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[h] IB DP Biology HL D3.3 Homeostasis Flashcards
[q] D3.3.1—Homeostasis as maintenance of the internal environment of an organism
[a] Variables are kept within preset limits, despite fluctuations in external environment.
Include body temperature, blood pH, blood glucose concentration and blood osmotic concentration as homeostatic variables in humans.
[q] D3.3.2—Negative feedback loops in homeostasis
[a] Students should understand the reason for use of negative rather than positive feedback control in homeostasis and also that negative feedback returns homeostatic variables to the set point from values above and below the set point.
[q] D3.3.3—Regulation of blood glucose as an example of the role of hormones in homeostasis
[a] Include control of secretion of insulin and glucagon by pancreatic endocrine cells, transport in blood and the effects on target cells.
[q] D3.3.4—Physiological changes that form the basis of type 1 and type 2 diabetes
[a] Students should understand the physiological changes, together with risk factors and methods of prevention and treatment.
[q] D3.3.5—Thermoregulation as an example of negative feedback control
[a] Include the roles of peripheral thermoreceptors, the hypothalamus and pituitary gland, thyroxin and also examples of muscle and adipose tissue that act as effectors of temperature change.
[q] D3.3.6—Thermoregulation mechanisms in humans
[a] Students should appreciate that birds and mammals regulate their body temperature by physiological and behavioural means.
Students are only required to understand the details of thermoregulation for humans.
Include vasodilation, vasoconstriction, shivering, sweating, uncoupled respiration in brown adipose tissue and hair erection.
[q] D3.3.7—Role of the kidney in osmoregulation and excretion
[a] Students should understand the distinction between excretion and osmoregulation.
Osmoregulation is regulation of osmotic concentration.
The units for osmotic concentration are osmoles per litre (osmol L−1).
[q] D3.3.8—Role of the glomerulus, Bowman’s capsule and proximal convoluted tubule in excretion
[a] Students should appreciate how ultrafiltration remove solutes from blood plasma and how useful substances are then reabsorbed, to leave toxins and other unwanted solutes in the filtrate, which are excreted in urine.
[q] D3.3.9—Role of the loop of Henle
[a] Limit to active transport of sodium ions in the ascending limb to maintain high osmotic concentrations in the medulla, facilitating water reabsorption in the collecting ducts.
[q] D3.3.10—Osmoregulation by water reabsorption in the collecting ducts
[a] Include the roles of osmoreceptors in the hypothalamus, changes to the rate of antidiuretic hormone secretion by the pituitary gland and the resultant switches in location of aquaporins between cell membranes and intracellular vesicles in cells of the collecting ducts.
[q] D3.3.11—Changes in blood supply to organs in response to changes in activity
[a] As examples, use the pattern of blood supply to the skeletal muscles, gut, brain and kidneys during sleep, vigorous physical activity and wakeful rest.
[q] Homeostasis
[a] Regulation of a constant internal environment Homeostasis allows for fluctuations within acceptable limits
This is despite changes in the external environment
[q] Negative Feedback
[a] The reversal of a change in order to return to optimal conditions (or steady state).
Most systems work through negative feedback. Information fed back turns the system off.
Examples include blood glucose regulation and ADH
[q] Positive Feedback
[a] The reinforcement of a change, once one is detected, to move further away from the steady state.
Are not as common as positive feedback destabilizes the system and is often harmful.
Beneficial examples include transmission along neurons and oxytocin
[q] Regulation of Blood Glucose
[a] Normal level in the blood is between 60-100 mg/dl
Glucose levels that are too high result in hyperglycaemia
If the glucose level is too low, hypoglycaemia can occur
[q] Pancreas
[a] Both an exocrine and an endocrine gland
Exocrine glands secrete digestive enzymes
Islets of Langerhans contain two cell types of interest
– β cells secrete insulin
– α cells secrete glucagon
[q] Insulin
[a] Secreted from β cells in the pancreas
•Try to remember: ‘You are studying the IB course’
Insulin – Beta cells
•Binds to receptors on cell surface membranes altering the permeability of membranes to glucose
[q] 3 main target organs for insulin
[a] Liver: enzymes speed up conversion of glucose into glycogen (glycogenesis)
Muscle Tissue: glycogenesis stimulated
Adipose (fat storage cells): breakdown of lipids prevented here
[q] Glucagon
[a] Secreted from α cells in the pancreas
•Antagonistic to insulin
•Be careful not to confuse glucagon and glycogen. Try to remember: ‘Glucagon is the hormone’
Main effects:
•Activates enzymes in liver cells to speed up glycogenolysis (conversion of glycogen to glucose)
•Increases production of glucose from amino acids and fatty acids.
[q] Compare Insulin and Glucagon
[a]
[q] Type 1 Insulin-dependent or early onset diabetes
[a] Physiological changes: Constant thirst, Undiminished hunger, Excessive urination, Lack of insulin production
Risk factors: Can affect any age but more typically diagnosed in early childhood or adolescence
Prevention and treatment: No cure, Regular measurement of blood glucose level, Combination of:
– Injection of insulin into the bloodstream daily
– Dietary modifications
– Regular exercise
[q] Type 2 Insulin-independent or late onset diabetes
[a] Physiological changes: Mild – sufferers usually have sufficient blood insulin but insulin receptors on cells have become insensitive to insulin (insulin-resistant).
In response, β cells increase production of insulin and become exhausted.
Risk factors: Family history, lifestyle, body weight, diet, age
Prevention and treatment: Usually can be reversed with a combination of weight loss, healthy diet, and regular physical activity.
[q] Thermoregulation
[a] Thermoregulation is an example of negative feedback control
– Coordinated by the nervous system
– Birds and mammals detect changes in temperature using specialised neurons called thermoreceptors
[q] Types of Thermoreceptors
[a] Peripheral thermoreceptors are found in the skin
Central thermoreceptors are found inside the body
The hypothalamus integrates signals from both the peripheral and central thermoreceptors to regulate body temperature
[q] Hypothalamus in Thermoregulation
[a] Constantly monitoring the temperature of the blood flowing through it
Thus monitors our body temperature – should remain ~ 36.5℃/37℃
Receives information about temperature through peripheral and central thermoreceptors
Initiates behavioural and physiological responses to bring the temperature back to the set point
[q] Pituitary gland in Thermoregulation
[a] In cooler environments the body will lose heat
Signals from the peripheral thermoreceptors to the hypothalamus cause it to increase metabolic rate and therefore increasing heat production
[q] Production of Thyroxin
[a] The hypothalamus stimulates the pituitary gland.
1. The pituitary gland releases thyroid stimulating hormone (TSH) to stimulate the thyroid gland.
2. The thyroid gland produces a hormone called thyroxin. The primary role of thyroxin is to increase the metabolic rate of body cells, which will result in more heat.
3. The production of TSH is an example of a negative feedback loop. The release of TSH is decreased and stopped if the levels of thyroxin produced are very high.
4. When the body temperature increases, signals from peripheral and central thermoreceptors stop the hypothalamus from producing TSH.
[q] Thermoregulation in Low Temperatures: Skin Hair
[a] Skeletal muscles continually contract and relax involuntarily (shivering) to produce heat. This heat is transferred to the blood.
Hair erector muscles attached to the base of the hair follicle contract, causing the skin hair to stand up.
This traps a layer of air between the skin and the hair which helps to insulate the body.
[q] Thermoregulation in Low Temperatures: Capillaries
[a] Smooth muscles of arteries can adjust their diameter
Peripheral blood vessels undergo vasoconstriction (narrowing of those blood vessels)
Allows the conservation of heat by keeping the blood flow close to the core and vital organs e.g. heart , brain
[q] Thermoregulation in Low Temperatures: Fat-storing brown adipose tissue
[a] Brown adipose tissue cells are full of mitochondria.
Normally, during aerobic respiration in mitochondria, monomers are broken down, and these energy-producing reactions are coupled with ATP production.
Uncoupled respiration – mitochondria in brown adipose tissue cells can release energy without producing ATP, to increase body heat when it is cold outside.
[q] Thermoregulation in High Temperatures: Skin Hair
[a] Hair erector muscles attached to the base of the hair follicle relax and the hair lies flat.
The layer of insulating air is reduced and more heat is lost by conduction or direct air movement.
[q] Thermoregulation in High Temperatures: Role of Capillaries
[a] Smooth muscles of arteries can adjust their diameter
Peripheral blood vessels undergo vasodilation (widening of those blood vessels)
More blood to brought to the surface of the body.
Allows more heat loss at the surface via convection and conduction.
[q] Thermoregulation in High Temperatures: Sweating
[a] Sweat is produced when the body needs to lose heat (temperature has increased above the set point)
In low humidity and moving air, evaporation occurs and causing cooling
[q] Excretion
[a] Process of metabolic waste removal from an organism.
Performed by the excretory system: Composed of kidneys, lung, skin, large intestine, liver
Mammals excrete nitrogenous waste in the form of urea
[q] Osmotic Regulation
[a] Process of regulating osmotic concentration
– Water is constantly being gained or loss (sweat, urination, egestion, reabsorption) and needs to needs to be regulated in the body
Osmotic concentration: concentration of solute particles per unit volume
– Measurement unit: osmoles per litre (osmol L−1)
– Osmoles: 1 mole of a substance dissolved in water
[q] Kidney Structure
[a] Blood enters via the renal artery
Blood is filtered (removal of metabolic waste) and exits via renal vein
Urine exits renal pelvis via ureter towards bladder
[q] Nephrons
[a] Specialized functional units that perform:
– Filtration (removal of metabolic waste from blood)
– Excretion (removal of waste)
– Osmoregulation (regulate osmotic concentration)
[q] Ultrafiltration
[a] 1. Glomerulus is a knot of capillaries
2. Blood enters glomerulus via the afferent arteriole (wider) and exits via efferent arteriole (narrower)
3. Difference in diameter creates high pressure within glomerulus → ultrafiltration of solutes from blood into filtrate
Capillaries are fenestrated → allows for solutes in blood to flow out
4. Podocytes wrap around capillaries and form filtration slits to prevent blood cells from entering filtrate
5. Filtrate collected in Bowman’s capsule and continues to proximal convoluted tubule
6. Filtrate contains solutes like ions, waste, toxins, sugars, and water
[q] Podocytes
[a] Cells of the epithelial lining of Bowman’s capsule and wrap around capillaries to form filtration slits that are size selective
– Prevents red and white blood cells and other large proteins from entering filtrate
– Allows solutes such as urea, toxins, amino acids, salts, glucose, and water into filtrate
[q] Epithelium
[a] Refers to the outer lining of a surface or inner lining of a cavity.
[q] Endothelium
[a] Refers to inner lining of blood vessels. Fenestrated
[q] Selective Reabsorption
[a] Filtrate contains useful substances → needs to be reabsorbed into bloodstream
– Water, glucose, amino acids, ions
Occurs in proximal convoluted tubule (PCT)
Toxins and other unwanted solutes remain in filtrate to be excreted
[q] Selective Reabsorption: Reabsorption of selective solutes
[a] Sodium potassium pumps actively transport Na+ out of PCT into blood
Helps maintain low concentration of Na+ in PCT cell
Sodium diffuses passively from filtrate into PCT cell through the cotransporter proteins
Also transports glucose and amino acids from filtrate
Sodium dependent glucose cotransporters as an example of indirect active transport (concentration gradient of Na+ set up using active transport)
Passive diffusion of glucose and amino acids from PCT cell back into blood
Water moves passively via osmosis (follows movement of ions)
[q] PCT Structure to Function
[a] Microvilli (protrusions of cell membrane) to increase surface area to maximize diffusion and reabsorption
Cells of PCT wall contain many mitochondria to power active transport
PCT wall is only one cell thick to decrease diffusion distance from filtrate to blood.
PCT cells are connected by tight junctions to prevent leaking of larger materials
[q] Osmoregulation
[a] Filtrate from PCT travels down descending limb of Loop of Henle and then back up ascending limb
Active transport of sodium ions out of filtrate in ascending limb into fluid filled medulla region
Creates high osmotic concentrations in medulla (a “salty” environment)
Filtrate continues to collecting duct
Final reabsorption of water from filtrate in collecting ducts
Water passively exits filtrate into medulla through aquaporin channels
– High Na+ concentration in medulla maintained by ascending limb
Concentrated filtrate (urine) collects in renal pelvis and exits kidney via ureter into bladder
[q] Osmoreceptors
[a] Located in hypothalamus and detect changes in solute concentration in blood
Dehydration or high sodium levels → high solute concentration in blood detected by osmoreceptors → stimulate release of antidiuretic hormone (ADH) from pituitary gland → increase in number of aquaporins present in collecting duct membrane → higher water reabsorption from filtrate
[q] Osmoregulation: Aquaporins
[a] Stored in intracellular vesicles of collecting duct cells
Increase in ADH levels → vesicles fuse with collecting duct cell membrane → aquaporins insert into cell membrane → water reabsorbed from filtrate
Decrease in ADH levels → cell membrane invaginates → aquaporins stored back into vesicles
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