subject guide notes

D3.3.1—Homeostasis as maintenance of the internal environment of an organism

organisms have developed mechanisms to maintain a relatively stable internal environment despite changes to the external environment

homeostatic mechanisms aim to keep the variables in the body within optimal conditions, despite changes in environment

examples of internal conditions that’re monitored & maintained in humans include:

  • body temperature

  • body pH

  • blood glucose concentration

  • blood osmotic concentration

D3.3.2—Negative feedback loops in homeostasis

there is negative control in homeostasis, instead of positive, because the body has to resist changes to its internal environment, and introduce mechanisms to return the internal environment back to its preset limits

  • if the internal environment exceeds these limits, it can hv consequences for organism’s health & bodily functions

    • ex; hypothermia is when the internal body temp is low, and causes bodily functions to fail

D3.3.3—Regulation of blood glucose as an example of the role of hormones in homeostasis

remember - cells in body use glucose as an immediate energy source during respiration

exocrine tissue of pancreas produces enzymes for digestion

endocrine tissue of pancreas produces insulin & glucagon

  • Islets of Langerhans produces these 2 hormones

the 2 hormones are responsible for maintaining & controlling blood glucose concentrations

D3.3.4—Physiological changes that form the basis of type 1 and type 2 diabetes

insufficient insulin production = high blood glucose levels

diabetes causes hyperglycaemia

in ppl with diabetes, the cells cannot obtain enough glucose from the blood

kidneys begin to filter excess glucose from the blood

  • they also draw water from blood to dilute the urine, which causes dehydration

    • so the person becomes continually thirsty

hyperglycaemia caused by untreated diabetes can result in:

  • nerve damage

  • cardiovascular damage

  • blindness

type 1 diabetes is an autoimmune disorder

  • the immune system attacks the beta cells of the pancreas

    • inability to produce insulin

typically diagnosed in childhood or adolescence - so type 1 is sometimes called early onset diabetes

individuals can manage their blood glucose through a combination of regular exercise, dietary modifications or insulin therapy

type 2 diabetes is when insulin is being produced, but the cells are insensitive to it

  • since the cells are resistant to insulin, the glucose cannot enter the cells & remains in the blood = high glucose levels

beta cells in pancreas produce more insulin and can become tired

most of the time, it can be reversed by modifications to diet, regular physical exercise or moderate weight loss

D3.3.5—Thermoregulation as an example of negative feedback control

thermoregulation coordinated by nervous system & is centred in the hypothalamus

hypothalamus & pituitary gland work together to regulate production & secretion of hormones

using thermoreceptors, birds, animals & humans can detect changes in temperature

  • in skin - peripheral thermoreceptors

  • inside body - central thermoreceptors

hypothalamus stimulates pituitary gland

pituitary gland releases thyroid stimulating hormone (TSH) to stimulate the thyroid gland

when stimulated, thyroid gland produces hormone called thyroxine

if thyroxine levels are high, the production of TSH is low

  • example of negative feedback loop

when body temp increases, signals from the thermoreceptors signal to the pituitary to stop production of TSH

i think this process only happens in cold temps

D3.3.6—Thermoregulation mechanisms in humans

in cold conditions

skeletal muscles will undergo shivering, which increases body temp

muscles on skin contract, in order to make hairs on skin stand up

  • this traps heat in the layer of air between the skin & hair

vasoconstriction of blood vessels also reduces blood flow to peripheral blood vessels, keeping blood close to centre of body, conserving heat

also, mitochondria in brown adipose tissue can release energy without producing ATP - is called uncoupled respiration

  • increases body temp

hypothalamus also stimulates release of thyroxin

when it gets warmer

hairs on skin lie flat

glands in skin secrete sweat

  • when the water in sweat evaporates, it carries heat from the body (called evaporative cooling)

vasodilation of the blood vessels closest to surface of skin, bring more blood to surface of body

  • blood will carry heat to surface of body to encourage heat loss via convection & conduction

D3.3.7—Role of the kidney in osmoregulation and excretion

kidneys are responsible for osmoregulation (maintaining balance between water loss & water gain) & of excreting toxic waste products

blood enters kidney via the renal artery, & each artery forms smaller arterioles, which connect to capillaries

within the nephron, solute & water present in blood, leave capillaries & enter Bowman’s capsule

  • the filtrate contains monomers, ions & waste

as filtrate moves through the nephron, useful solutes are absorbed by the capillaries

  • nephrons absorb back most of the water & ions to regulate water & ion balance of blood

  • waste left in tubule & collected in bladder

    • waste products & water are called urine

D3.3.8—Role of the glomerulus, Bowman’s capsule and proximal convoluted tubule in excretion

ultrafiltration of blood is first stage of urine production

blood enters the nephron through the afferent arterioles

  • it will flow through the glomerulus where it is filtered to form urine

  • blood exists through the efferent arterioles

glomerulus is also enveloped by podocytes & composed of intertwined capillaries

capillaries are also covered in basement membrane

  • during ultrafiltration, the slits in the capillary allow blood, small proteins, salts and nutrients to flow out, but the basement membrane prevents white & red blood cells from passing through

reabsorption occurs in the proximal convoluted tubule, which absorbs back the following solutes from the filtrate to the blood

  • all the monomers (such as amino acids and glucose)

  • most of the water & ions

the reabsorption of substances from the glomerular filtrate involves the membrane transport mechanisms of diffusion, facilitated diffusion, osmosis and active transport:

  • The pumps use ATP for active transport to shuttle Na+ (out of the tubule) and K+  (into the tubule). As a result of this the Cl–are attracted to the space outside the tubule because of the positively charged Na+.

  • Glucose and amino acids are absorbed along with Na+ from the filtrate by specific carrier proteins down their concentration gradient. Since absorption of glucose and amino acids into the proximal convoluted tubule is powered by active transport of Na+ into blood, it is referred to as secondary active transport.

  • The glucose and amino acid concentration within the proximal convoluted tubule cells increases as they are absorbed from the filtrate. This concentration is higher than that of blood plasma, thus both glucose and amino acids are reabsorbed into blood by diffusion. Microvilli in the tubule wall cells greatly increase the surface area, which in turn enhances the diffusion process

D3.3.9—Role of the loop of Henle

both parts of the loop of Henle are designed to reabsorb the ions & water that weren’t reabsorbed by the proximal convoluted tubule

for the descending limb

  • doesn’t absorb sodium

  • has aquaporins that allow water to move from the filtrate to the blood

    • water moves because of osmosis?

      • the urine in the loop of Henle has a low solute concentration & the surrounding fluid has a high solute concentration

  • as urine moves further down this limb, more water is reabsorbed

for ascending limb

  • solutes such as sodium, chloride & potassium are actively transported out of urine & into surrounding fluid

  • has no aquaporins = is impermeable to water

D3.3.10—Osmoregulation by water reabsorption in the collecting ducts

distal convoluted tubule reabsorbs more water before the filtrate enters collecting ducts

collecting duct is responsible for osmoregulation & reabsorbs water based on body’s hydration status (if body is dehydrated or not)

osmoreceptors, located in hypothalamus, can detect changes in osmolarity of blood

if they detect high osmolarity (indicating high sodium & increased dehydration), the hypothalamus releases antidiuretic hormone (ADH)

ADH acts on the collecting duct and mobilises the aquaporins from the intracellular vesicles to the cell membrane

  • increase in aquaporins = allows for more water reabsorption from filtrate

D3.3.11—Changes in blood supply to organs in response to changes in activity

during sleep or rest, overall activity is reduced. so:

  • low blood supply to major organs

  • heart rate at normal or below normal levels (cuz not much demand for blood)

  • blood flow to muscles is redirected to other essential functions (such as digestion, tissue repair and growth)

during increased activity, organs hv higher demands for organs & oxygen, so:

  • high blood supply to major organs, such as lungs

  • heart rate is above normal levels (cuz high demand for blood)

  • blood flow to digestive organs is mostly redirected to skeletal muscles

  • after heavy exercise, may be more blood flow to kidneys to support regeneration & regulate blood values

blood vessels in certain systems can vasodilate, while others vasoconstrict, and this redirects the blood

  • for example, after a large meal, most of the blood is diverted to stomach via vasodilation of vessels of digestive system, & vasoconstriction of other vessels