hormones and homeostasis

hormone → definition

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

hormones

• together with the nervous system help to coordinate various activities within the body

• made of either protein or steroids (lipid)

• some are also involved in homeostasis

exocrine glands

• glands with ducts

• produce a secretion that is carried by the duct

  • e.g.: the salivary gland has a duct that carries the saliva to the buccal cavity

endocrine glands

• ductless glands produce hormones and secrete them directly into the bloodstream

• the blood then carries the hormone to the target organ or tissues

endocrine system

• some glands produce only hormones and are hence purely endocrine glands

  • e.g.: testes: secrete testosterone into the bloodstream which then distributes it to the rest of the body

• pancreas produce hormones (insulin and glucagon) as well as pancreatic juice → both an endocrine and an exocrine gland

pancreas as both a endocrine and exocrine gland

• a group of cells called the islet of langerhans: secrete hormones (insulin and glucagon) involved in glucose regulation directly into the blood.

• there are other cells that produce bicarbonate and digestive enzymes. These are secreted into the duodenum through the pancreatic duct

endocrine glands → example

1. hypothalamus produces ADH, pituitary gland stores and releases ADH

2. pancreas (islets of Langerhans)

hypothalamus produces ADH, pituitary gland stores and releases ADH

• hormone: anti-diuretic hormone (ADH)

• function: osmoregulation

• target organ(s): walls of collecting duct of kidney nephron

pancreas (islets of Langerhans)

• hormone: insulin and glucagon

• function: blood glucose regulation

• target organ(s):

  • insulin: liver and muscle cells

  • glucagon: liver cells only

homeostasis → definition

the maintenance of a constant internal environment

importance of homeostasis

• ensures a stable internal environment in the organism, with minimal unavoidable disturbances

• allow the organism to function:

  • more efficiently as cells are maintained in an internal environment with optimal conditions

  • with a degree of independence from the external environment as the organism is not adversely affected by changes in the external environment

• examples in the body

  1. regulation of blood glucose concentration

  2. temperature regulation

  3. regulation of blood plasma water potential

internal environment

1. cellular level

2. tissue level

cellular level

• composed of cytoplasm, whose constituents are controlled by:

  • 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

  • enzyme activity, which is controlled by the rate of protein synthesis

tissue level

• the immediate environment surrounding cells of multicellular organisms is the extracellular fluid. In mammals → tissue fluid

• tissue fluid fills the space between cells (intercellular spaces) → formed when the higher blood pressure at the arterial ends of the capillaries forces blood plasma out of the capillaries

  • tissue fluid = blood plasma minus proteins

  • it provides cells with the medium in which they live

features of the internal environment to be kept constant

1. temperature

2. pH

3. concentration of respiratory gases (oxygen and carbon dioxide)

4. concentration of essential molecules e.g. glucose

5. concentration of ions (which affect the water potential)

6. concentration of toxic substances e.g. nitrogenous waste products that arise from protein metabolism

components of homeostatic control system

• reference point/set point represents the optimal level in a homeostatic control system

• consists of:

  • receptor

  • control centre

  • effector

receptor

• detects the stimulus (any change or deviation) from the reference point

• this information is then relayed to the control centre

control centre

• information is compared with the reference point

• if there is a deviation, the control centre sends an appropriate signal to the effector

effector

• serves to carry out the appropriate response 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 → negative feedback

principles of homeostasis

• can be achieved through negative feedback → mechanism that counteract changes in the internal environment and restores it to the reference point

• for negative feedback to take place:

  1. reference point to be maintained

  2. stimulus → change in the internal environment

  3. receptor to detect the stimulus

  4. self-regulatory corrective mechanism to bring about the reverse effect of the stimulus

importance of regulating blood glucose level

• glucose is the ideal substrate for cellular respiration → the preferred fuel molecule for both cardiac and skeletal muscle’s

• is the only metabolic fuel molecule used by the brain → a drastic decrease in blood glucose level could lead to fainting, convulsions, coma and finally death

regulation of blood glucose level

• regulated by 2 hormones secreted from the islets of Langerhans of the pancreas:

  • glucagon is secreted from the alpha (α) cells and helps increase blood glucose concentration

  • 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 → 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.

response to a rise in blood glucose levels

1. Blood glucose levels increase above the reference point of around 90 mg/ 100 ml (stimulus).

2. The rise in blood glucose level is detected by islets of Langerhans (receptor) in pancreas.

3. 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).

4. 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.

5. These actions decrease blood glucose concentration until it returns to the reference point (negative feedback).

6. 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.

response to a fall in blood glucose levels

1. Blood glucose levels decrease below the reference of around 90mg/100ml (stimulus).

2. The fall in blood glucose level is detected by islets of Langerhans (receptor) in pancreas.

3. This triggers the secretion of glucagon (a hormone,) by the α-cells in the islets of Langerhans of the pancreas. (control centre)

   • Glucagon will be transported by the blood to the liver. (signal)

4. 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)

5. Glucose is released into the blood stream, hence increasing blood glucose concentration until it returns to reference point. (negative feedback)

6. This return to reference point is detected by the α-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.

what if there’s not enough insulin

• diabetes mellitus

  • type 1 diabetes → pancreas fails to produce enough insulin

  • type 2 diabetes → person’s body cells no longer respond to insulin produced by the pancreas

• signs and symptoms: increase in blood glucose, glucose found in urine, excessive thirst and urination, tiredness, loss of weight

• treatment

  • 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)

mammalian skin

• forms a protective covering over the body surface

• acts as an excretory organ as well as a regulator of body temperature

• consists of:

  • hairs, sweat glands, temperature receptors, blood vessels and fatty tissue

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

• 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.

rise in temperature

• A rise in bodily temperature can be due to:

  1. 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.

  2. 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.

• 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 hormone

response to rise in temperature

• arterioles in the skin dilate (vasodilation) while shunt vessels constrict to allow more blood to flow through blood capillaries under the skin surface

  • more heat is lost through the skin by radiation, convection and conduction

• 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

• the metabolic rate of the body slows down, thus less heat is produced within the body

• 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.

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.

• In the brain, the hypothalamus sends out nerve impulses to the relevant body parts.

response to fall in temperature

• arterioles in the skin constrict (vasoconstriction) and shunt vessels dilate to allow less blood to flow through blood capillaries under the skin surface

• less heat is lost through the skin by radiation, convection and conduction

• sweat glands become less active, and the production of sweat decreases ⇒ less latent heat of vaporisation is lost from the body

• the metabolic rate of the body increases, thus more heat is produced within the body

• sometimes the above reactions are not sufficient to prevent a drop body temperature ⇒ shivering (a reflex) occurs

  • the spasmodic contraction of the muscles increases heat production

• these actions increase blood temperature until it returns to reference point

• this return to normal is detected by the thermoreceptors; the removal of the stimulus will stop the homeostatic action

regulation of blood plasma water potential

• The water potential of blood depends on the amount of water and salts in the plasma.

• There are special receptor cells in the hypothalamus of the brain called osmoreceptors.

• Osmoreceptors are sensitive to changes in water potential of the blood.

• The amount of water in the blood plasma is controlled by anti-diuretic hormone (ADH).

• ADH is produced by the hypothalamus in the brain and is released by the pituitary gland. It causes an increase in water reabsorption at the kidney tubules.

importance of blood plasma water potential regulation

• Any drastic change in water potential will affect the cells in the body.

  • If the blood plasma is too dilute, water molecules will enter the cells by osmosis. The cells will swell and burst.

  • If blood plasma is too concentrated, water molecules will move out of the cells by osmosis. The cells will become dehydrated, shrink and are thus unable to carry out their metabolic functions.

• The composition of tissue fluid must be kept within very narrow limits. This ensures that tissue fluid is kept at a constant water potential

response to rise in the blood plasma water potential

• When water potential in the blood plasma increases above the reference point (e.g. large intake of water/ drinking), osmoreceptors in the hypothalamus detect the change and stimulates the pituitary gland.

• Pituitary gland releases less ADH into the bloodstream, causing the walls of the distal convoluted tubule and collecting duct to be less permeable to water.

• Kidney tubules reabsorb less water back into the blood capillaries.

• More urine is produced. Urine is also more dilute.

  • These actions decrease the water potential of blood plasma until it returns to the reference point.

• This return of water potential to reference point is detected by the osmoreceptors. Secretion of ADH returns to norm.

response to fall in blood plasma water potential

• When water potential in the blood plasma decreases to below the reference point (e.g. loss of water through sweating), osmoreceptors in the hypothalamus detect the change and stimulates the pituitary gland.

• Pituitary gland releases more ADH into the bloodstream. ADH makes the walls of the distal convoluted tubule and collecting duct more permeable to water.

• Kidney tubules reabsorb more water back into the blood capillaries.

• Less urine is produced. Urine is also more concentrated.

• These actions increase the water potential of blood plasma until it returns to normal levels.

• This return of water potential of blood plasma to reference point is detected by the osmoreceptors. The increased secretion of ADH stops.