Homeostasis

  • Homeostasis regulates a range of variables in humans

    • body temperature: 36 - 37.5 C

    • blood pH: 7.35 - 7.45

    • blood glucose levels: 3.9 - 5.6 mmol/L

      • levels increase after a meal but return to normal within a few hours

    • blood osmotic concentration: 275 - 295 mOsm/kg of water

      • blood osmotic concentration - measure of concentration of solutes in the blood

  • negative feedback - response that reverses a detected change to return body to homeostatic equilibrium within a tolerable range of set point

    • homeostatic variable increases → negative feedback decreases variable → returns it to set point

    • homeostatic variable decreases → negative feedback increases variable → returns it to set point

  • positive feedback - response that amplifies a detected change

    • variable is increasing/decreasing → positive feedback further increases/decreases variable

    • variable does not return to a set point

  • hormones - chemical messengers released by endocrine glands

    • transported to target cells via bloodstream

  • Regulation of blood glucose levels

    • Increase

      • blood glucose levels increase after a meal → pancreas detects change → beta cells in islet of Langerhans secrete insulin into bloodstream

      • insulin travels to target cells (liver, muscle cells) → insulin binds to insulin receptors on surface of target cells → cells remove glucose from bloodstream

      • liver cells convert glucose into glycogen for storage

        • muscle cells use increased quantities of glucose for aerobic respiration

    • decrease

      • blood glucose levels decrease during excercise → pancreas detects change → alpha cells in islets of langerhans secrete glucagon into bloodstream

      • glucagon travels to liver cells → glucagon binds to glucagon receptors on the liver cells

      • glucagon triggers liver cells to convert glycogen into glucose → glucose released into bloodstream

    • example of negative feedback loop

  • Diabetes

    • Type 1

      • autoimmune disease

        • beta cells are damaged

        • body no longer produces insulin

      • risk factors = mainly genetic/autoimmune

      • cannot be prevented

      • treatment

        • blood sugar monitoring

        • injections of insulin

        • benefit from healthy diet + regular excercise

    • Type 2

      • beta cells produce insulin → liver and muscle cells DO NOT RESPOND

        • aka target cells are resistant to insulin

      • risk factors

        • excess body weight

        • lack of excercise

        • diet high in sugars/fats

        • genetics

      • prevented via healthy diet + lifestyle

      • treatment

        • lifestyle changes

          • healthy diet

          • excercise

        • medicines

  • thermoregulation - process by which an organism maintains body temperature within a narrow range

    • involves negative feedback loops

    • peripheral thermoreceptors - monitor body temperature

      • sense a change in body temperature → send nerve signals to thermoregulatory center of hypothalamus (in brain)

      • hypothalamus - monitors body temperature → initiates range of negative feedback responses → warms/cools body as required

    • body temperature drops → hypothalamus sends nerve signals to skeletal muscles → shivering

      • muscles = effectors

        • stimulated by motor neurons

    • pituitiary gland secrets thyroid stimulating hormone → regulates secretion of thyroxin from thyroid gland → controls metabolic rate

      • increase in thyroxin → increased rate of metabolism → warm up body

    • adipose tissue (body fat) - under skin, acts as insulator + reduces heat loss

  • endotherms - animals that generate internal heat to maintain body temperature

    • ex, birds + mammals

  • ectotherms - rely on external sources of heat

  • vasoconstriction - narrowing of blood vessels

    • less blood flowing

    • body is cold → blood vessels near skin vasoconstrict → reduce heat loss through skin

  • vasodilation - widening of blood vessels

    • more blood flowing

    • body is hot → blood vessels near skin vasodilate → increase heat loss through skin

  • shivering - caused by rapid involuntary contraction/relaxation of skeletal muscles

    • occurs when cold

    • muscle contractions require ATP energy

      • generation of ATP by respiration → releases heat energy → warms up body

  • sweating - release of water from sweat glands

    • occurs when hot

    • water absorbs heat energy from skin → water evaporates → body cools down

  • body is cold → muscles in skin contract → pulls hair shafts upright + erect → upright hair traps air + acts as insulator → prevents heat loss

    • piloerection - hair erection

    • humans have sparse body hair → limited thermoregulatory effect

  • brown adipose tissue (BAT) - specialized type of adipose tissue containing high concentration of mitochondria

  • uncoupled respiration - involves aerobic respiration producing heat instead of ATP

    • protons return to matrix through another proton channel instead of ATP synthase → ATP not produced

  • body is cold → brown adipose tissue produces heat via uncoupled respiration → body is heated up

  • water molecules form hydrogen bonds w each other

    • water has high latent heat of vaporization

      • latent heat of vaporization - energy required to convert liquid water to gas

    • water = excellent coolant

      • requires a lot of heat energy to evaporate

  • human behaviors to regulate body temperature

    • use of shelter

    • moving into shade/cooler areas when hot

    • adding/removing clothes

    • changing activity levels in response to external temperatures

    • hydration to cool down body

    • fans/ventilation

    • heating

    • planning activities to avoid colder/hotter parts of day

  • excretion - removal of metabolic waste from body

    • kidneys excrete urea

  • osmoregulation - maintenance of internal balance between water + dissolved material

    • regardless of environmental conditions

    • kidneys maintain osmotic balance btwn concentration of blood + tissue fluids + cytoplasm of cells

    • unit for osmotic concentration = osmol L-1

  • Nephron - functional units of kidney that filter blood + produce urine

    • structure includes

      • glomerulus

      • bowman’s capsule

      • proximal convoluted tubule

      • loop of henle

      • distal convoluted tubule

      • collecting duct

  • Ultrafiltration of blood

    • produces filtrate that enters Bowman’s

      • occurs in glomerulus

    • glomerulus - cluster of capillaries in bowman’s capsule

      • blood enters glomerular capillaries at high pressure

        • capillaries are much narrower than the arteriole that brings blood to the nephron

        • Afferent arteriole - brings unfiltered blood

          • thicker

        • Efferent arteriole - takes away filtered blood

          • thinner → higher pressure

      • glomerular capillaries are fenestrated → 20% of blood plasma leaks out

        • fenestrated - have many large pores in walls

      • glomerulus prevents large molecules from entering nephron

        • Stays in blood

          • RBCs/WBCs

          • platelets

          • proteins

            • antibodies

            • clotting factors

            • prothrombin

            • fibrinogin

      • glomerulus adaptations

        • fenestrated capillaries

        • basement membrane - glycoprotein that filters blood plasma

        • podocytes - surround capillaries in bowman’s capsule

    • glomerulus produces glomerular filtrate → filtrate enters nephron through bowman’s capsule

      • glomerular filtrate does not contain large molecules

      • BUT other materials are in same concentration as blood entering glomerulus

        • H2O

        • glucose

        • amino acids

        • salts/NaCl

        • urea

      • many useful substances from blood plasma are in glomelular filtrate

    • useful substances reabsorbed in bloodstream as filtrate passes through proximal convoluted tubule

  • selective reabsorption

    • large volumes of glomerular filtrate produced

      • contain water + nutrients that the body doesnt wanna lose

    • 1 liter of filtrate produced every 10 minutes by kidneys

    • proximal convoluted tubules - selectively reabsorb nutrients + most of water in filtrate

    • all glucose + amino acids in filtrate taken out

      • actively transported or facilitated out of proximal convoluted tubule → capillaries surrounding nephron

        • 80% of minerals = actively transported out of tubules → capillaries

      • high concentration of solutes outside tubule → 80% of water in filtrate leaves proximal convoluted tubule (osmosis)

    • adaptations of convoluted tubules - reabsorb nutrients + water

      • folded → increases surface area

      • microvili → increases surface area

      • one cell thick → rapid transport

      • protein pumps in plasma membrane → active transport

      • many mitochondria produce ATP via aerobic respiration → active transport

      • proximal convoluted tubules surrounded by capillaries → transport nutrients + water away from kidney

  • loop of henle - reduces volume of filtrate

    • transports water + sodium ions into medulla (kidney)

      • proximal convoluted tubule reabsorbs 80% of water BUT leaves a lot of water to be reabsorbed

    • ascending + descending limbs have different permeabilities to water + sodium

      • descending

        • permeable to water

          • has many aquaporins

            • water leaves by osmosis into high sodium concentration of medulla

        • impermeable to sodium

      • ascending

        • actively transports sodium into medulla

          • aka permeable to sodium

          • high sodium ion concentration in medulla → medulla is hypertonic compared to filtrate

        • impermeable to water

  • distal convoluted tubule - balances blood pH + mineral concentration

    • fluid that enters the DCT = dilute urine

    • urine enters collecting duct after leaving DCT

  • all useful substances + most water has been reabsorbed → remaining filtrate = urine

    • urine has high concentration of metabolic waste (ex. urea)

  • osmoregulation - maintenance of internal balance btwnn dissolve materials and water

    • osmoreceptors - monitor osmotic concentration of blood

      • located in hypothalamus

      • hypothalamus - regulates concentration of urine by stimulating pituitary to release antidiuretic hormone (ADH) when osmotic concentration of blood is too high

    • collecting ducts = differentially permeable to water

    • osmotic concentration of blood is too high

      • hypothalamus stimulates pituitary → pituitary release ADH into blood → ADH binds to receptors on cells lining collecting ducts

      • aquaporins switched from vesicles to cells’ plasma membranes → collecting duct is permeable to water

      • action of loop of henle → medulla has high solute concentration → water leaves by osmosis

    • osmotic concentration of blood is low

      • hypothalamus inhibits secretion of ADH → absence of ADH → aquaporin switch from plasma membrane to vesicles of cells in collecting duct → reduces permeability of collecting ducts

      • water is not reabsored as urine passes through collecting duct

      • dilute urine produced w low concentration of urea

    • negative feedback loop

  • nephrons - filter blood to form a glomerular filtrate

    • filtrate passes through nephron → converted to urine

  • sleep

    • sympathetic + parasympathetic nervous systems - involved in regulating blood supply to organs depending on body’s activity

      • body metabolic rate is low during sleep → parasympathetic nervous system changes blood supply to maintain basic body functions

        • blood flow to muscles reduced

          • not engaged w physical activity

        • blood flow to gut reduced

          • digestion slows during sleep

        • blood supply to brain + kidneys = constant

  • vigorous activity

    • energy + oxygen demands of body increase during vigorous activity → sympathetic nervous system becomes active

      • blood flow to muscles increased

        • engage in physical activity

      • blood flow to gut reduced

        • blood flow to muscles prioritized

      • blood supply to brain constant

      • blood flow to kidneys reduced

  • wakeful rest

    • there is a balance btwn sympathetic + parasympathetic systems during low level activities

      • blood flow to muscles low

        • BUT higher than sleeping

      • blood flow to gut stable

        • blood flow increases after a meal

      • blood supply to brain constant

        • support cognitive function

      • blood flow to kidneys stable