Homeostasis

Question 1

Homeostatic Conditions:
- SaO2 (arterial O2 saturation)
- Blood pH
- Stimulus for respiratory rate
- PaO2
- PaCO2

Answer 1

• SaO2: 93-95%
• pH: 7.3-7.4
• Stimulus for RR: increased CO2 in blood
• PaO2: 80-100mmHg (how well O2 moves from lungs into blood)
• PaCO2: 34-45mmHg (how well lungs remove CO2 from body)

Question 1

Homeostasis Definition

Answer 1

Maintenance of relatively stable internal environment despite changes in external environment

• dynamic process (oscillations around set point)

Question 2

Positive Feedback

Answer 2

• Amplification
• A = increased B = Increased A etc.
• MUST HAVE INBUILT REGULATORY SHUT OFF SIGNALS !

E.g. Blood clotting, myometrial contraction (labour), APs

Question 2

Negative Feedback

Answer 2

• Regulate variable around set point (self-regulating)
• Response to stimulus = shut off stimulus

E.g. Core body temp

Increased temp = more heat loss and less heat production = reduced body temp

Decreased body temp = less heat loss and more heat production = increased body temp

Question 3

Feedforward Regulation

Answer 3

Central common elicits response before or without changes in regulated variable

E.g. Intention to exercise = increased HR

Question 4

Theory of why homeostasis exists

Answer 4

• Maintenance of protein shape

Question 5

Elements of Negative Feedback
- Variable
- Receptor
- Control centre
- Effectors

Answer 5

• Variable: needs regulation
• Receptor: detects changes in variable
• Control centre: identifies when variable has shifted from set point
• Effectors: produce response to adjust variable

Question 6

Sensory Receptors role in homeostasis + 3 types

Answer 6

Detect changes in internal environment - turn physical stimulus into biological signal

• Stretch (mechanoreceptors)
• Temperature (thermosensors)
• Chemoreceptors

Question 7

Stretch Receptors
- Baroreceptors
- Osmoreceptors

Answer 7

Baroreceptors: detect stretch of vessel wall (change in pressure)
= regulate BP

Osmoreceptors: detect stretch of cell membrane (change in cell volume)
= regulate body fluid

Question 8

Temperature Receptors

Answer 8

Measure core body temperature

Question 9

Chemoreceptors

Answer 9

Detect change in CO2 and pH levels
= changes in ion flux across cell membranes
= stimulate receptors to release NT to stimulate sensory neuron

Question 10

Efferent Signal Types

Answer 10

Neuronal (ANS)
Chemical (Endocrine, Chemokine)

Question 11

Homeostatic Effectors

Answer 11

Smooth Muscle
- blood vessels (diameter)
- GIT (motility)

Cardiac Muscle
- HR

Glands
- Secretion levels + glucose release endocrine, paracrine, liver)

Skeletal Muscle
- RR + shivering

Renal Collecting Ducts
- Ion absorption and water reabsorption

Question 12

Negative Feedback Regulation in Endocrine System
- Physiological vs Endocrine Driven

Answer 12

Physiological:
- Direct body responses to a stimulus
- E.g. lowering body temperature

Endocrine:
- Hormones used to regulate internal body conditions
- E.g. Blood glucose levels

Question 13

Cushing’s Disease
(Homeostatic Dysregulation)

Answer 13

Disorder due to excessive levels of cortisol hormone.

Aetiology:
- Pituitary adenoma (excessive ACTH secretion)
- Adrenal adenoma (excessive cortisol)

ACTH and Cortisol:
1. CRH released
2. CRH stimulates ACTH release from pituitary
3. ACTH stimulates cortisol production
4. More cortisol reduces hypothalamus release of CRH
= -ve feedback

Question 14

Serology for ACTH vs Cortisol

Answer 14

High ACTH but low cortisol
*Adrenal damage -> not responding to ACTH production
*Addison’s Disease

High ACTH and high cortisol
*Pituitary damage = excess ACTH = excess cortisol
*Cushing’s Disease

Low ACTH but high cortisol
*Adrenal damage = excess cortisol (disproportionate to ACTH)
*Cushing’s disease

Low ACTH and low cortisol
*Hypopituitarism – hypopituitary insufficiency -> not enough ACTH = reduced cortisol

Question 15

Thermoregulation and Fever
- Homeostatic variables
- Factors causing normal variation
- Define Fever

Answer 15

Hypothalamus = thermoregulatory centre

Core body temp = 37 celcius
Normal variation = 36-37.5

Variable Factors:
- menstrual cycle
- age
- circadian rhythm

Fever = resetting of set point

Question 16

Physiological Thermoregulatory Responses (Metabolic)

Answer 16

• Non-shivering thermogenesis
• Shivering thermogenesis
• Skin blood flow (vasoconstriction and vasodilation)
• Sweating (sympathetic)

Question 17

Steps of Thermoregulation

Answer 17

1. Variable = change in body temp
2. Receptors = Thermoreceptors detect change in body temp and send afferent signals to hypothalamus
3. Control centre = Hypothalamus identifies when change in body temp is outside homeostatic range and stimulates physiological responses:

TOO HIGH = sweating, vasodilation
TOO LOW = shivering, non-shivering thermogenesis, vasoconstriction

4. Feedback: thermoreceptors detect induced changes in body temp and relay info back to hypothalamus. Once body temp returns to normal range = shut off hypothalamus response.

= negative feedback

Question 18

Fever
- Function
- Process

Answer 18

Functions
- enhance innate and adaptive immune responses
- create environment less ideal for bacteria and viruses (denature)

Process:
1. Infection infiltrates and detected by immune system
2. Chemicals released to stimulate endothelial cells to produce COX2
3. COX2 = PG secretion
4. PG = inflammation and raised body temp

Question 19

Anti-pyretic Drug Function

Answer 19

Block COX2 receptors

= inhibits PG secretion
= stops fever and inflammation