Homeostasis Flashcards
What is human physiology?
The study of normal human function and the mechanisms controlling this
Define the term homeostasis
The maintenance of steady states within the body by coordinated physiological mechanisms
What does the body need to function normally?
Stable internal environment
What is feedforward control?
When responses made in anticipation of change
What is feedback control?
When responses made after change occurs
What is positive feedback control?
When change occurs, signal produced initiates further change eg. child birth - the more oxytocin the body produces, the more oxytocin is produced and the faster the contractions become
The initial change is amplified
Less common than negative feedback control
What is the main tupe of homeostatic control in the body?
Negative feedback control
What happens when homeostasis is disrupted?
i.e. homeostatic mechanisms are deficient, inappropriate, or excessive
Disease or death
How does negative feedback promote stability?
Regulation of a controlled variable by monitoring the information flowing through a closed loop
Give examples of variables controlled at cell membrane level
Water and electrolyte concentration within and outwith cells
Membrane potential
What are the three steps through which a physiological control system maintains homeostasis?
- Sensing a variable move outwith accepted limits
- Integrating information with other relevant info
- Initiating response to return variable to within accepted limits
What are the three steps through which a physiological control system maintains homeostasis?
- Sensing a variable move outwith accepted limits (sensor)
- Comparing value sensed with set point (control center)
- Initiating response to return variable to within accepted limits (effector)
What is negative feedback control?
When a change occurs, signal produced initiates counteraction, when no action is needed, no signal produced
No news, is good news
What is negative feedback control?
When a change occurs, signal produced initiates counteraction, when no action is needed, no signal produced
No news, is good news
Give two examples of variables controlled by negative feedback
Blood pressure
Body temperature
What is blood pressure?
outwards pressure exerted by blood on vessel walls
What is systolic blood pressure?
Pressure exerted on walls of aorta and systemic arteries when heart contracts
<140 mm Hg
What is diastolic blood pressure?
Pressure exerted on walls of aorta and systemic arteries when heart relaxes
<90 mm Hg
What does pulse mean?
The difference between systolic and diastolic blood pressures
Usually between 30 to 50 mmHg
What does the mean arterial blood pressure (MAP) mean ?
Average arterial blood pressure during one cardiac cycle
Normal range = 70 to 105 mm Hg
How can mean arterial blood pressure be estimated?
MAP = [ ( 2 x diastolic ) + systolic ] /3
MAP = diastolic + (pulse/3)
MAP = Stroke volume x heart rate x systemic vascular resistance
Why is it important to control MAP?
If < 60 mm Hg, pressure not high enough to perfuse internal organs
If >105 mmHg, pressure is too high and could damage blood vessels or put extra strain on heart
What is the baroreceptor’s role in short-term regulation of MAP?
Senses change in blood pressure and sends information to medulla via 9th and 10th
Firing rate increases when MAP increases, and decreases when MAP decreases
What is cardiac output?
Volume of blood pumped by each ventricle of heart per minute
What is systemic vascular resistance?
Sum of resistance of all vasculature in systemic circulation
What is a normal systolic/diastolic pressure under resting conditions?
systolic - < 140 mm Hg
diastolic - < 90 mm Hg
What is a normal heart rate?
60 to 100 bpm
What is a normal MAP?
70 to 105 mm Hg
What makes up systemic arterial blood pressure?
Systolic and diastolic blood pressures
Define Hypertension
Clinic blood pressure of 140/90 mmHg or higher
Day time average of 135/85 mmHg or higher
Why is MAP not average of systolic and diastolic blood pressures?
Diastole = 2/3 of time during cardiac cycle Systole = 1/3 of time during cardiac cycle
What is stroke volume?
Amount of blood pumped by left ventricle into aorta per contraction
How is Cardiac output (CO) calculated?
CO = stroke volume x heart rate
What type of receptors are baroreceptors?
Mechanoreceptors
What 3 things can regulate MAP?
Heart rate
Stroke volume
Systemic vascular resistance
What is autorhythmicity?
The heart’s ability to beat without any external stimuli
What alters heart rate?
Autonomic nervous system
What increases the heart rate?
Stimulation from sympathetic nerves using noradrenaline on beta1 receptors
What is tachycardia?
Fast heart rate
What is bradycardia?
Slow heart rate
What decreases heart rate?
Stimulation from parasympathetic vagus nerve (10th cranial nerve)
using acetylcholine
What kind of receptors does acetylcholine act on?
Muscarinic
What increases stroke volume?
Increased contractile strength of heart
What stimulates contractile strength of heart to increase?
increasing stroke volume
Sympathetic nerves in ventricular myocardium
Are there many parasympthetic nerves in the ventricular myocardium?
No
Can stroke volume be controlled intrinsically?
Yes, through Starling’s Law of the heart
Which blood vessels produce the most systemic resistance?
Arterioles
What regulates the systemic vascular resistance?
Smooth muscle in vasculature
What does contraction of smooth muscle in blood vessels cause?
Vasoconstriction
What does relaxation of smooth muscle in blood vessels cause?
Vasodilation
How does vasoconstriction affect SVR and MAP?
Increases SVR and MAP
How does vasodilation affect SVR and MAP?
Decreases SVR and MAP
What is vasomotor tone?
When smooth muscles in blood vessels are constantly partially contracted
What causes vasomotor tone?
Sympathetic nerves constantly causing low level of stimulation, meaning noradrenaline is constantly released
Is vasomotor tone affected by any parasympathetic nerves?
No
Except in penis and clitoris
How does increased /decreased sympathetic discharge affect vasomotor tone?
VT increased - vasoconstriction
VT decreased - vasodilation
