Topic 8: Hormones and Homeostasis Flashcards
A. Hormones
Definition of Hormones
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
B. Glands
Exocrine Glands
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)
B. Glands
Endocrine Glands
Ductless glands that produce hormones and secrete them directly into the bloodstream, which then carries the hormone to the target organs or tissues.
C. Endocrine Glands, and its Respective Hormone, Function, Target Organs
Hypothalamus produces ____, pituitary gland stores and releases ____
Hormone: Anti-diuretic hormone (ADH)
Function: Osmoregulation
Target Organ(s): Walls of collecting ducct (of kidney nephron)
C. Endocrine Glands, and its Respective Hormone, Function, Target Organs
Pancreas (islets of Langerhans)
Hormone: Insulin and glucagon
Function: Blood glucose regulation
Target Organ(s): liver and muscle cells (insulin); liver cells only (glucagon)
D. Introduction to Homeostasis
Definition of Homeostasis
The maintenance of a constant internal environment.
D. Introduction to Homeostasis
Importance of Homeostasis
- 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.
D. Introduction to Homeostasis
Examples of Homeostasis
- the regulation of blood glucose concentration
- temperature regulation, and
- the regulation of blood plasma water potential
D. Introduction to Homeostasis
Internal Environment at the Cellular Level
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
D. Introduction to Homeostasis
External Environment at the Tissue Level
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.
D. Introduction to Homeostasis
Features (physiological parameters) of the Environment to be Kept Constant
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
E. Homeostatic Control
Reference Point
The reference point (or set point) represents the ptimal level in a homeostatic control system.
E. Homeostatic Control
Receptor
- 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.
E. Homeostatic Control
Control Centre
- 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)
E. Homeostatic Control
Effector
- 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.
F. Principles of Homeostasis
Negative Feedback
- 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
F. Regulation of Blood Glucose Concentration
Importance of Regulating Blood Glucose Level
- 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.
F. Regulation of Blood Glucose Concentration
Regulation of Blood Glucose Level
- 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.
F. Regulation of Blood Glucose Concentration
Response to a Rise in Blood Glucose Levels
- 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:
- Increases permeability of cell membranes to glucose, thus increasing rate of uptake of glucose from the blood by cells
- Increases rate of cellular respiration – increases the rate of oxidation of glucose in cells
- Stimulates liver and muscle cells to convert excess glucose to glycogen for storage (process is known as glycogenesis).
- 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.
F. Regulation of Blood Glucose Concentration
Response to a Fall in Blood Glucose Levels
- 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)
- Stimulates conversion of stored glycogen back to glucose in the liver
- 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.
F. Regulation of Blood Glucose Concentration
Lack of Insulin
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
F. Regulation of Blood Glucose Concentration
Symptoms of Diabetes Mellitus
increase in blood glucose, glucose found in urine, excessive thirst and urination, tiredness, loss of weight
F. Regulation of Blood Glucose Concentration
Treatment for Diabetes Mellitus
- 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)
G. Temperature Regulation
The Mammalian Skin
- 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
G. Temperature Regulation
Importance of Temperature Regulation
- 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.
G. Temperature Regulation
Reasons for a Rise in Temperature
- 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.
G. Temperature Regulation
Response to a Rise in Temperature
- 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.
G. Temperature Regulation
Reasons for a Fall in Temperature
- 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.
G. Temperature Regulation
Response to a Fall in Temperature
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.
