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Homeostasis

A presentation used to inform students in my Senior Biology Class about the nature of keeping internal balance.
by

Jeffery Wells

on 12 May 2014

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Transcript of Homeostasis

HOMEOSTASIS
Maintenance of a constant internal environment (within narrow limits) through regulatory mechanisms that compensate for a changing external environment
Unit #4 Overview
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In the study of homeostasis we will examine...
mechanisms to maintain internal balance
the role of the kidney in maintaining water and ion balance
hormonal and nervous control of homeostasis
What areas of the body are under homeostatic control?
Blood pH
Body Temperature
Blood Sugar
Water Balance
WHY THE NEED FOR SUCH CONTROL?
Enzyme Function
optimal conditions required to ensure proper functioning
Cell Function
water and ion balance within cell have affect on function
Feedback Loops
Mechanisms that help regulate homeostasis
Feedback loops consist of:
Sensors to detect changes in the internal environment
A comparator which fixes the set point of the system (e.g. body temperature). The set point will be the optimum condition under which the system operates
Effectors which bring the system back to the set point
Feedback control. Negative feedback stops the system over compensating (going too far)
A communication system to link the different parts together
Thermoregulation
The Endocrine System
The Nervous System
Kidney Function
The Immune System
Dynamic Equilibrium
condition that remains stable within fluctuating limits
Negative Feedback
Positive Feedback
prevent small changes from becoming too large
Reinforce a change whereas Negative FB resists change
move the system away from range
accomplishes a physiological task more rapidly
Hormone Oxytocin triggers uterine contraction that increases the strength of the next contraction
Mammals are homeothermic (36-38oC) and endothermic (body heat derived from metabolism)

Adaptations:
Behavioural: relocation, clothing
Structural: hair, fur, feathers
Physiological: shivering, hormones (adrenalin) to increase metabolic rate, sweating, vasoconstriction/ vasodilation
Balance between heat production/gain and heat loss
Thermoreceptors detect a change in temperature and send a signal to the hypothalamus

If body temp goes below 36 degrees
Inhibition of heat loss mechanism and activation of heat saving mechanisms (vasoconstriction, erection of body hair) and heat generating mechanisms: shivering, secretion of adrenaline

If body temp goes above 38 degrees
Inhibition of heat saving/generating mechanisms
Stimulation of heat loss mechanisms (vasodilation, sweating)
http://www.wordle.net/show/wrdl/2777653/Homeostatic_Mechanisms
Practical Applications?
Kidney Function in Waste Excretion
Excretion is the removal of metabolic wastes (i.e. CO2 and nitrogenous wastes)
Ammonia – major waste product; toxic (even in small amounts); end product of protein and nucleic acid catabolism
Humans excrete nitrogenous waste in the form of UREA which is less toxic than ammonia
Collects urine from renal medulla
Renal artery: contains oxygen, glucose, wastes
Renal vein: contains carbon dioxide
Carriers urine to bladder
removes waste and toxin
osmoregulation
regulates blood volume; thus blood pressure
Ion regulation
maintains blood pH
Na+, K+, HCO3, Ca2+, PO4
The Nephron
Functional Unit of the Kidney
Makes urine by filtering blood, removing small molecules and ions then reclaiming useful materials before excreting urine
Urine Formation
Filtration
Reabsorption
Secreti n
Blood from renal artery is forced into glomerulus under high pressure
Much of the liquid is forced out of the glomerulus into the Bowman’s capsule
Glomerular Filtrate – water, urea, salt, glucose and other chemicals filtered out of the blood
Glomerular Filtration Rate (GFR) = 125 ml of water and dissolved substances per minute
What is the GFR per hour? Per day?
return of substances from filtrate to blood
active release of substances by cells lining the nephron into the nephron tubule
Proximal Tubule
About 2/3 of salt and water filtered are reabsorbed by active transport and osmosis

Material Reabsorbed Material Secreted
Glucose H+
Amino acids Toxins
K+
HCO3-
Descending Loop of Henle
Permeable to water but NOT sodium; water is reabsorbed by osmosis resulting in decrease water in filtrate
Ascending Loop of Henle
Permeable to sodium but NOT water; NaCl diffuses out of thin segment and is actively transported out of thick segment
Distal Tubule
Secretion of K+ and H+ by active transport
Reabsorption of HCO3 to maintain pH
Reabsorption of NaCl and water
Collecting Duct
Cells are permeable to water which is therefore reabsorbed
Urea also reabsorbed to increase osmotic pressure of interstitial fluid
ADH
Anti
Diuretic
Hormone
– increases the permeability of collecting ducts to water which increases water reabsorption in the blood
When would this occur?
Decreased water intake or increased water loss
Response?
Osmoreceptors in hypothalamus detects pressure change leading to release of ADH
When ADH is released less urine is produced
ALDOSTERONE
regulates blood pressure by adjusting blood volume.
Drop in blood pressure (BP) or blood volume in the afferent arteriole detected by the Juxtaglomerular Apparatus (JGA)
Response?
Stimulus?
JGA release RENIN
Angiotensin II
constricts arterioles = dec. blood flow = inc. blood pressure
stimulates proximal tubule to reabsorb Na+
raises blood volume
stimulates adrenal gland to release Aldosterone
stimulates distal tubule to reabsorb Na+ and Water
Redundancy of ADH and Aldosterone?
Both: increase water reabsorption
Difference Maker: excessive loss of salt (i.e. diarrhea)
Reduces blood volume without an increase in osmolarity
In this case ALDOSTERONE SAVES THE DAY!!
responsible for the maintenance of homeostasis by way of a series of chemical messenger systems coordinated by the hypothalamus and pituitary gland
interprets stimuli from our senses and integrates an appropriate, immediate response
GENERAL FUNCTION OF NERVOUS SYSTEM
SENSORY
MOTOR
INTEGRATION
gathering information
monitoring changes inside and outside body (stimuli)
To process and interpret sensory input and decide if action is needed
A response to integrated stimuli
response activates muscles or glands
CELL TYPES
HOW DOES INFORMATION MOVE ALONG A NEURON?
SYNAPTIC CLEFT
FUNCTIONAL CLASSIFICATION OF THE NERVOUS SYSTEM
CENTRAL NERVOUS SYSTEM
(integration centre)
PERIPHERAL NERVOUS SYSTEM
(sensory and motor control)
Brain and Spinal Cord
Nerves outside the CNS
Brain Anatomy and Function
Cerebrum
Brain Stem
Cerebellum
receives sensory input
sends motor impulses to skeletal muscles
special senses (gustatory, visual, auditory and olfactory)
interpretation of speech and language
Midbrain
relay station for auditory and visual information
Pons
assist with breathing
sleep cycles
Medulla Oblongata
Heart rate control
Blood pressure regulation
Breathing
Swallowing
Vomiting
Coughing/sneezing
coordination of voluntary motor movement, balance and equilibrium and muscle tone
coordination of voluntary motor movement, balance and equilibrium and muscle tone
NEUROLOGICAL DISORDERS DUE TO BRAIN DAMAGE
Cerebellar Ataxi
Dystonia
Verbal Apraxia
EFFECTS OF THE ANS
Sympathetic NS prepares the body for stress
Increases HR
Increase release of glucose
Increases skin blood flow
Stimulates adrenal gland to release Epi

Parasympathetic NS returns body to normal
Astrocytes
Abundant, star-shaped cells
Brace neurons
Form barrier between capillaries and neurons
Control the chemical environment of the brain
Oligodendrocytes
Produce myelin sheath around nerve fibers in the central nervous system
Supporting Cells
Microglia Cells
phagocytic cells that digest debris
Ependymal Cells
circulate cerebrospinal fluid
Satellite and Schwann Cells
protect neurons and form myelin in PNS neurons
The Neuron
Anatomy of a Nerve Cell (Neuron)
Dendrite – receives sensory information
Axon – carries nerve impulses toward other neurons or effectors (gland or muscle)
Myelin sheath – insulation formed by Schwann cells
Nodes of Ranvier – areas along an axon between myelin
Neurilemma – protective membrane surrounding axon and assist with damage control
REFLEX ARC
Reflexes are involuntary
Reflex arcs aid in our safety since they operate extremely fast.
Components of reflex arc:
Receptor
Sensory neuron
Interneuron
Motor neuron
Effector

ELECTROCHEMICAL IMPULSE
What is an action potential?
How do nerve cell membranes become charged?
How does an action potential move along a neuron?
How does threshold level of a neuron relate to interpretation of signal intensity?
How do nerve cell membranes become charged?
Rich supply of positive and negative ions both inside and outside the nerve cell. A membrane potential is created due to:
unequal concentrations of ions
the selective permeability of the neuron membrane
Large ions cannot cross
High [Na+]
High [K+]
Adequate concentrations of Na+ and K+ are maintained by the sodium -potassium pump
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html
What is an Action Potential?
voltage difference across a nerve cell membrane when the nerve is excited.
At Rest (Resting Potential): -70 mV
Outside (+), inside (-) – due to high [Na+] on outside and large negatively charged molecules on the inside (proteins, amino acids, phosphates)
Membrane is more permeable to K+; K+ diffuse out faster than Na+ diffusing in
Movement of ions controlled by gated ion channels
In Response to a Stimulus...
Depolarization
Repolarization
Undershoot/Hyperpolarization
K+ gated channels close
Na+ gated channels open = allows increase flow of Na+ into the axon
K+ gated channels open; K+ moves out of the axon
Na+ gated channels close
slow K+ gates remain open
more K+ out = inside more negative

Refractory Period - a second AP cannot be generated at this time
How does an AP travel along a neuron?
After Na+ ions diffuse into the axon they become attracted to adjacent negative ions causing depolarization of the adjacent membrane.
Myelin and Saltatory Conduction
Myelin sheaths surrounding the axon allows for faster signal propagation than an unmyelinated axon – process known as saltatory conduction
http://www.brainviews.com/abFiles/AniSalt.htm
Threshold and Intensity
each neuron has a specific threshold which must be reached in order to be excited

This threshold must be reached or the action potential will NOT fire at all (All-or-None Principle)

More intense the stimulus = greater frequency of neuron firing
Therefore, the greater number of impulses reaching the brain = the greater the intensity of response
Cooking Frogs
How do AP's pass from nerve to nerve, nerve to muscle, nerve to gland?
When a wave of depolarization reaches the presynaptic terminal, it triggers the opening of special Ca2+ channels.
Calcium triggers release of neurotransmitters (NTs), such as Acetylcholine, by exocytosis into synaptic cleft.

NTs bind to either excitatory or inhibitory receptors on the post-synaptic terminal.

Once NTs does its job, it is broken down by specific enzymes = Acetylcholine (Ach) is broken down by cholinesterase

Electrocution
Myasthenia Gravis
autoimmune disease characterized by fatigue and muscular weakness, especially in the face and neck, that results from a breakdown in the normal communication between nerves and muscles caused by the deficiency of acetylcholine at the neuromuscular (nerve-muscle) junctions.
Medical Intervention?
Patients with MG are given cholinesterase inhibitors to allow Ach to stay in the synaptic cleft and interact with available receptors
Full transcript