ can be
part of more than one system – for example, the pancreas is also part of the endocrine
system that produces hormones.
The largest level in the hierarchy the organism level – an organism is any living
individual. All the parts of the human body functioning with one another constitute the
total organism – one living person.
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The Body’s Organ Systems
There are eleven organ systems within the human body. They are the Integumentary
System, the Skeletal System, the Muscular System, the Nervous System, the Endocrine
System, the Cardiovascular System, the Lymphatic System, the Respiratory System,
the Digestive System, the Urinary System and the Reproductive (Male and Female)
System.
The Interrelationship of Body Organ Systems
The 11 body organ systems inter-relate with each other almost continually – For
example, the integumentary system (the skin) protects the body as a whole from the
external environment, while the respiratory system takes in oxygen and the digestive
system takes in nutrients from that external environment.
These substances are
distributed by the blood (cardiovascular system) to all body cells and the metabolic
wastes are eliminated by the urinary system, the digestive system and the respiratory
system.
Eight Functions to Maintain Life
The human organism must generate eight functions necessary to maintain life – these
functions are:
1.
Maintain boundaries
2.
Movement (fight or flee)
3.
Responsiveness (i.e. remove hand from a hotplate)
4.
Digestion
5.
Metabolism
6.
Excretion
7.
Reproduction
8.
Growth
Survival Needs
In order to survive, the human organism also needs nutrients, oxygen, and water, as
well as maintaining a normal body temperature and living in an atmosphere that is
conducive to life.
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Homeostasis
What is homeostasis? Homeostasis is a state of balance within the body. The human
body has the ability to maintain that state of balance or equilibrium internally within
relatively narrow limits. For example, body temperature must be maintained within set
boundaries to sustain life – if the body temperature gets too low, hypothermia and
eventually death will occur.
Homeostatic Control Mechanisms
Continual communication within the body must occur for the achievement and
maintenance of homeostasis. This communication is mostly commonly undertaken by
the nervous and endocrine systems.
Outside events or factors that occur may upset the homeostatic balance within the body.
For example, putting your hand on a hot surface, such as a cook top. When this occurs,
the receptor (nerve cells) that is monitoring the outside environment responds to the
stimuli of the body part touching a hot surface and sends a message along the afferent
pathway to the control centre (the brain).
The control centre analyses the input it
receives and determines what response or course of action is appropriate.
It then
sends a message along the efferent pathway to the effector, which will then stimulate
the muscles to move the hand off the hot surface. This is an example of a reflex arc
(produced by the nervous system) where the hand is jerked away from a painful
stimulus.
Two other examples of homeostatic control mechanisms are those of negative feedback
and positive feedback mechanisms.
The negative feedback mechanism works to
maintain homeostasis within the body, regardless of what is happening in the external
environment. The nervous system (more correctly, the autonomic division of the motor
nervous system) continually monitors what is going on in the body and when changes
need to be made to maintain homeostasis, the nervous system can either put those
changes into place itself, or call upon the endocrine system to help.
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For example, if someone was to consume a large chocolate bar and a 600ml bottle of
soft drink in a short period of time (about fifteen minutes), it would quickly affect the
body’s blood glucose levels (BGL’s).
Normal blood glucose levels are maintained
around 90mg of glucose per 100ml of blood – when we consume food, particularly food
high in carbohydrates (sugars) the BGL rises rapidly, disrupting homeostasis (or the
normal ‘balance’ of blood glucose levels).
Because high BGL’s are not ‘normal’ for the body, the negative feedback mechanism
recognises that there is something ‘wrong’ or ‘negative’ happening (which is why it is
called the negative feedback mechanism).
The nervous system stimulates the
pancreas (part of the endocrine system) to secrete insulin into the blood – insulin
accelerates the uptake of glucose by the cells and also encourages the liver to store
excess glucose as glycogen. Within a short period of time, the blood glucose levels drift
back towards the normal range and the stimulus for insulin diminishes and is eventually
shut off. Therefore we can say that negative feedback mechanisms recognise the need
for a change as the result of a stimulus (in this case the rising BGL’s) and once that
change brings the BGL’s back into a ‘normal’ range, that stimulus is shut off.
In a positive feedback mechanism, the response exaggerates the original stimulus so
that the output is accelerated – for example, when there is a break in the lining of a
blood vessel, platelets cling to the site of the injury and release chemicals that attract
more platelets. This rapidly growing pileup of platelets initiates the sequence of events
that finally forms a clot. So we can say that positive feedback mechanisms cause the
original stimulus (need for platelets) to be increased or exaggerated until a result is
achieved (more and more platelets arrive at the injury site until a clot is formed that
seals the ruptured blood vessel).
Childbirth is also a good example of a positive
feedback mechanism, as the labour contracts continue to increase (initial stimulus is
exaggerated) until a result (the birth) is achieved.
Homeostasis is so important that most disease is regarded as a result of a homeostatic
imbalance.
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A Frame of Reference for Anatomical Studies
The language of Anatomy can sometimes be confusing and even intimidating, but
anatomists use universally accepted terms to identify body structures.
To describe
parts of the body, we need an initial point of reference – this is called the ‘Anatomical
Position’.
In this position, the body is erect and standing with feet slightly apart. The palms of the
hands are facing forwards and the thumbs face away from the body. When health
professionals use directional terms in relation to a body, it is assumed that the body is in
this position and when we refer to left or right, it is the specific side(s) of the body being
viewed that we are referring to (not our own left or right).
Directional Terms are used to explain precisely where a body part or structure is, in
relation to another. The common directional terms are – Superior, Inferior, Posterior
(Dorsal)**, Anterior (Ventral)**, Medial, Lateral, Proximal, Distal, Superficial (External)
and Deep (Internal).
Superior means towards the head or upper part of the body. Inferior means towards the
feet or the lower part of the body. Posterior (Dorsal)* is towards or at the back of the
body. Anterior (Ventral)* is towards or on the front of the body. (** Anterior and Ventral
and Posterior and Dorsal are synonymous in humans but not in animals. For example,
the ventral surface is the ‘belly’ of a dog, and the dorsal surface is on their back.
Similarly, the dorsal fin is found on the back of a shark). Medial is towards or at the
midline of the body. Lateral is away from the midline or on the outer side of the body.
Proximal is closer to the origin of the body part or the attachment point of a limb to the
body trunk. Distal is farther from the origin of the body part or the attachment point of a
limb to the body trunk. Superficial (External) is towards or at the body surface. Deep
(Internal) is aware from the body surface or more internal.
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Regional Terms
Body Planes are sectional surfaces used in anatomical studies. The major body planes
are Frontal, Sagittal and Mid-Sagittal and Transverse. The Frontal plane divides the
body into anterior and posterior parts. It may also be called the Coronal plane. The
Sagittal plane is a vertical plane that divides the body into left and right parts. A sagittal
plane that lies exactly in the midline is called a Midsagittal plane. A Transverse (or
horizontal) plane runs horizontally from left to right, dividing the body into superior and
inferior parts – this may also be referred to as a ‘cross section’.
Body Cavities
Within the Axial part of the body there are two large cavities called the ‘dorsal body
cavity’ and the ‘ventral body cavity’. The dorsal (or posterior) body cavity protects the
nervous system organs and is divided into two cavities – the cranial cavity encasing the
brain and the vertebral or spinal cavity enclosing the spinal cord.
The ventral (or anterior) body cavity houses a group of internal organs collectively called
the ‘visceria’, and has two major subdivisions – the thoracic cavity and the
abdominopelvic cavity. The thoracic cavity, surrounded by the ribs and muscles of the
chest is further subdivided into the lateral pleural cavities, each housing a lung; and the
mediastinum, which has the pericardial cavity (enclosing the heart) and surrounds the
esophagus, trachea, etc.
The abdominopelvic cavity is separated from the thoracic cavity by the diaphragm and
has two parts – the abdominal cavity and the pelvic cavity - although they are not
physically separated by a muscle or membrane wall.
The abdominal cavity contains
the stomach, intestines, spleen and liver. The pelvic cavity contains the bladder, some
of the reproductive organs and the rectum.
References:
Marieb, E.N. (2012). Essentials of Human Anatomy & Physiology, (10th ed.). San
Francisco, USA: Pearson International.
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