Lecture 12 - Excretory System Flashcards
Main ideas of excretory sysem
- Maintains volume, concentration and composition of extracellular fluids (interstitial fluid)
- determines water balance of cells that those fluids bathe
- thorough exchange between blood vessels and interstitial fluid –> blood volume and pressure - Excretes waste
- metabolic wastes of cell carried through bloodstream to kidneys
Cells and osmosis
Volume depends on osmosis
- volume of cells depends on whether they take up water or lose it to extracellular fluids
- depends on difference in solute concentration on the two sides of the membrane
osmolarity: moles of solute in liters of solvent
Osmotic Regulation Across a Membrane
WATER IN:
If solute concentration in cytoplasm is greater than that in extracellular fluid, water moves into cells
- swell and possibly burst
- HYPOTONIC conditions
- cell is HYPEROSMOTIC relative to extracellular fluid
WATER OUT:
If solute concentration in extracellular fluid is greater than that of cytoplasm, water moves out of cells
- Shrink
- HYPERTONIC conditions
- cell is HYPOOSMOTIC relative to extracellular fluid
Controlling Fluid Osmolarity and Composition
- OSMOLARITY: must maintain osmolarity of extracellular fluid within appropriate range for homeostasis
- volume and salt concentration must be kept within certain limits - SOLUTE COMPOSITION: must maintain appropriate solute composition
- how much AND what
- SAVE some substances, solutes valuable in short supply –> re-absorption (glucose)
- ELIMINATE other substances, solutes in excess, toxic waste products –> secretion (urea, excess sodium)
Marine animals
- hypoosmotic to sea water
- gain water and salt from food and drinking salt water
- excrete salt ions from gills
- also excrete small amount of urine containing water and ions
Freshwater animals
- constantly take in water by osmosis
- gain some salts from food
- lose salts by diffusion and maintain water balance by excreting large amounts of dilute urine
Terrestrial Animals
Adaptations to reduce water lost
- eating moist food and producing water metabolically through cellular resp
Can vary depending on diet
- special structures aid in proper osmotic balance with sepcific environment
Ex. Salt intake: herbivores must conserve salts, but birds that eat marine animals excrete excess sodium
Ex: water intake: desert animals urine is so concentrated it can form crystals
Flexible adaptation
ex vampire bat - blood meals, lots of water –> rapidly eliminate water to maximize consumption. But meals may be few and far between so then conserve body water with highly concentrated urine
Challenges faced by:
Marine animals
Freshwater animals
Terrestrial animals
Marine: salt concentration is too high
freshwater animals:
- water is plentiful, but salt mus tbe conserved
- must constantly bail out the excess water entering their body
- produce copious amounts of very dilute urine
Terrestrial
- msut conserve both salts and water
- can vary depending on diet, specialized structures aid in proper osmotic balance
specificity - want to secrete some ions and maintain others
ALL need to get rid of ammonia waste
Excretion of Nitrogen
- in addition to maintaining salt and water balance, animals must eliminate the waste products of metabolism
carb and fat metabolism:
- end products are H2O and CO2, easy to eliminate
protein and nucleic acid metabolism:
- end products: H2O, CO2 and nitrogenous waste
- NH3 is TOXIC
Three types of nitrogenous waste
- ammonia
- urea
- uric acid
differ in toxicity and energy costs of producing them
Ammonia
NH3
- highly toxic but diffuses rapidly in water
- animals that excrete nitrogenous wastes as ammonia need to access lots of water
- continuously excreted
- lost from blood to environment by diffusion across gill membranes
- for animsla that cannot continuously excrete ammonia, its buildup would be toxic so must convert it to urea or uric acid
- fish
Urea
- the liver of mammals and most adult amphibians converts ammonia to the less toxic urea
- the circulatory system carries urea to the kidneys, where it is excreted
- conversion of ammonia to urea is energetically expensive
- large loss of water
*mammals
Uric Acid
- relatively nontoxic and does not dissolve readily in water
- it can be secreted as a semisolid paste with little water loss
- uric acid is more energetically expensive to produce than urea
*insects, land snails, many reptiles and birds
Vertebrate Excretory organ and its functional unit
Kidney
Nephron
Processes of urine formation
most excretory systems produce urine by refining a filtrate derived from body fluids
- filtration
- reabsorption
- secretion
- excretion
Filtration
1.
- at start of each nephron is a dense ball of capillaries called a glomerulus
- highly permeable to water, ions, and small molecules
- impermeable to large molecules
- blood pressure drives the movement of water and small solutes out of the glomular capillaries and into the nephron
Friltration –> Bowman’s Capsule
- 5
- glomerulus filters blood to produce a fluid (renal filtrate) that lacks cells and large molecules
- filters fluid into bowman’s capsule (beginning of nephron, encolses glomerulus)
- the filtrate produced in Bowman’s capsule contains salts, glucose, amino acids, vitamins, nitrogenous wastes and other small molecules
Reabsorption
- In renal tube
- all of the nephron past the Bowman’s capsule = renal tube, several sub divisions
- converts renal filtrate into urine
- capillaries run alongside renal tube
Reabsorption:
- specific ions, nutrients, water, are reabsorbed out of renal filtrate and are returned to the blood
- glucose, amino acids, most NaCl
Secretion
3.
- additional waste substance that the body needs to excrete are transported into the renal tubule
- ex: drugs (penicillin), H+ ions
Excretion
- processed filtrate (urine) of the individual nephrons enters collecting ducts and is delivered to a common duct leaving the kidney
- goes to bladder
Organization of the kidney
- two regions
- outer cortex
- inner medulla
nephron = major functional unit
~ 1 million nephrons per kidney
- nephron empties into ureter which leads to bladder
Organization of nephrons within the kidney
- glomeruli and Bowman’s capsule
- located in the outer laywe (cortex) of the kidney - Proximal concoluted tube
- also located in the cortex - loop of henle
- renal tubule straightens and descends into inner core of kidney (medulla)
- makes a hairin tirn and ascends back to cortex
- peritubular capillaries run into the medula in paralles - distal convoluted tube
- ascending limb of loop of henle becomes the distal convoluted tube when it reaches the cortex - collecting duct
- distal convoluted tubes of many nephrons join
- descend back down through the medulla
- join with other tubes to leave the kidney for the bladder
Nephron uses movement of water and ions to concentrate urea
- most of the water and solutes filtered by the glomerulus are re-absorbed and do not appear int he urine
- 98% of the fluid filtered out of the glomerulus is returned ot the blood
- concentration f urea
- modulation of other solute concentrations
Proximal Convoluted Tubule
- proximal convoluted tube is responsible for most of the reabsorption of water and solutes
- active transport of Na+ (Cl- follows), glucose, amino acids, and other valuable solutes –> nulk reabsorption of solutes
- active transport of solutes causes water to follow osmotically (passive –> flows out through aquaporins)
- reabsorbs ~75% of fluid that initially enters nephron
Osmolarity of fluid in PCT
- osmolarity is not changed
- solute leaves but water follows in same proportion
- fluid that enters the loop of henle has the same osmolarity as blood plasma, although its composition is different (removed vlauable solutes)
- needs to make urine that is more concentrated than blood plasma (must now go through loop of henle and colelcting duct)
How urea becomes predominant solute
Osmolarity in loop of henle
- loop of henle increases osmolarity of the extracellular fluid as you move deeper into the medulla
- different segments of the loop of henle have different properties
3 limbs:
- thin descending
- thin ascending
- thick ascending
Thin descending limb
- permeable to water, but not to Na+ or Cl-
- as descend deeper into medulla, higher salt concentration in interstitial fluid
- outside is saltier than renal fluid
- high osmolarity drives the water to flow our of renal tube
- flows out through aquaporins
- filtrate becomes increasingly concentrated
Thin ascending limb
- permeable to salt but not water
- at base of loop of henle, salt concentration in interstitial fluid is maximal. water had flowed out of renal fluid to make it osmotically balanced
- as fluid flows up, it is flowing into areas that have a lower salt concentration in interstitial fluid
- outside is less salty than renal fluid
- salf diffuses out of the renal tube into interstitial fluid passively
now:
- less water from thin descending, less salt from thin ascending, still urea
Thick ascending
- active transport of Na+ and Cl-
- not permeable to water
- as renal fluid continues to moce up ascending limb, active transport continues to pump salt out
- gives the interstitial fluid of the medulla its high salt concentration
now:
less water from thin descending, even less salt from thin ascending, STILL urea
Distal convoluted tube
- continues to pump NaCl out
- permeable to water
- water follows the flow of salts out of the tubule
- fine tuning the ionic composition of the urine
- large numbers of active transporters for many different ions
- adjusts amount reabsorbed based on physiological needs of the individual (ex by hormone regulation)
Collecting Duct
- when the renal fluid enters the collecting duct, the composition is very different from the start (proportion of water to solute is very similar, but makeup is very different)
- major solute is now urea
- as renal fluid flows down collecting duct, it is concentrated
- descending into the increasingly high-salt interstitial fluid of the medulla
- water flows out by osmosis (passive)
- byt the bottom of the colelcting duct, urine is greatly concentrated with urea as its major solute
- ability of a mammal to concentrate its urine is determined by the maximum concentration gradient it can establish in the renal medulla
Recycling of urea
- as water leaves the collecting duct, some urea also leaks out into the interstitial fluid of medulla
- adds to osmotic potantial
- urea diffuses back into loop of henle and is returned to collecting duct
Summary of Reabsorption
- proximal converted tubule –> bulk of reabsorption of solutes by active transport
- loop of henle
- Thin descending limb- removal of water, passive
- thin ascending - removal of salt, passive
- thin ascending - removal of salt, active - distal convoluted tubule –> fine tuning ionic composition of solutes based on physiological needs
- collecting duct –> removal of water (passive), concentration of urine
Renal failure
- loss of kidney function results in retention of salts and water, leading to high blood pressure
- retention of urea in blood
- decreasing pH
Dialysis
- patients blood flows through many small channels made of semipermeable membranes
- dialysis solution flows on other side of membrane, through which molecules can diffuse
- concentration of molecules that need to be conserved must be at the same concentration in blood and dialysis fluid
- concentration of molecules that need to be removed from the blood are zero int eh dialysis fluid
Hormones that regulate kidney function
- antidiuretic hormone (ADH)
- renin-antiotensis-aldosterone system (RAAS)
- atrial natriuretic peptide (ANP)
Blood osmolarity
- the osmolarity of the urine is regulated by nervous and hormonal control
- osmoreceptors in the hypothalamus monitor blood osmolarity
- an increase in osmolarity triggers the release of (ADH) from the posterior pituitary (ex after profuse sweating)
- ADH helps to conserve water by controlling water reabsorption
ADH
- triggers cells in the colelcting duct to insert aquaporins in their plasma membrane
- increase permeability of these membranes to water
- more water is reabsorbed from the collecting duct fluid into the interstitial space
- higher the levels of AHD, greater the number of aquaporins
ADH regulated by blood pressure
- ADH also responds to signals about blood pressure
- stretch receptors in wall of aorta and cartoid arteries are activated when blood pressure is high
- -> inhibit release of ADH
- -> less water reabsorbed, which decreases blood volume and thus lowers blood pressure
- if blood pressure falls too low, no longer inhibit ADH release
- -> more ADH –> more water reabsorption –> increased blood volume –> increased blood pressure
dehydration and alcohol
- alcohol inhibits ADH release
- water is not reabsorbed
- excessive urination and dehydration
Blood pressure decrease and glomular filtration rate
(GFR)
- blood must be supplied to the kidneys at adequate blood pressure in order for glomeruli to filter blood
- drop in blood pressure near the glomerulul causes the release of renin
- activates RAAS (renin-angiontensin-aldosterone system)
RAAS
- drop in blood pressure triggers release of renin
- RENIN triggers formatino of angiotensis II (or simply angiotensin)
- ANGIOTENSIS, raises blood pressure
- constricts blod vessels at glomerulus
- constricts peripheral blood vessels all over body
- stimulates thirst to increase blood volume
- stimulates adrenal gland to release ALDOSTERONE
ALDOSTERONE
- stimulates sodium reabsorption form the kidney
- therby making reabsorption of water more effective
- thus increases blood volume and pressure
Blood Pressure: increase
- atrial natriuretic peptide (ANP) opposes the RAAS
- ANP is released in response to an increase in blood volume and pressure and inhibits the release of renin
ANP acts during high blood pressure
- when blood volume is high, heart muscle fibers are overly stretched
- heart releases a hormone called atrial natriuretic peptide (ANP)
- ANP decreases the reabsorption of sodium from the kidney
- less water is absorbed
- -> more to pass to urine
- -> blood mlowered
- -> blood pressure lowered
