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Demonstrate understanding of how an animal maintains a stable internal environment
Credits: 3 Assessed: Internal Lit/ Num Standard: Literacy
Introduction
Animals can maintain a stable internal environment - homeostasis - despite fluctuations in the external environment. This standard requires you to investigate how one of the following control systems operates in maintaining homeostasis:
body temperature
Blood pressure
Osmotic balance
Blood glucose levels
Respiratory gas levels and balance in body tissues.
You will need to carry out research to obtain information on the chosen control ( homeostatic) system so that you have knowledge and understanding of the:
purpose of the control system - ie the selective advantages to the animal.
components of the control system - these may include specific cells/tissues/organs/body systems/also specific chemicals, such as enzymes and hormones.
mechanisms of the control system- how the control system responds to the normal range of environmental fluctuations.
Mechanisms of the control system involve interaction and feedback between parts of the system including:
Biochemical Processes - the chemical reactions and pathways that occur in the body cells ( eg the ornithine cycle occurs in liver cells to detoxify the ammonia produced in the de-amination of amino acids from proteins in our diet).
Biophysical processes - the physical factors ( eg vasoconstriction and vasodilation) that affect biology and biochemistry.
You also need to obtain information on how the control system may be disrupted by factors in the internal and/or external environment.
Factors in the internal environment that may result in disruption and breakdown of control:
genetic conditions
metabolic disorders
Factors in the external environment that may result is disruption and breakdown of control include:
extreme environmental conditions - such as high temperatures, low oxygen levels, reduced water supply
disease or infection
drugs or toxins
If the system is unable to maintain or regain control, then death may result.
Homeostatic Feedback Mechanisms
Body cells function efficiently within a narrow range of conditions ( eg. pH, osmotic pressure, temperature) - therefore the internal environment must be kept relatively stable. maintaining a dynamically stable internal environment is known as homeostasis, the body has complex feedback control systems ( homeostatic mechanisms) to maintain homeostasis/keep the internal environment constant. Feedback control systems depend on different body systems/organs working together.
Homeostatic feedback control systems are typically cyclical, involving:
input ( also known as a stimulus) from the internal or external environment.
receptors/sensors - sense organs/sensory nerves or cells
controller - usually the brain ( especially the hypothalamus)
effectors- glands/muscles/organs.
output - the response/action of the effector.
Negative Feedback means that whenever a change occurs in the system, it sets off a corrective mechanism that acts to reverse the change and bring the system back to a set point ( the normal level). For eg when glucose levels are high, negative feedback results in the control system acting to reduce the levels of glucose by removing glucose from the blood by converting it to glycogen.
Negative feedback control is important in the control of the following body systems:
body temperature
blood pressure
Osmotic balance
blood glucose levels
respiratory gases level and balance in body tissues.
The efficiency of control systems decreases with aging. If a control system cannot cope with the extent of the change to the system or a control system fails through disease or injury, the resulting unstable internal environment may lead to various disorders and eventually death. Important disorders associated with loss of control of the various systems include;
hypothermia/hyperthermia ( body temperature)
hypoglycemia/hyperglycemia ( blood glucose)
Hypotension/ hypertension ( blood pressure)
diabetes mellitus ( blood glucose)
diabetes insipidus ( osmotic control)
Endocrine System
The endocrine system is essential in maintaining homeostasis. It is a series of ductless glands (endocrine glands) located throughout the body. The endocrine glands secrete messenger chemicals called hormones, many are proteins eg testosterone, and estrogen directly into the blood for transport to target tissues ( organs/glands/cells). Each endocrine gland secretes its specific hormone and each hormone has a specific target and produces a specific effect.
Pancreatic gland secretes hormone insulin which targets liver cells to convert glucose from the blood into glycogen for storage. It is responsible for the control of blood glucose levels.
The Pituitary gland secretes the hormone ADH ( antidiuretic hormone) which targets kidney tubules to reabsorb much of the water from the filtrate, returning the water to the blood ( so it is not lost from excretion in urine. It is responsible for the control of osmotic balance.
Thyroid gland secretes the hormone thyroxine which controls the body's metabolic rate. Thyroxine acts on cells, increasing the number and activity of the mitochondria by binding to a cell's DNA. This increases the metabolic rate eg increased metabolism of proteins/ carbohydrates/ fats/ vitamins, increased protein synthesis, increased cardiac output/heart rate/ breathing rate. It is responsible for the control of the metabolic rate
Adrenal glands secrete the stress hormones adrenalin, accurately known as epinephrine, and noradrenalin, accurately known as norepinephrine. They prepare the body for the "flight or fight response" by increasing cardiac output, constricting the blood vessels( vasoconstriction), and dilating the airways. The adrenals also secrete aldosterone which affects kidney function by regulating the solute concentration of the blood plasma. It is responsible for the control of blood pressure.
The pituitary gland is known as the " master gland" as it not only produces its own hormones ( eg ADH) but also produces hormones that control the activity of other endocrine glands eg FSH - controls the production of thyroxine by the thyroid. The pituitary is about the size of a pea and is located directly under the brain. It is closely connected with a small area of the brain called the hypothalamus, which has receptors for monitoring the level of many substances in the blood and therefore acts as the control center for many homeostatic functions eg body temperature, and water/osmotic balance. The hypothalamus integrates the action of all homeostatic mechanisms. In hormone-controlled homeostatic mechanisms, there may be a lengthy period before the corrective mechanism comes into effect, as many hormones are proteins so need to be synthesized ( gene expression activated) by the cells in the relevant endocrine gland. The hormone has to enter the blood for transport to the target tissue where it produces the corrective reaction. Most of the internal control systems in our body are under the control of hormones which in turn may be under the control of the pituitary, which in turn is under the control of the hypothalamus.
Nervous System
Our nervous system consists of three parts - the central nervous system ( CNS), the peripheral nervous system ( PNS), and the autonomic nervous system ( ANS).
CNS is made up of the brain and spinal cord.
PNS receives and responds to stimuli through the nerves of the skin, organs, and skeletal; muscles - involves most of the body's nerves.
ANS is under the subconscious control of the brain and regulates the actions of glands ( eg endocrine. sweat, salivary), smooth muscles ( eg pupil contraction, digestion) heartbeat, and respiratory rate. It also controls internal sensations such as hunger, thirst desire to urinate and defecate. The ANS has two systems, the sympathetic and the parasympathetic, which work in opposition to each other. In general
the sympathetic tends to speed up functions- acts as an accelerator.
the parasympathetic tends to slow down functions- acts as a brake.
Most internal organs are supplied with nerves from both systems to control their function eg sympathetic increases the heartbeat, and parasympathetic decreases the heartbeat. The ANS is an essential component in many homeostatic mechanisms.
Homeostatic Control of Blood Glucose
Blood Glucose Levels
Although most tissues can use fat as an alternative source of energy, the body's main fuel is glucose. The brain is unable to use anything else. Body cells require glucose for cellular respiration to provide the energy needed in the form of ATP for their processes- heat energy, a by-product is important in maintaining body temperature.
The concentration of glucose in the blood affects every body cell and its concentration is maintained at a set point between 4.4 and 8.0 mmol/l. Glucose may be ingested directly in food. Glucose is the end product of the digestion of starches and carbohydrates but may also be produced indirectly from the digestion of proteins and fats.
It is difficult to define what a normal range is for blood glucose and figures vary between sources. In addition the units of concentration vary from country to country. The following table is indicative of blood glucose levels in NZ.
Blood Glucose level
Blood Glucose level
Setpoint
On the low side
Too low ( hypoglycemic)
Too high ( hyperglycemic)
New Zealand
New Zealand
4.4 -8.0- mmol/L
3.0 - 4.4 mmol/L
less than 3.0mmol/L
more than 8.0mmol/L
USA
USA
80-144 mg/dL
54-79 mg/dL
less than 54 mg/dL
more than 144 mg/dL
The need for glucose is continuous, but the supply from the intestine is intermittent. For a few hours after a meal, the blood leaving the small intestine is rich in glucose. During a short fast ( eg at night while sleeping)the digestive system supplies little or no glucose to the rest of the body. Despite this, in a healthy person, the blood glucose remains within the set point. To maintain such a stable blood glucose level, the body has to store glucose after a meal and release it back into the blood after a fast.
The body has two ' storehouses: for glucose - the liver and the skeletal muscles.
After a meal, liver cells remove glucose from the blood and convert it to glycogen( the animal equivalent of starch in plants). During a fast, the process is reversed ( glycogen to glucose)
Glucose stores in the muscles are normally used only during vigorous exercise. During recovery, these glycogen stores are replenished from blood glucose.
The storage and release of glucose are under the control of hormones secreted by the pancreas. Glucose receptor cells in the pancreas monitor glucose concentrations in the blood plasma. the pancreas contains 1-2 million tiny clusters of cells called the islets of Langerhans ( part of the endocrine system). Three important types of cells associated with the control of blood glucose are found in the islets of Langerhans - alpha, beta, and delta cells.
Alpha cells secrete the hormone glucagon. Glucagon stimulates the release into the blood plasma of glucose from the glycogen stored in the liver cells- this action increases blood glucose levels. Glucagon also stimulates the release of fatty acids from fat tissue.
The increase in glucose levels ( and the presence of free fatty acids) in the blood in turn stimulates the release of insulin and inhibits glucagon - part of the feedback loop in the homeostatic control of blood glucose.
Beta cells are the most common of the cell types. They secrete the hormone insulin when stimulated by:
the hormone glucagon
the growth hormone somatotropin ( aka growth hormone, GH, produced by the pituitary)
high blood glucose levels ( occur after a meal) - the most important stimulator.
Insulin stimulates the uptake of glucose by body cells for cellular respiration and stimulates liver cells to convert glucose to glycogen for storage. When the liver's glycogen store is full, surplus glucose is converted into fat and stored under the skin. This action decreases blood glucose levels.
Beta cells cannot make insulin and their failure to produce the amounts sufficient to control blood glucose that results in diabetes mellitus.
Delta cells secrete the hormone somatostatin which is a strong inhibitor of somatotropin, insulin, and glucagon; however, the role of somatostatin in the homeostatic regulation of blood glucose is not clear. ( Somatostatin is also produced by the hypothalamus, where its function is to inhibit the secretion of somatotropin growth hormone).
In emergencies, adrenaline/epinephrine, secreted by the adrenal gland, acts as a powerful stimulator for the breakdown of glycogen to glucose - part of the "fight-or-flight" response.
Slides for Homeostasis
Homeostasis worksheet- Thermoregulation and flowchart
https://www.liveworksheets.com/ap1929072fl
Cloze passage - Thermoregulation
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