Study guide for ‘Homeostasis’ W.K. Leung P.6
Study guide for ‘Homeostasis’
c. Syllabus
Content / Suggested teaching activity / Learning objectives for students11.1 Homeostasis
???the need for the regulation of the internal environment and the concept of negative feedback.
11.2 Water balance
???the structure and functions of the mammalian kidney.
???the formation of urine as a result of ultrafiltration, reabsorption of solutes and water including the role of loop of Henle, and tubular secretion.
???the action of antidiuretic hormone (ADH). ???describe the effects of ADH.
???the biological principle of the dialysis machine (kidney machine).
???water balance in terrestrial flowering plants including adaptations shown by xerophytes.
11.3 Regulation of body temperature
???the importance of body temperature regulation.
???the meaning of ectotherms and endotherms.
???the role of the skin, circulation, muscles,
hypothalamus, nervous system, and hormones (adrenaline and thyroxine) in the regulation of body temperature.
???the physical and metabolic methods in the regulation of body temperature.
11.4 Regulation of blood glucose level
???the action of pancreatic hormones on blood glucose regulation. [Refer to Section 6 and 10.]
???the role of liver in blood glucose regulation. / ???Discuss why organs donated for transplantation
should be kept in ice-cold saline solution.
???Use audiovisual materials to show the structure of a dialysis machine, and discuss with students the scientific principles involved.
???Discuss alternative ways to dialysis machine, e.g. peritoneal dialysis, kidney transplant.
???Suggest a diet for patients with renal problems with reasons.
???Examine the structural adaptive features of xerophytes in relation to water balance.
???Examine the features of mammalian skin in relation to temperature regulation.
??Discuss how ectotherms and endotherms respond to hot and cold conditions.
??Construct a concept map to show the mechanism of temperature regulation.
??Construct a concept map to show the mechanism of blood glucose regulation. / ???point out the importance of a constant internal environment and the negative feedback mechanism involved in its maintenance.
???develop an appreciation of the interrelationships of various systems in maintaining a constant internal environment.
???relate the structure of the kidney to its functioning in regulation of water.?
???state the biological principle involved in dialysis machine.
???relate water balance in terrestrial flowering plants including xerophytes to their adaptations.
???point out the importance of the body
temperature regulation in metabolic activities, behaviour and ecological distribution of animals./
???distinguish between ectotherm and endotherm.
???describe how ectotherms and endotherms respond to hot and cold conditions.
???state the mechanism of temperature regulation in mammals.
???apply the negative feedback mechanism to explain the homeostatic control of body temperature.
???point out the importance of maintaining constant blood glucose level.
???explain the role of insulin and glucagon in blood glucose level regulation.
???apply the negative feedback mechanism to explain the homeostatic control of blood glucose level.
Suggested answers
0 1. Homeostasis of respiratory gases
Q.1 Role of stretch receptor
The stretch receptors in the lung tissues mainly functions as a brake in the inspiratory process. Informing the respiration centre about the state of inflation of the lungs to prevent over-stretching.
Its inhibitory effects on the inspiratory centre (when the lungs are inflated) helps the body to establish a basic rhythm of the respiratory cycle.
Q.2 Role of chemoreceptors
Chemoreceptors are much more sensitive to changes in blood CO2 concentration than that of O2. They thus monitor the respiratory needs of the body and help to inform the respiratory centre about the metabolic state and need of the body.
This is important for adjustment / regulation of the ventilation rate (involving raising both the rate and depth of breathing when CO2 concentration in the blood rises) to the need of the body.
Q.3 Effects of Hyperventilation
Hyperventilation increases the tidal movements of air, carbon dioxide is expelled from the lungs and the carbon dioxide tension of the alveolar air decreases.
This reduces the carbon dioxide tension of the blood and increases the blood pH.
The low CO2 concentration in the blood cause cerebral blood vessels to constrict, reducing brain blood supply and produces symptoms of dizziness and fainting. The increase in blood pH and low CO2 conditions may also decreases the ventilation rate.
(Breathing into a paper bag can help since the person is rebreathing expired air, which is enriched with CO2, CO2 is retained in the blood.)
1 Excretion and Osmoregulation
P.1 Ammonia is highly toxic. Most aquatic animals get rid of ammonia by excreting it in very dilute solutions. Most terrestrial animals convert the ammonia to urea or uric acid, which conserves water because these less toxic wastes can be transported in the body in more concentrated form.
Land snails, insects, birds, and some reptiles excrete uric acid as the major nitrogenous waste. Because it is thousands of times less soluble in water than either ammonia or urea, uric acid can be excreted as a precipitate after nearly all the water has been reabsorbed from the urine. In birds and reptiles, the pastelike urine is excreted into the cloaca and eliminated along with faeces from the intestine.
P.2 The basement membrane of the capillary wall has the smallest pore size thus acting as the effective barrier for ultrafiltration.
P.7 The osmotic concentration of the filtrate before entering the Loop of Henle is isotonic to blood.
Na+ is actively transported out of the ascending limb of the loop of Henle. Since the ascending limb is relatively impermeable to water, the loss of Na+ in the filtrate make it actually hypotonic compared to the filtrate first entering the loop.
P.9 ADH promotes water reabsorption mainly by increasing the permeability of the collecting duct to water, so that more water can be reabsorbed by osmosis.
By increasing the permeability of Urea, ADH enhances that osmotic water uptake because urea also contributes to the hypertonicity of the medulla tissue, thus more water can be reabsorbed by osmosis.
Effect on Urine output by :
A) Drinking 1 L of hypertonic saline
Increase in ADH secretion helps to increase the proportion of water reabsorbed in the collecting duct.
At the same time, a smaller proportion of salt (NaCl) is reabsorbed. (this is due to the effect of another hormone called aldosterone)
The end result is excretion of a large volume of urine with high salt content / large volume of hypertonic urine (due to osmotic diuresis).
This helps to reduce the blood volume as well as decreasing the osmolarity of the blood back to normal.
B) Drinking 1 L of Distilled water
The end result is excretion of a large volume of urine with little salt content. This helps to reduce the blood volume & restore the osmolarity of the blood back to normal.
Common misconceptions:
1
/Most water in the glomerular filtrate is reabsorbed by the loop of Henle or the collecting duct.
/The proximal convoluted tubule is responsible for reabsorbing MOST water from the Glomerular filtrate (GF).
In fact, 80% of the water of the glomerular filtrate is reabsorbed in the proximal coiled tubule by osmosis, accompanying the reabsorption of glucose, amino acids and most mineral salts in this region. The amount of water reabsorbed in this region is relatively independent of the degree of hydration or dehydration of the body (obligate reabsorption of water).
With less than 20% of the original water content remaining, the collecting duct is important in regulating the final concentration of urine by controlling the amount of water reabsorbed along the collecting duet. Thus the collecting duct of the nephron serves a similar role as the colon of the alimentary canal by regulating the final water content of the product to be removed from the body, although most water is reabsorbed in the proximal coiled tubule. The amount of water reabsorbed in the collecting duct is regulated by the antidiuretic hormone secreted from the pituitary gland.
The main function of the loop of Henle is NOT to reabsorb water or mineral salts, but to establish a high osmotic concentration of sodium salts in the interstitial fluid towards the inner region of the medulla by active transport and the counter current mechanism.
2
/Urea is not reabsorbed in the renal tubules and is excreted completely in the urine
/Following the reabsorption of large volume of water in the proximal coiled tubule, the concentration of urea in the tubular fluid (GF) increases, establishing a diffusion gradient and urea diffuses from the tubular fluid into the surrounding tissue fluid and capillaries.
Because of this diffusion of urea back to the circulation, only about 50% of the filtered urea remains inside the renal tubule. Despite this reabsorption of urea, urea concentration in the tubular fluid increases steadily as relatively more water is reabsorbed.
3
/A shipwrecked sailor had drifted in the sea for some days without any fresh water supply. Could he survive longer by drinking sea water?
/sea water contains 3.5% of minerals, while the human kidney can produce a hypertonic urine that contains 2.2% minerals. Thus if the sailor drinks sea water, he needs to use extra water from his body to excrete the minerals taken in with the sea water. This increases the rate of dehydration of his body and hastens his death.
3. Homeostasis of Thermoregulation
P.1 Endotherms & Ectotherms
a. Oxygen consumption rate relates to aerobic respiration. The energy released by this is required in all metabolic reactions. The faster the rate of metabolism, the greater the consumption of oxygen.
b. As environmental temperature increases, less heat is required from metabolism to maintain a constant body temperature.
c. Body temperature of endotherms and thus optimum temperature for metabolic reactions lies between 35- 45°C. Endotherms readily adjust its rate of heat loss to equate that of heat gain by metabolism, thus allowing for heat loss and the insulating effect of body tissues, little demand is made for metabolic heat and a steady level of metabolism can be maintained.
d. Endotherms have a metabolism that generates heat at all times, though this is reduced to a minimum at environmental temperatures of more than 30°C. Ectotherms generates little internal heat and hence survive with a much lower metabolic rate. In general, ectotherms are less active than endotherms.
e. The curve should rise up to a maximum probably to around 40 to 45°C. It should then fall fairly rapidly. Most ectotherms, unless specifically adapted for high temperature, have a lethal temperature of around 40 45°C.
f. The curve will remain stable, that is flattened until lethal temperature is reached. Endotherms can maintain a lower temperature than the environment by regulating heat loss. This cannot continue indefinitely.
P.5 Role of Hypothalamus in thermoregulation
a. During this period the subject was allowed to equilibrate with his surroundings.
b. There is a direct relationship between these two variables which suggests that the rate of sweating is controlled by activity of the hypothalamus.
- There is a dynamic equilibrium of heat loss and gain at the skin before the the ingestion of ice that maintain a stable skin temperature. During the time when ice was being ingested, the rate of evaporative heat loss at the skin decrease due to the action of the hypothalamus (reducing heat loss), latent heat of evaporation is not being lost from the skin and this accounts for the observed rise in skin temperature.
P.5 Fever
'Fever' is due to the resetting of the hypothalamic 'thermostat' at a higher temperature. Until the core temperature rises to that temperature there is a discrepancy between 'normal' body temperature and the cold conditions.
In these conditions the body responds by shivering and the body continues to feel cold until the core temperature reaches the temperature of the ‘ hypothalamic thermostat’.
P.7 Adaptations to life at Low Temperature
a. The metabolic rate of the three animals is unaffected at environmental temperature above 26°C. Below that, the metabolic rate of the racoon increases very steeply so that at 0°C, its metabolic rate increased four folds.
The metabolic rate of the lemming does not rise until temperature falls below 20°C and then it increases less rapidly. The Eskimo dog shows no increase in metabolic rate until -20°C and below this, increase is more gradual.
b. Increased exercise & by involuntary shivering.
c. Muscle fatigue & heavy energy expenditure.
d. Radiation, convection & conduction (also evaporation of sweat or fluid from lungs & mouth).
e. Insulation of the body & by seeking warmer microhabitat.
f. Hair / fur.
g. By trapping air between layers of hair. Air is a bad conductor & air insulating layer is formed.
h. By ‘fluffing up’ or being raised to trap a thicker layer of air.
i. No. It would be expected that fur would be thickest in the Eskimo dog, then the lemming and least thick in the racoon.
Thick fur in racoon would make heat loss a problem at high temperature. The Eskimo dog is unlikely to be exposed to high environmental temperature.
j. Human compensate for lack of a covering of hair by wearing clothes that can adapt them for a wide range of temperature.
k. Fat cells in the dermis also provide an insulating layer.
l. The layer of fat is likely to be least developed in the racoon & most highly developed in the Eskimo dog.
A summary of Thermoregulatory responses
P.8 Adaptation to Life at High Temperature
Q. To reduce heat gain from absorption of solar energy.
a. This is the temperature of the tissues below a level of 2.5 cm beneath the surface of the skin. This temperature is normally determined by taking the rectal temperature.
Whereas the ‘core’ of body is maintained at a very narrow range of ‘stable’ temperature; temperatures in other regions of the body can vary tremendously depending upon position and the external temperatures.
b. i) 6°C - 7°C ii) 3°C - 4°C
c. This reduces the temperature gradient between the body temperature and the environment, so less heat is gained from the environment.
Allowing the body temperature to rise through reduction of sweat production, this saves precious water in the camel.
d. Radiation and convection.
e. If the temperature falls to 34°C at night, much of the day will pass before the body temperature reaches over 40 an sweating begins.
f. This reduces heat gain by acting as an insulating layer.