homeostatic regulation. learning objectives explain homeostasis discuss the relationship between...
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HOMEOSTATIC REGULATION
Learning Objectives•Explain homeostasis• Discuss the relationship between external and internal environments •List the main body fluid compartments, their constituents and their relationship to each other •Define equilibrium and steady state •Give examples of homeostatic mechanisms •Explain negative feedback •Define controlled variable, sensor, comparator and set point and give examples of each in a negative feedback loop •Discuss factors that may change set points •Define redundancy and hierarchy with respect to homeostatic control mechanisms •Explain positive feedback •Explain feed-forward regulation
Organ Systems in ReviewThe integration between systems of the body
External and Internal Environments“all the vital mechanisms, however varied they may be, have only one object, that of preserving constant the conditions of life in the internal environment.” Claude Bernard (1857)
• The Basis of Physiological Regulation
• A Stable Internal Environment Is Essential for Normal Cell Function
Environments
pH 7.4
40 mm Hg0.23 mm HgPCO2
PaO2 95 mm Hg160 mm HgPO2
37 °C-10 and +40 °CTemperature
InsideOutsideVariable
stability of the internal environment is the primary condition for a free and independent existence-By controlling its internal environment the organism is no longer at the mercy of the environment
pH ?/variable
Body Fluid Compartments and their Relationship
Blood Plasma 3 L
Interstitial 11 L
Intracellular 28 L
Transcellular 1 L
•most cases substances within the plasma must pass through the interstitial fluid before entering cells. •Therefore the interrelationships between these 4 compartments are crucial in underlying whole body homeostasis.
Body
Body Fluid Constituents
[Na+] = 145 [K+] = 4.5 [Cl-] = 116
[Protein] = 0 mM Osmolality 290 mOsm
[Na+] = 142 [K+] = 4.4 [Cl-] = 102
[Protein] = 1 Osmolality 290 mOsm
[Na+] = 15 [K+] = 120 [Cl-] = 20
[Protein] = 4 Osmolality 290 mOsm
Plasma Interstitial Cellular
•substances aren’t in equilibrium, but there is a balancethere is a difference between the basic constituents of the body-fluid compartments. This means that homeostasis is not about reaching equilibrium, but about maintaining a steady-state. Since the system is not necessarily in equilibrium energy expenditure is required to maintain a steady state.
To summarize:
Homeostasis is the maintenance of a steady state of the internal environment of the body.
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Homeostatic controlmechanisms
Feedback( flow of information along a closed loop )– Negative or Positive
• Negative – change is sensed and action taken to prevent further change e.g-regulation of secretion of hormones.
• Positive – change is sensed and action taken to amplify change (usually associated with a discrete end point, e.g. birth, ovulation)
Homeostatic Mechanisms
• Most homeostatic mechanisms are based on negative feedback
• specific terms that are used to describe the processes involved-
Controlled Variable SensorComparator, set point
Effectors
Blood Pressure Regulation
Blood Loss
Blood Pressure (controlled variable)
Baroreceptor (sensor)
Brain (comparitor)
Vasoconstriction ↑ Cardiac Output
(effectors)
Cardiovascular control
center – compares BP to
set point and adjusts
vascular tone and cardiac output
accordingly
Blood Glucose -ve Feedback
b-cell
Variable Blood Glucose
b-cellGlucose
Insulin secretion
Cells
“Gain” of a Control System
• The degree of effectiveness with which a control system maintains constant, conditions is determined by the gain of the negative feedback.
• For instance, let us assume that a large volume of blood is transfused into a person whose baroreceptor pressure control system is not functioning, and the arterial pressure rises from the normal level of 100 mm Hg up to 175 mm Hg.
• Then, let us assume that the same volume of blood is injected into the same person when the baroreceptor system is functioning, and this time the pressure increases only 25 mm Hg. Thus, the feedback control system has caused a “correction” of –50 mm Hg—that is, from 175 mm Hg to 125 mm Hg. There remains an increase in pressure of +25 mm Hg, called the “error,” which means that the control system is not 100 per cent effective in preventing change.
QAssume that excess blood is transfused into a patient whose
arterial baroreceptors are nonfunctional and blood pressure increases from 100 to 150 mm Hg.
Then, assume that the same volume is blood is infusedinto the same patient under conditions where his arterialbaroreceptors are functioning normally and bloodpressure increases from 100 to 125 mm Hg. What isthe approximate feedback “gain” of the arterial baroreceptors
in this patient when they are functioning normally?A) -1.0B) -2.0C) 0.0D) +1.0E) +2.0
Positive Feedback
• This is when instead of the comparator causing the controlled variable to come back to normal it potentiates the error signal and the controlled variable moves further away.
• There are few normal physiological events that are controlled by positive feedback.
Positive Feedback
Contraction
oxytocin
All steps in this process produce an increase in the next step leading to a loop of stimulation. The positive feedback loop is broken when the baby is expelled from the uterus and hence the step involving pressure against the cervix has been removed.
Feed-forward Control
• Anticipation of change – gets the body ready for change
• e.g. heart rate and ventilation can increase even before exercise begins
• Or salivation and digestive enzyme production begins before a meal is eaten
Redundancy
• Homeostatic mechanisms are important – therefore often there is more than one control mechanism
• If one mechanism fails – then there is a backup system (e.g. control of cutaneous blood vessels by both cardiovascular control center and temperature control center, or blood pressure)
Hypovolemic Shock
B.P. falls
Kidney Juxtaglomerular cells
Aortic arch Carotid sinusRenin
Activity drop
Hypothalamus Posterior Pituitary
Medulla oblongata
Angiotensinogen in blood
Angiotensin I
LUNG
Angiotenin II
ACE
Adrenal Cortex
Heart rate contractility
Blood Vessels
Kidney Salt water conservation
Aldosterone
ADHSympathetic output
Inc. volumeInc. vasc. resistance
Inc. B.P.
In response to a bacterial infection my body's thermostat is raised. I start to shiver and produce more body heat. When my body temperature reaches 101 degrees, I stop shivering and my body temperature stops going up. This is an example of:
A) Negative feedbackB) A malfunctioning control systemC) Positive feedbackD) A negative impact
Which of the following is an example of a positive feedback?
A) Shivering to warm up in a cold winter stormB) A cruise control set on your car applies more gas when going up a hillC) You sweat on a hot summer's day and the blood vessels in your skin vasodilateD) You get cut and platelets form a clot. This in turn activates the fibrin clotting system and more blood forms clots
Where is the body's "thermostat" found?A) Within the nervous system, in the HypothalamusB) Within the integumentary system, in the skinC) Within the brain, in the corpus callosumD) Within the Urinary system, in the kidneys
What system has little to contribute to the homeostasis of the organism?A) Urinary SystemB) Reproductive SystemC) Respiratory SystemD) Nervous System
Summary I
• Homeostasis – maintenance of a stable internal environment
• Steady state – unchanging with time • Equilibrium – when parameters are
maintained in an energetically favorable situation
• Redundancy – more than 1 system to control a variable (backup systems)
Summary II
• Negative feedback – feedback causes a perturbation to be minimized or reversed with view to keeping parameter at a set point
• Positive feedback – amplification of a deviation (usually defined end point)