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

• e.g. blood glucose concentration, osmoregulation, pH, temperature

D3.3.2 Negative feedback loops in homeostasis

• Positive vs negative feedback

• Negative feedback mechanisms are the main mechanisms of homeostasis

  • disadvantage: large amounts of energy are used to maintain homeostasis

D3.3. Regulation of blood glucose

• Regulated by hormones insulin and glucagon

  • Secreted by cells in the pancreas into the bloodstream

• Two cell types in the islets of Langerhans

  • alpha cells synthesize/secrete glucagon if blood glucose is low

    • stimulates breakdown of glycogen → glucose in liver cells

  • beta cells synthesize/secrete insulin when blood glucose is high

    • stimulates uptake of glucose in multiple tissues → used in cell respiration

    • e.g. skeletal muscle and liver

    • Conversion of glucose to glucagon

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

• Consistently elevated blood glucose levels → presence of glucose in urine → tissue/protein damage

• Impaired water reabsorption in urination → frequent urination

• Type 1:

  • Inability to produce sufficient insulin

  • Autoimmune disease → immune system attacks of beta cells in islet of Langerhans

  • Treated by testing blood glucose regularly and injecting insulin when it is too high

• Type 2:

  • Deficiency of insulin receptors/glucose transporters → Inability to process/respond to insulin

  • Treatment: diet/lifestyle adjustments

D3.3.5 Thermoregulation as an example of negative feedback control

• Thermoregulation: Control of core body temperature

  • Humans: roughly 37ºC

• Negative feedback → generate or lose heat

• Monitored by thermoreceptors

  • peripheral thermoreceptors located in the skin → external temperature

  • central thermoreceptors located in hypothalamus

• Hypothalamus monitors body temperature through thermoreceptors

  • Secretes thyrotropin-releasing hormone (TRH) → pituitary gland

  • pituitary gland releases thyroid-stimulating hormone

    • simulates thyroxin secretion by thyroid

    • Increases metabolic rate of cells

  • Hypothalamus can increase/decrease generation of metabolic heat by increasing/decreasing secretion of TRH

• Some tissues act as effectors of temperature change

  • brown adipose tissue generates heat at a rapid rate

D3.3.6 Thermoregulation mechanisms in humans

• Both birds and mammals are thermoregulators

• Responses to cold:

  • Vasoconstriction: The narrowing of blood vessels → reduces blood flow to skin and minimizes heat loss

  • Shivering: Muscles contract to cause movement + generate heat

  • Uncoupled respiration: Brown adipose tissue generates heat energy through respiration instead of ATP production

  • Hair erection: Makes hair thicker and insulates heat

• Responses to heat:

  • Vasodilation: Blood vessels widen → more blood flows to the skin → more heat loss

  • Sweating: Sweat secreted by glands in the skin → evaporation causes cooling

D3.3.7 Role of the kidney in osmoregulation and excretion

• Osmoregulation: Maintaining the osmotic concentration of body fluids

  • concentration of solutes in a fluid → affects movement of water

  • Kidney varies the relative amount of water and salts removed in urine

• Excretion: Removal of toxic waste products of metabolism from the body

• Nephron: Basic functional unit of the kidney

  • Tube with one layer of cells

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

• Ultrafiltration is the first stage in urine production

• Glomerulus: A ball-shaped network of blood capillaries

  • Blood enters through afferent arteriole

  • Blood leaves through efferent arteriole

  • Fluid is filtered out through the walls of blood capillaries

    • through pores called fenestrations

• Glomerular filtrate: fluid forced out of blood plasma

  • Plasma proteins and blood cells are not filtered out - not as permeable because they are slightly larger molecules

• Filter unit only allows small/medium-sized molecules to pass

  • 1st layer: Membrane that supports capillaries’ walls, made of negatively charged glycoproteins → prevents plasma proteins from being filtered out

  • 2nd layer: Inner layer of Bowman’s capsule - podocytes (cells) with branches (foot processes) that wrap around the capillaries of glomerulus → prevent small molecules from being filtered out of blood

• Bowman’s Capsule: cup-shaped structure

  • Outer wall: impermeable layer of cells → helps transport glomerular filtrate to proximal convoluted tubule

• Proximal convoluted tubule

  • Convolutions increase length of nephron → takes longer for filtrate to flow through → more reabsorption of substances in the filtrate

• Reabsorbtion methods:

  • Sodium ions: Active transport from filtrate to space outside tubule

  • Chloride ions: Diffuse to space outside tubule - charge gradient from active transport of sodium ions

  • Glucose: Moved by cotransporter proteins in outer membrane of tubule cells (uses concentration gradient of sodium ions)

  • Water: Osmoses from the solute concentration gradient

D3.3.8 Role of the loop of Henle

• Kidney cortex: Contains all glomeruli + bowman’s capsules + Proximal/distal convoluted tubules

  • same osmotic concentrations as most other tissues

• Medulla: contains loops of henle and collecting ducts

  • concentration gradient - rises near the center of kidney

• Loop of Henle: Establish/maintain the osmotic concentration gradient in the medulla

  • Filtrate enters from proximal convoluted tubule

  • flows towards center of kidney (descending limb)

  • → flows back to medulla (ascending limb)

• Ascending limb: walls are impermeable to water

  • sodium ions pumped out of filtrate into interstitial fluid (between cells of medulla)

  • interstitial fluid becomes hypertonic relative to filtrate (higher solute concentration)

• Descending limb: walls are impermeable to sodium ions, permeable to water

• More solutes than water are reabsorbed in loop of Henle

  • Filtrate → distal convoluted tubule - hypotonic (lower solute concentration than normal body fluids)

D3.3.10 Osmoregulation by water reabsorption in the collecting duct

• Osmotic concentration is regulated by removing small/large amounts of water from urine

  • Varying permeability of water in cells in the distal convoluted tubule and collecting duct → number of aquaporins in the plasma membrane

• Hypothalamus monitors osmotic concentration of blood

  • Too high: causes pituitary gland to secrete antidiuretic hormone (ADH) → more aquaporins moved into cells of DCT and CD

• High blood osmotic concentration → more water reabsorption

• Low blood osmotic concentration → less water reabsorption