Optional Biology Notes. Dr. M. J. Butler. 1
Chapter 19: Animal Tissues and Organs (20)
Sections 19.1, 19.6 will not be tested
This chapter introduces animal anatomy and physiology. Note that the feedback
mechanisms that control body parameters such as temperature, blood sugar
levels etc are negative feedback mechanisms, they act to keep parameters
within accepted limits. BUT, there is also such a thing as a positive feedback
mechanism, and this acts to drive a parameter ever "higher". There are very few
of these in the human body, an example is the positive feedback mechanism
that drives ever increasing contractions of the uterus during childbirth, these
only cease when the child is born and the pressure on the uterine wall is
relieved and this signals contractions to stop. Positive feedback mechanisms are
The nervous and the endocrine systems are the two systems that are largely in
control of homeostasis in the human body.
Animal Tissues, Organs, and Homeostasis
A tissue is a group of cells that assemble and interact to perform a specific function.
An organ is a number of different tissues assembled together to perform a specific
function. Examples are hearts, lungs, liver.
Organ systems consist of a number of different organs organized to perform a specific
function. Examples are the skin and the circulatory system.
There are four types of animal tissue: epithelial, connective, muscle, nervous
Epithelial tissues form surfaces, external and internal, they are underlain by connective
tissue called basement membrane. Epithelial cells are generally flattened or cube
Epithelium forms sheets of cells that provide covering surfaces inside and on the surface
of the body (the skin). These include the lining of the lungs, digestive tract, blood vessels,
and the reproductive and urinary tracts
Tight junctions between epithelial cells prevent fluids leaking from cell to cell in tissues
because they seal adjacent cells together. Tight junctions are found in stomach
epithelium where they prevent acid leakage. Gap junctions between epithelial cells are
protein complexes that form pores that permits flow of cytoplasm between cells (as in Optional Biology Notes. Dr. M. J. Butler. 2
cardiac cells). Adhering junctions of epithelial cells are protein “spot welds” that hold
cells together (as in skin).
NB, be careful of the careless terminology used in some textbooks with regards to cell to
cell junctions, they often write of making seals between adjacent cell walls, but animal
cells do not have walls.
Glands are derived from epithelium, there are two kinds, exocrine which release
substances such as mucus, tears, saliva, oil, milk etc through ducts, and endocrine glands
which release hormones directly from their surfaces into the body fluids - and do not
Connective tissues, with the exception of blood, connective tissues are generally
produced by fibroblast cells that produce fibrous connective proteins called collagen and
elastin. These proteins are generally embedded in secreted polysaccharides that form the
ground tissue in which the fibres and cells are embedded. Tendons and elastic ligaments
are connective tissues. Special connective tissues include bone, fat (so called adipose
tissue), and blood.
Muscle and Nervous tissues are dealt with in detail in later chapters.
Specific Organ systems are discussed in detail in later, but note here that there are a
number of body cavities that contain specific organ systems.
The skin is an example of an organ system; it is the largest part of the integument (body
covering). Skin is a complex layered multifunctional organ system which functions in
protection against the sun, desiccation, entry of infectious agents, maintenance of body
temperature by means of sweat glands and nervous control of blood flow in small vessels
near the skin.
The outermost region of the skin is the epidermis - consisting mostly of dead flattened
keratinocyte cells that provide waterproofing. The underlying dermis region is live and
is mostly dense connective tissue and supports the epidermal layer as well as containing
nerve endings. Sweat and oil glands are of epidermal origin but are embedded in the
Homeostasis. This is a KEY physiological concept. Homeostasis can be defined as the
maintenance of a constant internal physiological environment by a cell and/or an
Homeostasis is a dynamic process that involves the interrelationship of numerous factors
to maintain a given biological process within appropriate narrow limits.
The nervous system and the hormonal system are the two key systems that control
homeostasis, and it is the mechanism of negative feedback that operates to accomplish
this control. These systems are covered in other chapters. Optional Biology Notes. Dr. M. J. Butler. 3
Feedback mechanisms maintain physiological status. Most physiological processes that
require maintenance of a narrow range of some aspect, such as body fluid pH,
temperature, ionic strength, etc are controlled by negative feedback mechanisms
undertaken by hormones and the nervous system.
In negative feedback a parameter under control is perceived by a sensor to have
increased or decreased beyond a given optimal range, as soon as this is perceived the
negative feedback mechanism acts to bring that property back within the acceptable range
by decreasing and reversing the direction of the change.
The classic example is body temperature. If the body senses that you are too hot you
begin to sweat, evaporation of the sweat on the skin causes you to cool down to the
acceptable range of body temperature, once this happens the body senses a normal body
temperature and stops the sweating process. Thus the body has acted to decrease the
change in the property that it sensed, this is negative feedback.
If the body senses it is too cold it causes shivering to occur, this requires metabolic
activity that produces heat (shivering of muscles), once the body senses that enough heat
has been produced to bring back the body to the acceptable temperature range, shivering
stops. The body reacted to a change in the property it sensed and reversed the direction of
that change - thus negative feedback has occurred.
There is also positive feedback, but this is not homeostatic, it drives a certain parameter
out of a normal range - it accelerates the change, the rapidly increasing uterine
contraction rate during childbirth is an example; release of oxytocin from the pituitary
causes uterine contractions, signals generated by those contractions go to the brain which
causes more oxytocin to be released, this means more contractions which in turn mean
even stronger signals to go to the brain from the uterus, to cause even more oxytocin to
be released and so on, the effect only ceases when the ba