13.1 Homeostasis
Homeostasis is the regulation of physical and chemical factors of the internal environment within normal ranges, ensuring that cells function under optimum conditions.
Learning Objectives
- Define homeostasis and explain the principle of negative feedback mechanism.
- Identify the physical and chemical factors regulated in the internal environment.
- Identify the organ systems involved in maintaining an optimal internal environment.
- Explain the regulatory mechanisms of body temperature, blood sugar, partial pressure of carbon dioxide, and blood pressure.
Flowcharts of Homeostatic Feedback Mechanisms in the Human Body
Negative feedback reverses any deviation from the normal range. Compare the dual loops for thermoregulation, blood glucose, carbon dioxide partial pressure, and blood pressure.
Thermoregulation
Hypothalamus coordinates physical and chemical responses to keep body temperature near the set point.
Blood glucose
Insulin lowers high blood sugar; glucagon raises low blood sugar.
pCO₂ regulation
Chemoreceptors detect pH changes and speed up breathing and heart rate.
Blood pressure
Baroreceptors adjust arteriole diameter and heart contraction strength.
Short Explanation
Definition
Homeostasis is the regulation of physical and chemical factors of the internal environment within normal ranges for cells to function in optimum conditions.
Physical factors regulated
Temperature, osmotic blood pressure, and blood pressure.
Chemical factors regulated
pH value, concentration of minerals, and blood sugar concentration.
Negative feedback
When a factor exceeds the normal range, regulatory systems act to bring it down. When it falls below the normal range, regulatory systems act to increase it.
Organ systems involved
Body temperature involves the integumentary, nervous, circulatory, muscle and endocrine systems. Blood sugar involves endocrine, circulatory and digestive systems. pCO₂ involves respiratory, circulatory and nervous systems. Blood pressure involves circulatory and nervous systems.
Try to Answer First
Answer in your mind, then press “Check Answer”.
What is the role of insulin when blood glucose concentration increases above normal after a heavy meal?
Explain why shivering occurs when an individual is exposed to extremely cold environments.
How do the carotid bodies and aortic bodies respond to an increase in cellular respiration during vigorous exercise?
Common Mistakes
- Confusing vasodilation with vasoconstriction: students often mistakenly state that arterioles constrict in hot environments instead of dilating.
- Misidentifying the hormone secreting cells: stating that alpha cells produce insulin and beta cells produce glucagon. The correct pairing is beta cells → insulin; alpha cells → glucagon.
- Misunderstanding the role of the erector muscle: thinking that erector muscle relaxation causes hair to stand erect, whereas erector muscle contraction pulls the hair upright.
- Attributing carbon dioxide detection to oxygen levels: chemoreceptors primarily monitor the concentration of H⁺ ions (pH) resulting from dissolved carbon dioxide, not the drop in oxygen concentration.
- Confusing glucagon with glycogen: glucagon is the hormone secreted by the pancreas; glycogen is the insoluble storage polysaccharide stored in liver and muscle cells.
Concept Misunderstandings
Homeostasis means maintaining internal factors at an absolutely fixed, unvarying point.
Factors fluctuate within an acceptable normal range around a set point; homeostatic mechanisms constantly make minor adjustments via negative feedback.
Sweating cools down the body simply by producing water on the skin.
Cooling only occurs when the sweat evaporates, absorbing latent heat of vaporisation from the skin.
Adrenaline and thyroxine are involved in physical methods of thermoregulation.
Hormonal controls of metabolic rate by adrenaline and thyroxine are classified specifically as chemical methods.
Summary
- Homeostasis maintains physical factors (temperature, blood pressure, osmotic pressure) and chemical factors (glucose, pH, mineral ions) within optimum ranges via negative feedback.
- Thermoregulation is coordinated by the hypothalamus: high temp → vasodilation, sweating, hair lowered, decreased metabolic rate; low temp → vasoconstriction, shivering, hair erect, increased metabolic rate.
- Blood sugar regulation (75–110 mg/100 ml): high blood glucose → beta (β) cells release insulin → glucose converted to glycogen and fat; low blood glucose → alpha (α) cells release glucagon → glycogen broken down into glucose.
- pCO₂ regulation: high CO₂ → lower pH detected by central and peripheral chemoreceptors → medulla oblongata signals faster contraction of intercostal muscles, diaphragm, and heart → excess CO₂ expelled.
- Blood pressure regulation: baroreceptors in aortic arch and carotid arteries signal the cardiovascular centre → regulates arteriole diameter (vasodilation/vasoconstriction) and heart contraction strength.
Short Activity
Objective: check your understanding of homeostatic feedback mechanisms.
A. Objective Quiz
Which of the following responses occurs through chemical methods when a person's body temperature drops below the normal range?
What is the physiological effect of glucagon secretion by the alpha (α) cells of the pancreas?
B. Fill in the Blanks
Pressure receptors located in the aortic arch and carotid artery that detect changes in blood pressure are called .
The normal concentration range of blood glucose in a healthy human body is mg/100 ml.
C. Matching / Drag and Drop
Drag each physiological stimulus to its corresponding regulatory response. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Stimulus to Its Effector Response
Write the correct stimulus next to each response.
| Correct Effector Response | Stimulus / Condition |
|---|---|
| 1. Arteriolar vasodilation occurs and cardiac muscles contract weakly to reduce resistance and pressure. | |
| 2. Arterioles dilate and sweat glands secrete sweat which absorbs heat as it evaporates. | |
| 3. Beta (β) cells secrete insulin to convert excess glucose into stored glycogen and fat. | |
| 4. Intercostal muscles, diaphragm, and cardiac muscles contract faster to expel gas. |
Answer choices: Blood pressure rises above normal • Body temperature rises above normal • Blood glucose levels increase after a meal • Partial pressure of carbon dioxide increases
