SYLLABUS REFERENCE

[B5.5] explain the concept of homeostasis;
[B5.6] explain the concept of feedback mechanisms;
[B5.7] discuss the regulation of blood sugar;
[B5.9] distinguish between heat and temperature;
[B5.10] discuss the regulation of temperature;

WHAT IS HOMEOSTASIS?

This is the maintenance of a constant internal environment. This involves controlling physical and biochemical conditions, such as:

blood sugar concentration
body temperature
concentration of various ions, such as sodium vs potassium
pH (influenced by carbon dioxide levels)

The purpose is to keep conditions in and around the organism's cells constant.

If this is not done, conditions would not be optimum, and cells would stop working. For example:

if carbon dioxide in the blood & tissues get too high, it can change their pH, making it more acidic. A wrong pH can denature the enzymes working in those tissues, rendering them ineffective. This in turn can lead to death, as multiple crucial biochemical reactions can slow down or come to a dead stop.
if body temperature gets too high, this can also denature the enzymes and all of the cell's biochemical activity halts, leading to death
if we lose too much heat, cellular reactions slow down to the point of being unable to maintain life.

Maintenance of a constant internal environment is accomplished via several homeostatic mechanisms, all of which:

use negative feedback loops
are coordinated by the nervous and/or endocrine system.

NEGATIVE VS POSITIVE FEEDBACK MECHANISMS

Both types of feedback mechanisms exist in human beings. Both are defined by how the system responds to change in a particular factor.

Positive Feedback

The system detects a change in a particular factor, and responds by taking action to INCREASE that change.

Since the body is most dependent on maintaining a constant environment, this feedback mechanism is understandably rare. And, where present, they tend to be temporary, lasting minutes to a few months. Here are some examples of where positive feedback is actually required:

The muscular contractions of childbirth is stimulated by a hormone called oxytocin. As the birth canal stretches, it stimulates the production of more oxytocin. This then stimulates stronger & more frequent contractions to push the baby out, which stretches the birth canal further, and so on. This feedback loop continues until the baby is out.
Breastfeeding a baby stimulates the production of more milk. This continues for several months, till the baby is weaned.
Platelets that are attracted to a wound and stimulates the clotting process also release chemicals that attract more platelets to the wound. Those platelets in turn also release chemicals that attract even more platelets to the wound, and so on. This continues until the blood clot completely seals the wound.

Negative Feedback

The system detects a change in a factor and initiates an action or series of actions that brings the factor back to its normal levels.

For Example

Increased carbon dioxide in the blood causes a response by the medulla in the brain to increase breathing rate. This then lowers the CO₂ levels.

The level of carbon dioxide was the STIMULUS. The stimulus is detected by SENSORY CELLS (DETECTOR), which then triggers a RESPONSE.

Now let's take a look at THREE conditions that are maintained in the human body, and the homeostatic mechanisms that work to keep them constant...

Regulation of Blood Glucose

Homeostasis keeps the levels of glucose constant in the blood, whether we eat large or small quantities of carbohydrates.

This regulation depends on two hormones, both of which come from a part of the pancreas known as the islets of Langerhans:

insulin - secreted by beta cells
glucagon - secreted by alpha cells

Insulin LOWERS blood glucose concentration and glucagon RAISES blood glucose concentration.

If Blood Glucose Concentration Increases...

This is detected by receptor cells in the islets of Langerhans. In response, the beta cells are stimulated to secrete insulin.

The insulin passes into the blood
It stimulates several types of cells & tissues to increase their uptake of glucose from the blood:

In most cells, glucose is the priority fuel for cellular respiration
In the liver and skeletal muscle, the glucose is converted to insoluble glycogen for storage
In fat cells (adipocytes), it stimulates the conversion of glucose to fat for storage.

The concentration of glucose in the blood falls to normal levels.

If Blood Glucose Concentration Decreases...

This is detected by receptor cells in the islets of Langerhans. In response, the alpha cells are stimulated to secrete glucagon.

The glucagon passes into the blood.

It causes the liver cells to break down glycogen to glucose
It stimulates the production of glucose from amino acids or fat

The glucose is released into the bloodstream
The concentration of glucose in the blood increases to normal levels.

Regulation of Water Concentration (Osmoregulation)

Our body fluids have a relatively constant composition, whether we drink a little, or a lot of water.

Maintaining consistent water and mineral salt concentration in body fluids also helps to maintain consistent levels in the cells.

Active mechanisms to regulate water levels are centred in the kidneys. These organs do this by controlling the water content of the urine it produces. They do so under the influence of anti-diuretic hormone (ADH), which is secreted by the posterior pituitary gland.

If Water Content in the Blood is Too Low...

Osmoreceptors in the hypothalamus of the brain detects the high blood concentration (hypertonic).

The posterior pituitary gland is stimulated to secrete ADH. The hormone is then carried to the kidneys via the blood. Once there:

It increases the permeability of the distal convoluted tubules & collecting duct to water.
More water leaves the glomerular filtrate in the tubules, passing into nearby capillaries and surrounding kidney tissues.
The blood is diluted to a normal level, and a smaller volume of concentrated urine is produced.

If Water Content in the Blood is Too High...

The osmoreceptors in the hypothalamus are not triggered by the diluted blood (hypotonic). It therefore does not stimulate the posterior pituitary gland to secrete ADH.

In the absence of ADH:

The walls of the distal convoluted tubule & collecting duct remains less permeable to water.
Less water leaves the glomerular filtrate in the tubules and more water goes on to the collecting duct.
The blood increases concentration to normal levels since it does not receive more water, and a larger volume of dilute urine is produced.

Regulation of Body Temperature

Whatever the external temperature, the human body temperature must be regulated in the normal range, which is around 37℃.

Detection of Body Temperature

Many structures in the skin are concerned with maintaining a constant body temperature in humans. These structures are part of & are controlled via the nervous system.

The main detector of temperature change are temperature receptors in the hypothalamus in the brain. This is also known as the thermoregulatory centre in the brain. It monitors the temperature of the blood passing over the receptors.

Other detectors are temperature receptors in the skin. They detect body & environmental temperatures. They are responsible for when we feel hot or cold. Note, however, that the temperature at the surface does not matter as much. Our skin temperature can vary enormously without causing harm. The focus is on core body temperature.

Therefore, these other detectors don't control the involuntary body temperature regulating mechanisms. Instead, they prompt humans to take voluntary actions, such as rubbing hands together and jumping up and down if we feel cold.

If Body Temperature Increases...

This can be caused by one or more of the following:

excessive heat in the surroundings
strenuous exercise
infection

The thermoregulatory centre detects the change, and sends nervous impulses to several tissues & cells to produce the following responses:

Sweat glands releases sweat onto the surface of the skin.

The water in the sweat evaporates, using heat from the body
The body loses heat and cools down

The arterioles in the skin dilates

More blood flows from the arterioles to the blood capillaries near the skin's surface
More heat is lost from the blood to the skin's surface, from where it radiates into the atmosphere

Hair erector muscles that move our body hair all relax, and the hair lies flat.

This has no significant effect in humans, but can be quite helpful for the hairy and furry mammals

Metabolic rate drops, so less heat is produced.

If Body Temperature Decreases...

The thermoregulatory centre detects the change and initiates responses that work on heat production and heat retention.

Heat Production

Increased muscle action, whether via physical activity or involuntary shivering.

The muscles under the skin contract & relax rapidly, producing shivering.
This increases respiration, which also produces heat energy.

The liver is a large organ that performs many metabolic activities. Such activity produces a lot of heat, which is then transferred to the blood. It is therefore an important source of heat.
Metabolic rate speeds up, producing more heat.

Heat Retention

The layer of subcutaneous fat below the skin acts as an insulating layer against the cold.
Vasoconstriction retains heat in your body:

The muscular tissue in the skin's arterioles contract, reducing their diameter
Less blood flows through the arterioles to the blood capillaries near the skin's surface
Less heat is lost through the skin

Hair erector muscles contract. This pulls hair upright, trapping an insulating layer of air. Apart from producing goosebumps, it has little effect in humans. It is, however, quite helpful to the more hairy and furry mammals.
Sweat production is greatly reduced.

Search This Blog

CSEC Biology Notes

HOMEOSTASIS [CSEC HSB]