Homeostasis Lecture 2 + 3 + 4

DR STEYN lectures

What are the body fluid compartments?

[ECF, ICF and internal environment]

• Extracellular fluid (ECF) = fluid outside all cells

  • two compartments = plasma + interstitial fluid

• Intracellular fluid (ICF) = fluid inside all body cells

• Internal environment = Refers to the ECF that surrounds and is in direct contact with body cells

What are the 10 factors homeostatically related?

1. Concentration of O2

2. Concentration of CO2

3. Concentration of waste products

4. Concentration of H2O

5. Concentration of Salt

6. Plasma volume

7. Blood pressure

8. Temperature

9. pH

10. Concentration of nutrient molecules

Explain how homeostasis is restored when blood pressure is increased

1. Increased Blood Pressure → The baroreceptors (pressure sensors in blood vessels) detect the rise in blood pressure.

2. Baroreceptor Firing Increases → More signals are sent to the brain (medulla oblongata).

3. The brain responds by:

   • Activating cardiac inhibitor centers (slows heart rate).

   • Inhibiting cardiac accelerator centers (reduces heart contraction strength).

   • Inhibiting the vasomotor center (causes blood vessel relaxation or vasodilation).

4. Effects:

   • Heart rate (HR) and stroke volume (SV) decrease, leading to lower cardiac output.

   • Vasodilation widens blood vessels, reducing resistance.

   • Blood pressure drops back to normal.

5. Homeostasis is restored.

Explain how homeostasis is restored when blood pressure is decreased

1. Decreased Blood Pressure → The baroreceptors detect the drop in blood pressure.

2. Baroreceptor Firing Decreases → Fewer signals are sent to the brain.

3. The brain responds by:

   • Inhibiting cardiac inhibitor centers (stops slowing the heart).

   • Activating cardiac accelerator centers (increases heart rate and contraction strength).

   • Activating the vasomotor center (causes vasoconstriction—blood vessels narrow).

4. Effects:

   • Heart rate (HR) and stroke volume (SV) increase, leading to higher cardiac output.

   • Vasoconstriction narrows blood vessels, increasing resistance.

   • Blood pressure rises back to normal.

5. Homeostasis is restored.

How is homeostasis restored when blood glucose levels increase

1. High blood glucose levels detected by insulin-secreting cells of the pancreas.

2. Insulin secreting cells of the pancreas are stimulated to release insulin into the blood.

3. Response:

   1. Body cells absorb glucose

   2. Liver takes up glucose and stores it as glycogen.

4. Blood glucose levels decease to normal level.

How is homeostasis restored when blood glucose levels decrease

1. Low blood glucose levels detected by the glucagon releasing cells of the pancreas

2. Glucagon releasing cells of the pancreas are stimulated to release glucagon into the blood

3. Response: Liver breaks down glycogen and releases glucose into the blood.

4. Blood glucose levels increase back to normal

How is homeostasis restored when temperature increases above normal?

1. Body temp increases above normal

2. Thermoreceptors send signals to the hypothalamus (control centre) which detects the change in temperature and sends impulses to the effectors.

3. The skin blood vessels dilate and sweat glands secrete sweat.

4. More body heat is lost to surroundings and body temp drops to normal.

How is homeostasis restored when temperature levels decrease below normal.

1. Body temp decreases below normal

2. Thermoreceptors send signals tot he hypothalamus (control centre) which detects the change in temp and sends nerve impulses to the effectors

3. Response:

   1. Skin blood vessels constricts and sweat glands become inactive

   2. if body temp continues to drop, hypothalamus sends signals to muscles to contract involuntarily → which causes body heat.

      Body temperature increases back to normal

Explain how homeostasis is restored when pH levels decease below normal

Response to Acidosis (Low pH):

• Cause: Increased H+ concentration in the blood, which makes pH in blood more ACIDIC (lower)

• • Effect on the body:

    1. Stimulates brain and arterial receptors.

    2. Increases respiration rate to expel CO₂ (which is acidic).

    3. Decreased blood CO₂ leads to reduced H₂CO₃ (carbonic acid), helping restore pH.

    4. The kidneys compensate by increasing H+ excretion in urine.

  • Result: Blood pH is restored to normal.

Explain how homeostasis is restored when pH levels increase above normal

3. Response to Alkalosis (High pH):

• Cause: Decreased H+ concentration (therefore pH INCREASES bc less H+) in the blood.

• Effect on the body:

  1. Stimulates brain and arterial receptors.

  2. Decreases respiration rate to retain CO₂.

  3. Increased blood CO₂ raises H₂CO₃, which lowers pH.

  4. The kidneys compensate by increasing OH- excretion and retaining H+.

• Result: Blood pH is restored to normal

What is a negative feedback system?

A control mechanism that counteracts a change in the body to maintain homeostasis by reversing changes from the normal range. E.g. body temp regulation or glucose levels.

What is a positive feedback system?

A control mechanism that amplifies (increases) a change until a specific outcome is achieved. Example: Oxytocin release during childbirth.

What is a feedforward system?

A control mechanism that anticipates a change and prepares the body before it happens. Example: Salivation before eating.

List all the steps that occur in a homeostatic control system

1. Stimulus

2. Change detected by receptor

3. Information is sent to control centre (through afferent and efferent pathways)

4. Control centre sends impulses to effectors

5. Response occurs

What is the cause of homeostatic imbalance?

• Malfunctions of body systems

• Aging (body’s control system is less efficient + increased risk of illness)