Topic 8: Hormones and Homeostasis

Question 1

A. Hormones

Definition of Hormones

Answer 1

chemical substance produced in minute amounts by glands, carried by the blood, which alters the activity or one or more specific target organs (eventually broken down by the liver)

• help to coordinate various activities within the body (together with the nervous system)
• made of either protein or steroids (lipid)
• some are involved in homeostasis

Question 2

B. Glands

Exocrine Glands

Answer 2

Glands that produce secretions that are carried by a duct

e.g., the salivary gland has a duct that carries the saliva to the buccal cavity (exocrine gland)

Question 3

B. Glands

Endocrine Glands

Answer 3

Ductless glands that produce hormones and secrete them directly into the bloodstream, which then carries the hormone to the target organs or tissues.

Question 4

C. Endocrine Glands, and its Respective Hormone, Function, Target Organs

Hypothalamus produces ____, pituitary gland stores and releases ____

Answer 4

Hormone: Anti-diuretic hormone (ADH)
Function: Osmoregulation
Target Organ(s): Walls of collecting ducct (of kidney nephron)

Question 5

C. Endocrine Glands, and its Respective Hormone, Function, Target Organs

Pancreas (islets of Langerhans)

Answer 5

Hormone: Insulin and glucagon
Function: Blood glucose regulation
Target Organ(s): liver and muscle cells (insulin); liver cells only (glucagon)

Question 6

D. Introduction to Homeostasis

Definition of Homeostasis

Answer 6

The maintenance of a constant internal environment.

Question 7

D. Introduction to Homeostasis

Importance of Homeostasis

Answer 7

• Cells function well only within a narrow range of conditions. Hence, large changes can result in cells not functioning efficiently, or even dying. (enzymes in cells function only within a narrow range of pH and temperature.)
• Homeostasis ensures a stable internal environment in the organism, with minimal unavoidable disturbances.
• Homeostasis allows the organism to function: a) more efficiently as cells are maintained in an internal environment with optimal conditions; b) with a degree of independence from the external environment as the organism is not adversely affected by changes in the external environment.

Question 8

D. Introduction to Homeostasis

Examples of Homeostasis

Answer 8

1. the regulation of blood glucose concentration
2. temperature regulation, and
3. the regulation of blood plasma water potential

Question 9

D. Introduction to Homeostasis

Internal Environment at the Cellular Level

Answer 9

The cell is composed of cytoplasm, whose constituents are controlled by:

a) The cell membrane
* This partially permeable membrane permits only certain molecules and ions to enter and leave.

• The rates at which molecules are exchanged are controlled by diffusion gradients, osmotic gradients and active transport mechanisms.

b) Enzyme activity, which is controlled by the rate of protein synthesis

Question 10

D. Introduction to Homeostasis

External Environment at the Tissue Level

Answer 10

The immediate environment surrounding cells of multicellular organisms is the extracellular fluid. In mammals, it is known as tissue fluid.

Tissue fluid fills the space between cells (intercellular spaces). It is formed when the higher blood pressure at the arterial ends of the capillaries forces blood plasma out of the capillaries (Fig. 1).
* Tissue fluid = blood plasma minus proteins
* It provides cells with the medium in which they live.

Question 11

D. Introduction to Homeostasis

Features (physiological parameters) of the Environment to be Kept Constant

Answer 11

a) temperature

b) pH

c) concentration of respiratory gases (oxygen and carbon dioxide)

d) concentration of essential molecules e.g., glucose

e) concentration of ions (which affect the water potential)

f) concentration of toxic substances e.g., nitrogenous waste products that arise from protein metabolism

Question 12

E. Homeostatic Control

Reference Point

Answer 12

The reference point (or set point) represents the ptimal level in a homeostatic control system.

Question 13

E. Homeostatic Control

Receptor

Answer 13

• This detects the stimulus (any change or deviation) (e.g. increase in concentration of blood glucose) from the reference point (90 mg of glucose per 100 ml of blood).
• This information is then relayed to the control centre.

Question 14

E. Homeostatic Control

Control Centre

Answer 14

• Here, information is compared with the reference point.
• If there is a deviation, the control centre sends an appropriate signal (e.g. release of insulin or glucagon into the bloodstream) to the effector (e.g. liver or muscle cells)

Question 15

E. Homeostatic Control

Effector

Answer 15

• This serves to carry out the appropriate response (e.g. increase or decrease blood glucose level) based on the signal received from the control centre.
• The response from the effector counteracts the initial change/ deviation which results in an effect (new stimulus) that is picked up by the receptor. This information is relayed to the control centre and returns the system to normal, optimal conditions after comparison against the reference point.

Question 16

F. Principles of Homeostasis

Negative Feedback

Answer 16

• Homeostasis can be achieved through negative feedback.
• Negative feedback refers to the mechanism that counteract changes in the internal environment and restores it to the reference point.
• For Negative Feedback to take place there must be a:1) reference point to be maintained
  2) stimulus (a change in the internal environment)
  3) receptor to detect the stimulus
  4) self-regulatory corrective mechanism to bring about the revese effect of the stimulus

Question 17

F. Regulation of Blood Glucose Concentration

Importance of Regulating Blood Glucose Level

Answer 17

• Glucose is the ideal substrate for cellular respiration. It is the preferred fuel molecule for both cardiac and skeletal muscles.
• It is the only metabolic fuel molecule used by the brain. Hence, a drastic decrease in blood glucose level could lead to fainting, convulsions, coma and finally death.

Question 18

F. Regulation of Blood Glucose Concentration

Regulation of Blood Glucose Level

Answer 18

• Blood glucose level is regulated by 2 hormones secreted from the islets of Langerhans of the pancreas: a) glucagon is secreted from the alpha (α) cells and helps increase blood glucose concentration; b) insulin is secreted from the beta (β) cells and helps decrease blood glucose concentration
• Normal level of blood glucose is about 90 mg/ 100 ml of blood (fluctuating between 70 mg and 150 mg).
• Glucagon and insulin operate antagonistically: i.e., they each oppose the actions of the other.
  • e.g., glucagon stimulates the breakdown of glycogen to glucose while insulin promotes the conversion of excess glucose to glycogen.

Question 19

F. Regulation of Blood Glucose Concentration

Response to a Rise in Blood Glucose Levels

Answer 19

• Blood glucose levels increase above the reference point of around 90 mg/ 100 ml (stimulus).
• The rise in blood glucose level is detected by islets of Langerhans (receptor) in pancreas.
• This triggers the secretion of insulin (signal) by the β-cells of the islets of Langerhans of the pancreas. (control centre)
  • Insulin will be transported by the blood to the liver and muscles (effectors).
• Insulin secreted in the blood stream causes the following responses:
  
  1. Increases permeability of cell membranes to glucose, thus increasing rate of uptake of glucose from the blood by cells
  2. Increases rate of cellular respiration – increases the rate of oxidation of glucose in cells
  3. Stimulates liver and muscle cells to convert excess glucose to glycogen for storage (process is known as glycogenesis).
  4. Decreased break down of glycogen to glucose.
• These actions decrease blood glucose concentration until it returns to the reference point (negative feedback).
• This return to reference point is detected by the β-cells of the islets of Langerhans, which in turn, decreases secretion of insulin.
  • The circulating insulin is broken down in the liver and excreted by the kidneys.

Question 20

F. Regulation of Blood Glucose Concentration

Response to a Fall in Blood Glucose Levels

Answer 20

• Blood glucose levels decrease below the reference of around 90mg/100ml (stimulus).
• The fall in blood glucose level is detected by islets of Langerhans (receptor) in pancreas.
• This triggers the secretion of glucagon (a hormone,) by the -cells in the islets of Langerhans of the pancreas. (control centre)
  • Glucagon will be transported by the blood to the liver. (signal)
• Glucagon triggers the following response in the liver: (effector)
  1. Stimulates conversion of stored glycogen back to glucose in the liver
  2. Conversion of non-carbohydrate sources such as pyruvate, amino acids and glycerol to glucose in the liver (this is known as gluconeogenesis)
• Glucose is released into the blood stream, hence increasing blood glucose concentration until it returns to reference point. (negative feedback)
• This return to reference point is detected by the -cells of the islets of Langerhans, which in turn, decreases secretion of glucagon.
  • The circulating glucagon is broken in the liver and excreted by the kidneys.

Question 21

F. Regulation of Blood Glucose Concentration

Lack of Insulin

Answer 21

The person will suffer from diabetes mellitus (a disease).

(a) Type 1 diabetes – the pancreas fails to produce enough insulin
(b) Type 2 diabetes – the person’s body cells no longer respond to insulin produced by the pancreas

Question 22

F. Regulation of Blood Glucose Concentration

Symptoms of Diabetes Mellitus

Answer 22

increase in blood glucose, glucose found in urine, excessive thirst and urination, tiredness, loss of weight

Question 23

F. Regulation of Blood Glucose Concentration

Treatment for Diabetes Mellitus

Answer 23

• injection of insulin (for Type 1 only)
• controlled diet and exercise
• taking medicine (e.g Metformin lowers glucose production in the liver and increases body’s sensitivity to insulin)

Question 24

G. Temperature Regulation

The Mammalian Skin

Answer 24

• The skin froms a protective covering over the body surface.
• It also acts as an excretory organ as well as a regulator of body temperature.
• Main components: sweat gland, blood capillaries, hair, erector muscle, sense muscles

Question 25

# G. Temperature Regulation Importance of Temperature Regulation

Answer 25

* Enzymes in the body can only work within a certain range of temperature. * Changes in the body temperature may result in enzyme inactivation or even denaturation. * Hence, the body maintains a constant internal temperature by regulating heat gain and heat loss. * Heat is gained through the external environment and metabolic activities. * Heat is lost through radiation, convection and conduction of heat from the skin; evaporation of sweat from the skin and exhalation; and through defecation and urination.

Question 26

# G. Temperature Regulation Reasons for a Rise in Temperature

Answer 26

* Increase in temperature in the external environment: on a warm day, the rate of heat loss from the body is reduced **OR** heat is absorbed from a warmer external environment. * In both cases, thermoreceptors in the skin detect a rise in external temperature and nerve impulses are sent to the hypothalamus. * Increase in temperature in the internal environment: when you perform vigorous muscular activities, a great deal of heat is produced **OR** when you consume hot beverages or food. * A rise in blood temperature is directly detected by the thermoreceptors in the hypothalamus when warmer blood flows through it.

Question 27

# G. Temperature Regulation Response to a Rise in Temperature

Answer 27

* The hypothalamus will send out nerve impulses to the relevant body parts where corrective processes occur to restore the temperature back to normal. * Homeostasis of temperature is controlled by nerve impulses, and not by hormones. **Blood Vessels** * Arterioles in the skin dilate (vasodilation) while shunt vessels constrict to allow more blood to flow through blood capillaries under the skin surface. * Thus, more heat is lost through the skin by radiation, convection and conduction. **Sweat Glands** * Sweat glands become more active. * Increased production of sweat. * As more sweat evaporates from the surface of the skin, more latent heat of vaporisation is removed from the body. **Others** * The metabolic rate of the body slows down, thus less heat is produced within the body. * Rapid breathing occurs. **Result** * These processes decrease blood temperature until it returns to the reference point. * This return to reference point is detected by the thermoreceptors. The removal of the stimulus will stop the homeostatic action.

Question 28

# G. Temperature Regulation Reasons for a Fall in Temperature

Answer 28

* On a cold day, the rate of heat loss is increased, especially at the skin surface. * A drop in external temperature is detected by the temperature receptors in the skin which then send nerve impulses to the brain.

Question 29

# G. Temperature Regulation Response to a Fall in Temperature

Answer 29

In the brain, the hypothalamus sends out nerve impulses to the relevant body parts causing: **Blood Vessels** * Arterioles in the skin constrict (vasoconstriction) and shunt vessels dilate to allow less blood to flow through blood capillaries under the skin surface. * Thus, less heat is lost through the skin by radiation, convection and conduction. **Sweat Glands** * Sweat glands become more active. * Increased production of sweat. * As more sweat evaporates from the surface of the skin, more latent heat of vaporisation is removed from the body. **Others** * The metabolic rate of the body slows down, thus less heat is produced within the body. * Rapid breathing occurs. **Result** * These processes decrease blood temperature until it returns to the reference point. * This return to reference point is detected by the thermoreceptors. The removal of the stimulus will stop the homeostatic action.