Flashcards in Renal Deck (85):
How is blood supplied to kidneys?
-via the renal arteries (they arise from the aorta).
How does venous drainage of kidneys occur?
- via the renal veins into the inferior vena cava.
What are the functions of the kidney?
- excretion of waste products (nitrogen containing compounds urea and creatinine - which arise from the metabolism of proteins and muscle: these substances are measured in blood as a way of assessing renal function) and toxins.
- water, electrolytes and acid-base homeostasis are dependent on renal function.
- kidney also has endocrine functions: it secretes renin which plays a vital role in maintaining blood pressure.
- it also produces hormone erythropoietin which stimulates the production of RBC in bone marrow.
- renal tissue is responsible for he activation of vitamin D through hydroxylation at carbon 1 (1 alpha) and therefore has an important role in calcium metabolism.
What are the functional units for production of urine?
- Nephrons (healthy human kidney has around 1 million nephrons).
- each nephron can be regarded as a filter (the bowmans capsule) which drains into a lengthy tube.
- the filtrate is modified as it passes along the tube until it enters the collecting duct and is carried to the ureter for excretion.
Where does filtration take place?
- it takes place in the bowmans capsule (which is like a cup containing a knot of capillaries which are known as the glomerulus).
- blood is carried to the glomerulus by the afferent artery and leaves via the efferent artery.
- as it travels through glomerular capillaries blood is filtered: large structures such as blood cells and large proteins are retained within capillaries while water and smaller molecules pass through capillary walls to form the glomerular filtrate which is collected in bowmans capsule and then passes into proximal tubule.
What is the function of the distal tubules?
- most of the fine tuning of renal excretion takes place in the distal tubules.
- they are involved in acid-base homeostasis, sodium-potassium exchange and regulation of calcium and phosphate excretion.
Where is antidiuretic hormone secreted (ADH)?
- secreted by posterior pituitary gland.
What is major difference between intracellular and extracellular fluid?
- electrolyte content.
- sodium is main cation in extracellular fluid.
- potassium is main cation in intracellular fluid.
How can extracellular fluid be divided?
- 1/3 (5L) in blood circulation (intravascular fluid).
- 2/3 (10L) in spaces between cells (interstitial fluid).
Where does most water intake come from?
- drinks (1500).
- water content of food (600).
- water generated by metabolism during oxidation of glucose and fatty acids (400).
- = 2500ml.
Where does most water output come from?
- urine (1500).
- faeces (larger if dihorreha) (100).
- skin (400).
- lungs (500).
- = 2500mls.
What is the ureter?
- duct by which urine is carried from kidneys to bladder.
What is intake of fluid regulated by?
- thirst centre in hypothalamic region of brain.
- it responds to changes in extracellular osmolality and also large alterations of circulating fluid volumes.
What is excretion of water regulated by?
- antidiuretic hormone (ADH) which is released from the posterior pituitary gland.
- this is sensitive to changes in osmolality and also responds to alterations in circulating fluid volume.
What releases renin, why does it release it, and what is the function of renin?
- renin is released from juxtaglomerular cells in response to a fall in renal perfusion pressure.
- renin triggers the cascade which results in sodium and water retention.
- this expands extracellular fluid volume resulting in improved renal perfusion.
What is increased plasma osmality an indicator of?
- dehydrated patient.
How many kidneys do we have?
What is a ureter?
- tubes that carry urine from kidneys to bladder.
Which arteries/veins enter and leave the kidney?
- renal artery enters the kidney.
- ureter and renal vein leave the kidney at the helium.
On sectioning of the kidney what are the two distinct areas of tissue?
- outer cortex.
- inner medulla.
Urine which is produced by the renal tissue is collected where?
- urine is collected in a branched system of ducts.
- these empty via the renal papillae into the cup-like cavities (calyces) where the urine gets accumulated before passing into the bladder through the ureter.
What happens as glomerular filtrate passes through the proximal tubule?
- active transport mechanisms reabsorb glucose, amino acids, proteins, bicarbonate and other electrolytes.
- water is reabsorbed along with electrolytes following the osmotic gradient.
How is an osmotic gradient across the kidney created?
- differential reabsorption of electrolytes do water within the loop of henle and surrounding blood vessels which are known as the vasarecty creates an osmotic gradient across the kidney.
- this is known as counter-current mechanism.
Where are the most highly concentrated fluids (both within the tubule and blood vessels surrounding the tubule) found?
- they are found in the medulla at the tip of the loop of henle.
The creation of an osmotic or concentration gradient within the kidney is an essential mechanism for ...
- diluting and concentrating urine in the collecting ducts.
Tubular fluid enters collecting ducts which are responsive to ...
- antidiuretic hormone (ADH).
What is the function of ADH in the collecting ducts?
- ADH determines the degree to which collecting ducts are permeable to water.
What happens if collecting ducts are permeable/impermeable to water in response to ADH?
- if collecting ducts are permeable to water in response to ADH: water leaves ducts along concentration gradient and will not be excreted (in other words, a concentrated urine will be produced).
- if collecting ducts are impermeable to water due to the absence of ADH: water is retained within the collecting ducts and excreted as a dilute urine.
Describe the water content of the body.
- approximately 45L in adult.
- 1/3 outside cells (extracellular): 15L.
- remaining 2/3 inside cells (intracellular): 30L.
Distribution of water between intracellular and extracellular fluid compartments depends on ...
- the particles within fluid which exert an osmotic pressure.
- water moves across cell boundaries from a region of lower osmolality to a higher osmolality thus maintaining osmotic equilibrium between the two compartments.
- sodium is responsible for majority of osmotic activity in extracellular fluid (therefore the total body sodium determines extracellular fluid volume).
How can you measure osmolality in the lab?
- using an osmometer.
- this exploits the depression of freezing point which results from the presence of osmotically active particles in solution.
How can osmotic activity be estimated?
- by summing the concentration of major contributors to osmolality.
How can you estimate the osmolality of plasma?
- in plasma both are chemically dissociated cations so they have to be paired with an equal concentration of anions to maintain electrochemical neutrality.
- use the equation below to estimate osmolality of plasma:
2[Na] + 2[K] + urea + glucose.
- 2 = double concentration.
What is essential about water intake and output?
- they need to be equal.
How much water is lost everyday by evaporation from skin and excretion in expired air?
- almost a litre.
- these are sometimes known as insensible fluid losses as can note measured.
How does ADH regulate urinary water excretion?
- by opening pores in collecting ducts.
- which in turn allows reabsorption of water along a concentration gradient.
- both are triggered by;
- an increase in extracellular fluid osmolality.
- a decrease in circulating fluid volume.
What is AVP, and what is it another name for?
- arginine vasopressin = alternative name for ADH.
In which ways is sodium lost (daily output)?
- small losses in sweat and faeces.
- renal 80 to 180 mmol.
Where is sodium filtered?
- sodium is freely filtered at glomeruli and the majority must be reabsorbed by renal tubules (>99%).
Name a factor which increase sodium absorption.
- low glomerular filtrate rate (as in dehydrated patients) which results in increased sodium reabsorption.
What is important for sodium homeostasis?
- the renin-angiotensin system has an important role in sodium homeostasis.
- the renin-angiotensin system is activated in the presence of low blood pressure or low renal blood flow. This activates the renin-angiotensin system and aldosterone release which results in an increase in sodium reabsorption.
The presence of renin triggers what?
- the renin-angiotensin cascade.
Angiotensin 2 is an extremely powerful what?
- vasoconstrictor (increases bp).
If the pulse of a patient is rapid and their blood pressure is low, what does this suggest?
- that the patient is dehydrated.
If a patient is dehydrated, what will their plasma sodium levels be like?
- plasma sodium will be high.
What is water balance determined by?
- extracellular osmolality.
Extracellular osmolality and fluid volume are determined by?
- total body sodium.
Sodium balance is regulated by?
- renin-angiotensin system.
Renin secretion is regulated by?
- renal blood flow (i.e. extracellular fluid volume).
Where is the majority of potassium?
- within cells (intracellular cation).
What does the concentration of potassium within the extracellular compartment affect?
- concentration of potassium in extracellular compartment (which is what we are assessing when we analyse plasma) affects the conduction of impulses in nerves, muscle and cardiac tissue: hence clinical importance.
How is most daily potassium intake excreted?
- by the kidneys.
- major gastrointestinal losses can occur in patients with diaorreha (these individuals can become severely potassium depleted).
What is the daily intake of potassium?
- 50-100 mmol.
How is daily output of potassium achieved?
- GI tract.
- kidneys 50-100mmol.
Where is potassium filtered and reabsorbed?
- it is freely filtered at the glomeruli and is fully reabsorbed in the proximal tubule (nephron).
Potassium together with H ions excreted in exchange for reabsorbed sodium in the distal tubule, under the control of what?
- aldosterone increases sodium reabsorption and at the same time increases potassium secretion (or excretion) in the distal tubule.
What is creatine kinase?
- an enzyme produced by muscle.
- severe injury to muscle results in release of creatine kinase and an increase in levels in blood.
Water, sodium and ... homeostasis re strongly inter-related?
Renal, glomerular and tubular function are essential in maintaining ...
- water and electrolyte homeostasis.
What are the key endocrine regulators of water and electrolyte homeostasis?
- the renin-angiotensin system.
Sodium intake and output must also balance. Roughly how much sodium per day does the western diet contain?
- 100-200mmol of sodium a day.
- vast majority of this is excreted by the kidney.
- although the gastrointestinal tract can be a major site of electrolyte loss in patients with diahorrea.
Does lung function play a role in acid-base homeostasis?
- yes it plays a critical role.
The body is a net acid producer. It produces vast quantities of volatile acid, how is this excreted?
- it is excreted by the lungs.
- it is excreted in the form of carbonic acid (carbon dioxide and water), this is excreted by the lungs as CO2.
How is carbonic acid generated?
- by oxidation of carbohydrates and fats to CO2 and water.
The body produces small amounts of non-volatile acids, what are they and how are they excreted?
- these are excreted by the kidneys.
- lactic acid: produced by partial oxidation of carbohydrates.
- ketoacids: produced by partial oxidation of fats.
- also include inorganic acids such as: sulphuric and phosphoric acid which result from metabolism of sulphur and phosphate containing compounds.
How is carbonic acid produced?
- CO2 combines with water and under catalysing influence of enzyme carbonic anhydrase carbonic acid is produced in a reversible reaction.
What does carbonic acid dissociate into?
- H ion and bicarbonate ion.
How is carbonic acid produced?
- CO2 is produced in peripheral tissues from carbohydrate and fatty acid metabolism, where it diffuses into the RBCs within the capillaries which contain the enzyme carbonic anhydrase.
- assisted by this enzyme CO2 combines with water within red cells to form carbonic acid which dissociates into H+ and HCO3- (bicarbonate).
- bicarbonate diffuses out of RBC into plasma while H+ is temporarily buffered in cells by haemoglobin.
Describe the reverse reaction of carbonic acid production?
- the process is reversed on reaching the lung capillaries.
- CO2 concentration in alveoli is low, due to respiration and therefore CO2 can diffuse down the concentration gradient out of RBC through alveolar wall and is expired.
Is the reabsorption of filtered bicarbonate an important renal function?
Describe how reabsorption of filtered bicarbonate is carried out by the kidneys.
- bicarbonate ion is freely filtered at the glomerulus and normally fully reabsorbed in the renal tubules.
- however bicarbonate ions cannot cross the luminal membrane of the renal tubular cells: they are reabsorbed by conversion to CO2 in the tubular lumen.
- the renal bicarbonate reabsorption does not result in any net H+ excretion as the secreted H+ ions are required to "capture" bicarbonate ions.
- once all filtered bicarbonate ions have bee reabsorbed, secreted H+ is captured by urinary phosphate and ammonia buffers and is excreted in the urine.
Ammonia production by the renal tubular cells is regulated by ...
- an inducible enzyme, glutaminate.
- this can be used when requirement to excrete additional H+ ion.
Acids (or H+) must be temporarily buffered for transport around the body and also in urine, true or false?
What is a buffer pair?
- a buffer pair is a weak acid (acid which is only partially dissociated) together with its conjugate base,
- a buffer pair reduces the change in hydrogen ion concentration following addition of a strong acid or alkali.
What happens when you add a strong acid to a buffered solution?
- eventually the buffer base will be entirely consumed and the buffering capacity of particular buffer pair will be lost.
- however, so long as theirs buffer base around there will be just a small rise in hydrogen concentration with net consumption of buffer base.
There are a number of buffer pairs within the human body, what are they?
- extracellular compartment: carbonic acid/bicarbonate (buffer pair we can measure using blood gas machine). Proteins also have an important role as buffers.
- intracellular compartment: proteins including haemoglobin. Phosphates, monohydrogen and dihydrogen phosphates form important intracellular buffer pairs (important in urine).
- urine: phosphates. Ammonia. These buffers temporarily handle H+ until acids can be excreted by lung or kidney, they can't go on buffering it without excreting it as all buffer base will be used up.
Why is the carbonic acid and bicarbonate buffer pair important?
- because physiologically the buffer capability can be extended.
- 1) retention or excretion of carbonic acid as CO2 by lungs.
- 2) retention, generation and excretion of HCO3- (bicarbonate) by the kidneys.
What does a blood gas machine measure?
- it measures pH directly, partial pressure of CO2 directly.
- knows 2 of 3 unknowns, therefore can work the third out.
- it calculates bicarbonate concentration.
What will cause a fall in pH (acidosis)?
- a fall in bicarbonate (metabolic acidosis), or
- a rise in pCO2 (respiratory acidosis).
What will cause a rise in pH (alkalosis)?
- a rise in bicarbonate (metabolic alkalosis).
- a fall in pCO2 (respiratory alkalosis).
Give an example of metabolic acidosis.
- metabolic acidosis due to impaired acid excretion is seen in patients with renal failure.
Give an example of respiratory acidosis.
- respiratory acidosis implies there is impaired excretion of CO2.
- this is seen in patients with lung disease e.g. pneumonia.
Give an example of metabolic alkalosis.
- metabolic alkalosis implies a loss of acid.
- this is seen in patients with severe vomiting as they lose vast amounts of hydrochloric acid secreted in the stomach.
Give an example of respiratory alkalosis.
- respiratory alkalosis implies low partial pressure of CO2 due to hyperventilation.
- this is seen in patients with panic attacks.