Homeostasis

Homeostasis
Sarah Jones
http://whitelines.com

Homeostasis
The condition of a relatively stable internal
environment, maintained within narrow
limits.
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• External environment can vary greatly e.g.
hot/cold.
• Cells require a relatively stable internal
environment.
everestviewtravels.com.au

Kinnear and Martin, Nature of Biology, 2006

• Extracellular fluids – tissue fluid and plasma
are located outside cells.
• Intracellular fluid – cytosol (part of cytoplasm)
is located within all cells.
• Exchange does occur – nutrients, oxygen,
carbon dioxide etc.
storify.com

Exchange

Body Systems
• Most body systems contribute in some way to
the maintenance of homeostasis.
• The endocrine and nervous systems are the
main contributes to maintaining a stable
internal environment
sites.google.com

The nervous system sends messages to the
appropriate organs and the endocrine system
secretes chemical messengers so the change can
be corrected.
www.babs.unsw.edu.au

Negative Feedback Loop
When a change is detected an action occurs to
produce a change in the opposite direction e.g.
rise body temperature

Variables controlled include
• Nutrients
• Temperature
• Water
• Ions, Na+, Ca+, Cl-
• pH
• Blood volume
• Blood pressure
• Oxygen
• Carbon dioxide
• Red blood cells
www.huffingtonpost.co.uk

Events that occur to maintain a relatively
constant level of blood glucose in a non-diabetic
person.

Negative Feedback
Stage 1: Detecting change from a stable state. In
this stage a sensor detects a change in a specific
variable. The fact that there has been an
undesirable change is then transmitted to the next
part of the control system.
Stage 2: Counteracting change. An effector
receives the message that an undesirable change
must be counteracted and the variable restored to
its desired level.
http://iheartguts.com

http://anatomyandphysiologyi.com/homeostasis-positivenegative-feedback-mechanisms/

Positive Feedback
Oxytocin produced by the posterior pituitary
gland stimulates contraction of the uterus and
also stimulates the pituitary gland to produce
even more of the hormone.

Endocrine System
commons.wikimedia.org

Hormones
The chemical nature of a hormone influences
the way in which it interacts with its target cells.
Pancreatic Hormone

(a) Some hormones act on the cell that produces
them.
(b) Some hormones diffuse to nearby cells.
(c) Endocrine signals — carried by the
bloodstream.

Amino acid derivatives — these are made in
advance by a cell and stored in secretory vesicles
until required. They leave the cell by exocytosis
or, if a precursor, by simple diffusion. They have
a short life span.

• Steroid hormones — these are synthesised on
demand from precursors in a cell and leave
the cell by simple diffusion. They have a long
life span.
• Protein hormones and peptide hormones —
made in advance by a cell and stored in
secretory vesicles. They leave the cell by
exocytosis and have a short life span.
www.uic.edu

Hormones, classified on the basis of their
chemical nature.

Exocrine and Endocrine Glands
• Exocrine glands secrete into a duct that carries
the secretion to the body surface or to one of the
body cavities e.g. sweat glands, mucous glands,
salivary glands
• Endocrine glands secrete hormones into the
extracellular fluid that surrounds the cells making
up the gland. The secretion then usually passes
into the capillaries to be transported by the
blood.

Organs Producing Hormones
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Hormones are only able to influence cells that
have the correct receptor for the hormone.
www.studyblue.com

Types of Hormones
• Protein and amine hormones work by
attaching to receptor molecules on the
membrane of the target cell.
• Steroid hormones work by entering target
cells and combining with a receptor protein
inside the cell.

Hypothalamus
• Located at the base of the brain.
• It regulates many of the basic functions of the
body, such as body temperature, water
balance and heart rate.
• Many of its functions are carried out through
the pituitary gland.

Pituitary Gland
• Approximately 13 millimetres in diameter, but
it is absolutely vital to the normal functioning
of the body.
• The pituitary consists of an anterior lobe and a
posterior lobe, each of which functions
separately.
learninghumananatomy.com

• The anterior (front) lobe has no nerves
connecting it to the hypothalamus but it is
connected to it by a complex network of blood
vessels.
• The posterior (rear) lobe is not a true gland
because it does not secrete substances.
www.austincc.edu

Anterior Pituitary Gland
• Hormones released by the anterior lobe of the
pituitary include the following:
• Follicle stimulating hormone (FSH)
– Target Organ: Ovaries and Testes
– In females: growth of follicles in the ovaries
– In males: production of sperm

• Luteinising hormone (LH)
– Target Organ: Ovaries and testes
– In females: ovulation and maintenance of corpus
luteum
– In males: secretion of testosterone
• Growth hormone (GH)
– Target Organ: All cells
– Growth and protein synthesis
en.wikipedia.org

• Thyroid stimulating hormone (TSH)
– Target Organ: Thyroid gland
– Secretion of hormones from the thyroid
• Adrenocorticotropic hormone (ACTH)
– Target Organ: Adrenal cortex
– Secretion of hormones from the adrenal cortex
• Prolactin (PRL)
– Target Organ: Mammary glands
– Milk production

Posterior Pituitary Gland
• Hormones released by the posterior lobe of
the pituitary include the following:
• Note these are not manufactured in the
posterior lobe, they are produced in the
Hypothalamus.
• Antidiuretic hormone (ADH)
– Target Organ: Kidneys
– Reabsorption of water

• Oxytocin (OT)
– Target Organ: Uterus
– Contractions of uterus during childbirth
– Target Organ: Mammary glands
– Release of milk
www.austincc.edu

The Thyroid Gland
Located in the neck just below the larynx.
musom.marshall.edu

• Thyroxine
– Target Organ: Most
– Increases metabolic rate and therefore oxygen
consumption and heat production

The Parathyroid Glands
• There are four parathyroid glands.
• Each is about the size of a small pea and they
are embedded in the rear surface of the lobes
of the thyroid gland.
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• Parathyroid hormone
– Target Organ: Bones and Kidneys
– Increases level of calcium in blood
musom.marshall.edu

The Thymus
• Located in the chest just above the heart and
just behind the sternum (breastbone).
• It is largest in infants and children but begins
to decline after puberty.
musom.marshall.edu

• Thymosins
– Target Cells: T lymphocytes
– Stimulates development and maturation of T
lymphocytes

The Adrenal Glands
• There are two adrenal glands, one
immediately above each kidney
• Each adrenal gland has an inner adrenal
medulla and an outer adrenal cortex.
• Each adrenal gland is really two separate
endocrine glands.

Corticosteroids
• Aldosterone
– Target Organ: Kidney
– Increases reabsorption of sodium ions and excretion
of potassium ions
• Cortisol
– Promotes normal metabolism; helps the body deal
with stress; promotes repair of damaged tissues

Adrenal Medulla
• Adrenaline and Noradrenaline
– Target Organ: Most tissues
– Prepares the body for fight-or-flight responses;
reinforces the effects of the sympathetic nervous
system

The Pancreas
• Lies just below the stomach and alongside the
duodenum, the first part of the small
intestine.
• It is both an exocrine gland and an endocrine
gland.
• Within the pancreas are clusters of special
cells called islets of Langerhans
• The islets are the endocrine part of the
pancreas and they secrete two important
hormones.

• Insulin
– Stimulates uptake of glucose; lowers blood
glucose level
• Glucagon
– Target Organ: Liver and fat storage tissues
– Stimulates breakdown of glycogen and fat;
increases blood glucose level
www.emaze.com

Testes
• Androgens (Testosterone)
– Target Organ: Many tissues
– Stimulate sperm production; growth of skeleton
and muscles and male sexual characteristics

Ovaries
• Oestrogens
– Target Organ: Many tissues
– Stimulate the development of female
characteristics; regulate the menstrual cycle
• Progesterone
– Target Organ: Uterus and mammary glands
– Regulates menstrual cycle and pregnancy;
prepares mammary glands for milk secretion

Nervous System
• The human body is made up of trillions of
cells.
• Cells of the nervous system, called nerve cells
or neurons, are specialised to carry
"messages" through an electrochemical
process. The human brain has approximately
100 billion neurons.

Central Nervous System
Brain and Spinal Cord

Peripheral Nervous System
From Spinal cord to the rest of your body.
www.eplantscience.com

Somatic Nervous System
• Transmits messages to skeletal muscles.
• Also called voluntary nervous system.
www.studyblue.com

Autonomic Nervous System
• Transmits messages to smooth muscle, heart
muscle and glands.
• Also called involuntary nervous system – fight
or flight.
• Has two parts – sympathetic and
parasympathetic nervous systems.
knowingneurons.com

• Sympathetic nervous system causes heart
muscles to contract more quickly, and the
metabolic rate to rise.
• parasympathetic nervous system directs body
functions when we are relaxed e.g. it
enhances digestion and emptying of the
urinary bladder.

Nerve Cell
http://en.wikipedia.org/wiki/File:Complete_neuron_cell_diagram_en.svg

• Neurons have specialised projections called
dendrites and axons. Dendrites bring
information to the cell body and axons take
information away from the cell body.
• Information from one neuron flows to another
neuron across a synapse.
scientopia.org

Three Main Parts
• The body of the neuron
containing the nucleus, the
dendrites and the axon that
ends with synaptic terminals
that will communicate with
other neurons.
• The myelin sheath increases
the rate at which a nerve
impulse is conducted along the
axon.

• Communication of information between
neurons is accomplished by the movement of
chemicals across a small gap called the
synapse.
• Chemicals, called neurotransmitters, are
produced at the end of a neuron’s axon. They
carry the impulse across the synapse to the
next neuron.

• Acetylcholine is the most common
neurotransmitter in the brain and in the
peripheral nervous system at neuromuscular
junctions
• Some neurons send signals via neurohormones.
• These neurohormones are released into the
blood, travel to a target organ where the signal is
transduced and the receptor cell responds.

Neuron Types
• Sensory (affector) Neurons - Relay messages
from receptor to the brain or spinal cord.
• Motor (effector) Neurons - Relay messages
from the brain or spinal cord to muscles and
organs.
• Interneuron (connecting) - Connects the
various neurons within the brain and spinal
cord.

Relationship between different kinds of neurons.

Transmission of Nerve Impulse
• A nerve that is not responding to a stimulus is
said to be ‘resting’.
• A small difference exists between the
electrical charge on the inside and outside its
cell membrane.

• Stimuli can activate neurons so that they
transmit nerve impulses along their axons =
‘excited’.
• As the impulse moves along the axon, a
change occurs in the permeability of the
membrane so that positive ions move into the
cell. This results in the outside of the
membrane becoming negative compared with
the inside.
• After a nerve impulse has been transmitted by
a neuron, the original distribution of ions
across the cell membrane is restored.

Sodium Potassium Pump
www.highlands.edu

Action Potential
Temporary change in the charge distribution as a
nerve impulse passes. This charge involves the
movement of ions, such as Na+, from the
outside to the inside of the neuron. The time
scale shown is one millisecond.

Myelin Sheath
• Some axons have an outer covering of a fatty
substance known as myelin.
• Nerve impulses travel much faster along these
axons.
• The speed of a nerve impulse along a myelin-
covered axon is about 200 m/s compared with
a speed of about 0.5 m/s along an axon that
lacks a myelin sheath.

Schwann Cell
• Schwann cells are a variety of glial cell that
keep the peripheral nerve fibres alive.
• In myelinated axons Schwann cells form the
myelin sheath.
www.kullabs.com

Efferent and afferent division of the
peripheral nervous system
Afferent nerve fiber is the nerve fiber (axon) of
an afferent neuron (sensory neuron). It is a long
process (projection) extending far from the
nerve cell body that carries nerve impulses from
sensory receptors or sense organs toward the
central nervous system.

Efferent nerve fiber is the nerve fiber (axon) of
an efferent neuron (motor neuron). It is a long
process (projection) extending far from the
nerve cell body that carries nerve impulses away
from the central nervous system toward the
peripheral effector organs (mainly muscles and
glands).
https://en.wikipedia.org/wiki/Afferent_nerve_fiber

Reflexes – receptor to effector
classroom.sdmesa.edu
wikis.engrade.com

Importance of learned reflexes for
reducing injury and survival
www.latimes.com

Flight or Fight Response
The sympathetic nervous system plays a major
role in the flight or fight response. Overall, the
autonomic nervous system allows the body to
react smoothly and quickly to change by
coordinating the action of muscles and glands.

Control of blood pressure

Maintaining core temperature

The Brain
http://downloads.bbc.co.uk/tv/scienceclub/

Role of Cerebral Cortex
The largest part of the brain is the cerebrum,
which has a folded surface called the cerebral
cortex. The cerebrum is divided into two halves,
called cerebral hemispheres, which are
connected by axons.
theconversation.com

• Thalamus receives impulses from sensory
neurons and directs them to the various parts
of the brain where they are interpreted.
• Hypothalamus regulates the release of many
hormones as well as controlling many other
aspects of homeostasis.
• The hypothalamus plays a role in temperature
maintenance, water balance and blood
pressure as well as sensations such as hunger
and thirst.

Role of the Cerebellum
• Controls -
– posture and balance
– fine motor coordination of voluntary muscle
– subconscious movements
– stops movements being spasmodic, jerky and
uncontrolled
– allows smooth coordinated movements such
as those required for writing, playing a
musical instrument or using a computer
keyboard

Homeostasis

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