Flashcards
Aspects of Renal Physiology
Flashcards do conjunto (69)
- 1Kidneys are <1% body weight, but receive what percentage of cardiac output?~25%
- 2What is the functional unit of the kidney? How many are there?The functional unit of the kidney is the nephron, of which there are 2.5 million per kidney. These are lost on a daily basis and do not regenerate over life.
- 3Roughly what size are nephrons?~5 cm in length and ~5μm in diameter.
- 4How many litres of plasma do the kidneys process per day? How much of this ends up as urine?~180 L / day. ~1% of this ends up as urine.
- 5Given how many nephrons each kidney consists of, what volume of the plasma filtered in a day is filtered by an individual nephron?~1.5 drops
- 6How much sodium is reabsorbed in a day?~600g
- 7List some roles of the kidney.Excretion of waste products (e.g. urea); excretion of foreign compounds (e.g. xenobiotics); control of erythrocyte number via erythropoietin; production of active form of vitamin D; regulation of plasma osmolarity; long term acid/base balance (short term = breathing) and excretion of non-volatile acids; long term control of blood pressure via regulation of H2O and ions.
- 8Which two elements does a nephron consist of?Vascular and tubular.
- 9What is the force that drives renal filtration of plasma?Ventricular systole pumps blood to the kidneys under high enough pressure that plasma is filtered into Bowman's capsule.
- 10Describe the journey of plasma/solute that is filtered from the plasma.The blood enters the afferent arteriole and its diffuse capillary bed under pressure from each ventricular systole and each peristaltic movement, which is sufficient to cross the basement membrane of Bowman’s capsule. The filtered plasma travels through the glomerulus/capsule and the proximal convoluted tubule in the cortex, then the loop of Henle in the medulla, before returning to the distal convoluted tubule in the cortex, and emptying via collection ducts into collection tubules and eventually the renal pelvis, after much reabsorption and secretion of course.
- 11Complete the equation regarding renal excretion. Excreted = ...filtered + secreted - reabsorbed
- 12Urine osmolarity is the sum total of...?The solutes involved.
- 13What does hypotonic urine require for its production?Hypotonic urine requires Na+ reabsorption independent of H2O, which is achieved via Na+-pumps in the distal convoluted tubule, which is impermeable to H2O.
- 14What does hypertonic urine require for its production?Hypertonic urine, in which H2O reabsorption independent of Na+ occurs, is more difficult. Once filtered, the H2O is technically outside the body, so osmotic forces are required – a hypertonic area inside the body - to set up an H2O concentration gradient. This area is the medullary interstitium, maintained by the loop of Henle. There is a very low water concentration in the medulla (maintained by solutes pumped out) so the H2O concentration is higher in the tubule, forcing it to move out into the interstitium independently of solutes in the tubule.
- 15How is urine composition controlled?Hormonally (via renin-angiotensin, aldosterone etc).
- 16Which hormone is involved in the production of hypertonic urine?Anti-diuretic hormone (ADH).
- 17Where is ADH released from?The posterior pituitary (neurohypophysis).
- 18Which sensory cells are involved in the release of ADH?Osmoreceptors in the hypothalamus (specialised cells sensitive to H2O movement which swell or shrink in response to this and their activity is altered because they are nerve cells) and volume receptors in the right atrium.
- 19How does ADH aid H2O reabsorption independent of Na+?It acts to make the tubules more permeable to H2O.
- 20How does atrial natriuretic peptide (ANP) act to reduce blood pressure?When stretch receptors in the right atrium of the heart are activated by large volumes being returned to it, ANP is released from the atrium. It inhibits Na+ reabsorption, causing its excretion, which H2O follows down a concentration gradient, thus lowering the circulating volume and blood pressure.
- 21Which hormone is involved in the production of hypotonic urine? How does it aid this?To generate hypotonic urine, in which Na+ is reabsorbed independent of H2O, aldosterone is released from the adrenal cortex, where it acts on the kidneys to stimulate Na+ reabsorption via channels. Its release is sensitive to the plasma osmolarity and to angiotensin.
- 22Where is renin released from?The kidneys.
- 23Angiotensin release is sensitive to what?Blood pressure and plasma osmolarity.
- 24Which aspect of the nephron is the renin-angiotensin system based around?The juxtaglomerular apparatus.
- 25Which cells is renin released from?Renin is released from granular juxtaglomerular cells on the afferent and efferent arterioles, smooth muscle cells which no longer stretch/contract but secrete renin.
- 26What is renin release sensitive to?Stretch (physical pressure), sympathetic stimulation, and hormones in the blood.
- 27Which nephron structures is the distal convoluted tubule located between?The glomerulus, and the afferent and efferent arterioles.
- 28Where are the macula densa cells located?In the distal convoluted tubule.
- 29What do the macula densa cells do?Feedback on the production and release of renin, with regard to the blood volume and pressure.
- 30What is significant about the placing of the granular juxtaglomerular cells in the afferent and efferent arterioles and the macula densa cells in the distal convoluted tubule?Given their close proximity but very different functions, the macula densa cells can easily feedback on the effect of the renin release by the granular juxtaglomerular cells. The afferent arteriole possesses the majority of the granular juxtaglomerular cells, and responds to the stretch of the blood volume passing through. The renin release will increase Na+ reabsorption and therefore H2O reabsorption, restoring blood pressure. The macula densa cells can feedback to the granular juxtaglomerular cells as to how effective this has been, and whether the response should be prolonged or reduced.
- 31What is the substrate of the renin-angiotensin system?Angiotensinogen.
- 32Which signs of lowered blood pressure do the granular juxtaglomerular cells release renin in response to?Decreased renal perfusion pressure – baroreceptor, decreased Na+ at macula densa cells, and increased sympathetic nerve activity – β1.
- 33What does renin do to angiotensinogen, which is present in the blood?It cleaves the last 10 amino acids from angiotensinogen to form angiotensin I.
- 34Which enzyme converts angiotensin I to the octopeptide angiotensin II?Angiotensin converting enzyme (ACE).
- 35Which enzyme converts angiotensin II to angiotensin III?Aminopeptidase.
- 36List some physiological effects of angiotensin II which support blood pressure.Vasoconstriction; direct renal sodium retention; aldosterone secretion (H2O reabsorption following Na+ reabsorption); increased thirst; anti-diuretic hormone release; sympathetic activity facilitation to act on heart/vessels; increased cardiac contractility; and long-term cardiac and vascular hypertrophy.
- 37Which hormones are released to support/raise blood pressure?Anti-diuretic hormone, aldosterone, renin/angiotensin, and adrenaline.
- 38Which hormone helps to reduce blood pressure?Atrial natriuretic peptide (ANP).
- 39What is renal clearance?The volume of plasma from which a substance is removed, per unit time (typically ml/min).
- 40What is the renal clearance of urea?Curea = 60ml/min : every minute, 60ml of plasma is cleared of urea.
- 41What are the assumptions made when calculating renal clearance?That all substances in the urine come from the plasma, that blood flow into the kidney is equal to blood flow out of the kidney (given that a tiny amount of plasma is lost), and that if a substance appears in the urine the kidneys must have reduced its plasma concentration (i.e. venous concentration < arterial concentration).
- 42What does (U x V) / (P x T) stand for? What does it calculate?(urinary concentration of analyte x volume of urine) / (plasma concentration of analyte x time taken for urine to accumulate or be collected). This equation calculates the rate at which a volume of plasma has been cleard of the analyte.
- 43How would you calculate the total amount of the analyte excreted?Urinary concentration of the analyte x urinary volume
- 44How would you calculate the minimum volume of plasma that could have supplied the mass of excreted analyte?(urinary concentration x urinary volume) / plasma concentration
- 45Creatinine is filtered but not secreted or reabsorbed. True or false?True!
- 46If creatinine is filtered but not secreted or reabsorbed, what is its clearance equal to?The glomerular filtration rate.
- 47What is the glomerular filtration rate?~120 ml / min.
- 48If a substance undergoes net reabsorption, how will its clearance relate to the glomerular filtration rate?Its clearance will be less than the glomerular filtration rate.
- 49Why is urea, a waste product of protein degradation, reabsorbed?For the hypertonicity of the renal medulla.
- 50If a substance undergoes net secretion, how will its clearance relate to the glomerular filtration rate?Its clearance will be more than the glomerular filtration rate.
- 51What is para-aminohippurate (PAH) used for?PAH is filtered and fully secreted but not reabsorbed, and so its clearance is equal to the renal plasma flow, which it is used to asses, as well as to calculate renal blood flow.
- 52What is the equation used to calculate renal blood flow?Renal blood flow = renal plasma flow / (100 – packed cell volume) * 100
- 53What must be known in order to calculate renal blood flow?Renal plasma flow (established by measuring the clearance of PAH), the volume of erythrocytes in the blood, and the proportion of blood volume taken up by erythrocytes, the packed cell volume or PCV).
- 54What is an osmole?A mole of particles, whatever they are (different compounds).
- 55Roughly, what is the normal osmolarity of the plasma?~300 mOsm (milli osmoles / L)
- 56What does the osmolarity of plasma reflect? What does it determine?It reflects the concentration of all solutes dissolved in it, and this determines the water concentration and water movement by osmosis.
- 57Why are osmoles used instead of the molarities of glucose and NaCl?Because both are present and both have different molarities, and both have a similar effect on water, so they must be considered together, equally, which is best done by using the non-discriminatory measurements of osmoles.
- 58What can be used to calculate the osmolarity of urine and plasma? What is the principle behind this?A Freezing Point Depression Osmometer can be used to establish the osmolarity of these fluids. Each solute lowers the freezing point of water in a known and constant way, and so the difference between the fluid and the normal freezing point of water is telling of its osmolarity.
- 59What is osmolar clearance?The volume of plasma completely cleared of solutes in a given time.
- 60Having established the osmolarity of urine and plasma, what other information would be needed to calculate the osmolar clearance?According to (U x V) / (P x T), the volume of the urine and the time taken to accumulate/collect it is necessary to calculate the rate of clearance.
- 61If you produce 100ml of 300 mOsm (isotonic) urine, what is the minimum volume of plasma that must have been cleared?100ml (because it is isotonic).
- 62If you produce 100ml of hypotonic urine (~125 mOsm), what is the minimum volume of plasma cleared? What is the free water clearance?If you produce 100ml of hypotonic urine (e.g. 150 mOsm), then the osmolar clearance is 50 ml/hour (the smallest volume of plasma required to account for all the solutes in that 100ml of hypotonic urine is 50ml). The other 50ml is ‘free’ water – it is positive free water clearance. You can see that the kidneys are ‘dumping’ water, because only 50ml of the urine was required for the solutes, the other 50ml is ‘free’ and so may have been ‘dumped’ if blood pressure was too high, for example.
- 63If you produce 100ml of hypertonic urine (~600 mOsm), what is the minimum volume of plasma cleared? What is the free water clearance?If you produce 100ml of hypertonic urine (e.g. 600 mOsm) per hour, then the osmolar clearance is 200ml/hour. The urine is twice as ‘strong’ as the isotonic plasma, so twice the volume of plasma would be required to supply those solutes. Given that 200ml has been cleared, but only 100ml has been excreted, the remaining 100ml is ‘free’ water – but this is negative free water clearance, as it is retained in the body.
- 64What is the equation for calculating free water clearance?free water clearance = urine production rate - osmolar clearance
- 65Generally, are we in positive or negative free water states? Why?We are mainly in positive free water states, because we are well hydrated so water is excreted rather than retained.
- 66What is the filtration fraction?The filtration fraction is the proportion of the plasma that is filtered when it flows through the glomerular capillaries. This is ~20%.
- 67What isn't all plasma filtered at the kidneys, rather than just the filtration fraction?The full amount can’t be filtered, or only the erythrocytes would be left in the vessels and blood would be too viscous to flow. ~80% is unfiltered, and serves to dilute the erythrocytes.
- 68Why is urine 'naturally' acidic?Because we produce acids all the time, many are non-volatile, and urine is their primary mechanism of excretion, so many are present in urine.
- 69What is titratable acidity?Titratable acidity is the amount of alkali required to bring urine pH back to 7.4 (the more acid excreted, the more base required to return urine to pH 7.4).
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- From the 12/11/13 Human Physiology lecture.
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