Homeostasis Flashcards
What is homeostasis?
Maintenance of a steady internal environment about a set point.
What are the 5 components of a feedback system?
- Regulated factor (a set point)
- Detector/sensor (afferent path)
- Comparator/control centre (determines set point)
- Effector (returns variable to set point/ efferent path )
- Response
Why does the value of a controlled variable oscillate?
→ Time delay between sensing a change and its correction
What are some examples of chemical concentrations that need to be maintained by homeostasis?
- ions, eg. Na+ and Ca2+
- nutrients, eg. blood glucose concentration
- hormones
What are some physiological characteristics that are maintained by homeostasis?
- Blood Pressure
- Core Temperature
What happens to the body when you are hot?
- Vasodilation
- Sweating
What happens to the body when you are cold?
- Shivering
- Vasoconstriction
- Increased metabolism
How is temperature sensed?
- Core body temperature is the REGULATED FACTOR
- Changes sensed by hypothalamic or cutaneous temperature receptors
- Hypothalamus mediates effects
How does an infection affect the elements of the temperature homeostatic negative feedback loop?
Pyrogens (bacterial or viral infections) change setpoint to a higher level, causing fever.
When you have an infection, what are the benefits of a fever?
- Inhibits bacterial growth
- Speeds up metabolic reactions
- Increases delivery of WBC to infection sites
During a fever, how is the temperature increased?
- Blood flow shifted to core to conserve heat
- Shivering
- Chills stop when high temperature is reached
Describe the temperature-time course of a typical febrile attack.
- Actual body temperature lags behind the rapid shift in set point
- Regulation of body temp. maintained but less precise
How would the blood pressure negative feedback loop react to increased blood pressure? PART 1
- Baroreceptors senses high BP through the stretching of blood vessels
- Feedback into the cardiovascular control centre within the brain.
How would the blood pressure negative feedback loop react to increased blood pressure? PART 2
- Decreased sympathetic activity (and increased parasympathetic activity)
- Decreased CO and TPR, so low BP
Where is the control center for blood pressure and what does it do?
- Medulla
- Changes TPR and HR
What happens during hypertension to the set point?
- Set point is reset
- Baroreceptors are less sensitive
What releases hormones from the posterior pituitary?
Neurons in the hypothalamus
Where do other neurosecretory cells release their hormones?
→ Into the portal capillaries where they are transported to endocrine cells of the anterior pituitary
What is the role of vasopressin (ADH) in control of blood pressure?
- Causes vasoconstriction
- Increases renal fluid absorption, which increases blood volume.
- Both increase BP
Give an example of two control systems activated after a haemorrhage to help restore blood pressure.
- Baroreceptor reflex to increase CO and TPR
- Stimulation of ADH secretion to increase blood volume
What happens when blood pressure is decreased? PART 1
- Glomerulus kidney cells secrete renin
- Liver produces angiotensinogen
- Converted to angiotensin I
What happens when blood pressure is decreased? PART 2
- Angiotensin II formed by ACE in the lungs
- Acts on adrenal cortex - secretes aldosterone - kidney tubules add Na+ channels - more Na+ reabsorbed into extracellular space
What happens when blood pressure is decreased? PART 3
- Acts on hypothalamus - secretes ADH - causing insertion of aquaporins on collecting duct walls
- Act on arteries - constrict - increased BP
Describe the hormonal regulation of cortisol secretion. PART 1
- Neurosecretory cells in the hypothalamus release CRH which travels down the portal vessels into the anterior pituitary, where it stimulates ACTH secretion.
Describe the hormonal regulation of cortisol secretion. PART 2
ACTH will act on the adrenals to stimulate cortisol secretion.
What is the difference between negative feedback control and positive feedback control? PART 1
With negative feedback, if there is an increase in a controlled variable, the body will put in motion processes that will eventually decrease that variable and its effects.
What is the difference between negative feedback control and positive feedback control? PART 2
With positive feedback control, if there is an increase in a controlled variable, the body will instead amplify that to further increase that variable. Response of effector reinforces stimulus.
How does the blood clotting mechanism work?
→ Break or tear in blood vessel wall
→ Clotting occurs as platelets adhere and release chemicals
→ Feedback cycle ends
→ Clotting proceeds until break is sealed by newly formed clot
Describe the positive feedback control of uterine contractions in labour by oxytocin.
- Oxytocin stimulates contraction of the uterine muscles.
- Cervix dilates and activates stretch receptors.
- Action potentials send signals to the hypothalamus.
- Stimulates a further release of oxytocin.
