Function Of The Kidneys - Filtration, Reabsorption And Secretion Flashcards
What are the functions of the kidney?
Excrete waste and toxins Regulate blood ionic composition Main blood osmolarity Regulate blood pH Regulate blood pressure Regulate blood glucose Produce hormones
During glomerular filtration which molecules are filtered and which remain in the blood?
Water and low molecular weight molecules are filtered Plasma proteins (and non-protein elements which are bound to them) remain in the blood
What is the equation which related excretion to filtration, reabsorption and secretion?
Excretion = filtration - reabsorption + secretion
What is the term for the bolume of fluid filtered from the glomeruli into the capsule space per unit time?
The glomerular filtration rate
What are the forces influencing the glomerular filtration rate?
Glomerular blood hydrostatic pressure promotes filtration
Blood colloid osmotic pressure and capsular hydrostatic pressure oppose filtration
What is paracellular reabsorption?
A passive process whereby substances pass around tubule epithelial cells via tight junctions and into the interstitial fluid.
What is transcellular reabsorption and how does it occur?
Here reabsorption occurs through tubular cells and into the interstitial fluid. Y active transport, passive diffusion, facilitated transport of cotransport.
What molecules can move by paracellular reabsorption?
Water Calcium Chloride Magnesium Potassium Sodium
Why are some substances removed from the blood by secretion?
Because they are in excess
Because they are harmful substances
To maintain a healthy pH
Where does reabsorption and secretion mainly occur?
The proximal convoluted tubules
Why is the proximal convoluted tubule so highly permeable to water and ions?
Because it has a ‘leaky’ apical membrane on its tubular cells
Approximately how many of the sodium ions in the filtrate are reabsorbed at the proximal convoluted tubule?
Two thirds
By what mechanism is sodium reabsorbed in the proximal convoluted tubule?
Sodium diffuses into the eputhelial cells and is actively pumped out by Na/K-ATPases on the asolateral membrane into the interstitial fluid.
Why is the transport of sodium in the proximal convoluted tubule particularly important?
Because it sets up the electrochemical gradient which drives water reabsorption via osmosis and the reabsorption of other solutes by diffusion
How are nutrients such as glucose, amino acids, lactic acid and water-soluble vitamins reabsorbed in the proximal convoluted tubule?
By facilitated transport with sodium ions
How are bicarbonate ions reabsorbed?
They combine with hydrogen ions to form carbon dioxide and water which can freely pass through the membranes of the tubular cells. Once in the cells they break down into bicarbonate and hydrogen ions again and the bicarbonate ions are transported into the interstitial fluid by sodium/bicarbonate ion transporters.
Which transporter pumps hydrogen ions back into the lumen of the renal tubules so that they can allow the reabsorption of bicarbonate ions?
The hydrogen/sodium ion antiporter
How does the reabsorption of chloride ions help with the reabsorption of other ions?
Chloride ions are negative and so their reabsorption increases the negative charge of the interstitial fluid, and so sets up an electrochemical gradient that allows other positive ions to diffuse into the interstitial fluid
How are small proteins present in the filtrate reabsorbed?
They undergo endocytosis at the apical membrane of the tubule cells and are digested to amino acids which can then be transported across the basolateral membranes of the tubule cells and into the interstitial fluid
Describe the secretion of ammonium ions from the proximal convoluted tubule?
These ions result from the deamination of the amino acid glutamine and then are transported back into the lumen of the tubules bey the hydrogen/sodium ion antiporter which will accept ammonium ions instead of hydrogen ions
Describe the effects of the counter current flows in the loop of Henle.
In the ascending limb there is ion reabsorption without water reabsorption but these ions just sit in the interstitial fluid (because the vasa recta forms hairpin loops)making it hyperosmotic. This hyperosmality drives water reabsorption in the descending limb.
Calcium channels sit on the apical membrane of which part of the renal tubule to assist reabsorption?
The early DCT
Which ions are secreted by the late DCT and collecting tubules of the kidneys?
Potassium ions
Hydrogen ions
Which transport molecule is responsible for the secretion of potassium in the renal tubule?
Na/K-ATPase
Which transport molecule is responsible for the secretion of hydrogen molecules in the renal tubule?
H/K-ATPase
Which cells in the kidney produce renin?
Juxtaglomerular cells
Describe how the secretion of renin leads to the secretion of aldosterone by the adrenal glands.
Renin acts of angiotensinogen to produce angiotensin I which is then converted to angiotensin II by angiotensin converting enzymes. Angiotensin II then stimulates the adrenal glands to produce and secrete aldosterone.
Increased renin leads to increased aldosterone levels. What affect does this have on sodium and potassium reabsorption and secretion?
Increased aldosterone levels, increases sodium reabsorption and increases potassium secretion
What factors can lead to the increase secretion of renin and what are these factors sensed by?
Decrease in arterial pressure sensed by juxtaglomerular cells which act as intarrenal baroreceptors.
Salt depletion sensed by the macula densa
What affect does ANP have an sodium reabsorption and how does it achieve this effect?
It inhibits sodium reabsorption by increasing glomerular filtration rate and inhibiting aldosterone secretion
Which hormone will increase calcium ion reabsorption whilst simultaneously decreasing phosphate reabsorption?
Parathyroid hormone
What is the typical water intake for an adult at 23 degrees C undertaking ‘normal daily activities’ and where does this intake come from
1200ml from water
1000ml from food
300ml from metabolic processes
What is the typical water output for an adult at 23 degrees C undertaking ‘normal daily activities’ and where does this intake come from
1500ml from urine
100ml from sweat
200ml from faeces
700ml from insensible loss
What is insensible loss?
The loss of water through things such as respiratory water loss and leakage of water through the skin
What is the value of renal blood flow?
Around 650 ml/min
What is the renal filtration rate?
90-140 ml/min
How much of the resting cardiac output do the kidneys receive?
25%
What is the main cation presence in the extracellular fluid?
Sodium
What is the main cation present in the intracellular fluid?
Potassium
What is the main anion presence in the extracellular fluid?
Chloride
What is the main anion presence in the intracellular fluid?
Bicarbonate
What are the components of the glomerular filtration barrier?
Glomerular endothelium
Collagen-based glomerular basement membrane
Podocyte foot processes forming the filtration slit membrane
What is the normal glomerular filtration rate?
100ml/min
Describe the absorptive process in the ascending loop of Henle which is driven by the sodium/potassium/2 chloride transporter?
The Na/K-ATPase sets up a concentration gradient which allows the transport of sodium, potassium and two chloride molecules through a single transporter into the tubular cell. Potassium is then secreted back into the lumen which creates a positive charge in the lumen which drives the transcellular absorption of magnesium and calcium
Describe the action of the ENaC channel in the DCT?
The action of the channel is driven by Na/K-ATPase and the channel transports sodium into the cell which creates a negative change in the lumen of the tubule which drives the secretion of potassium ions into the lumen
Which chemical will be released by the macula densa to cause arteriolar constriction when increased tubular flow is sensed?
Adenosine
What is the role of ANP and BNP in kidney function?
These directly inhibit sodium reabsorption in the DCT and also inhibit renin and ADH secretion
Normally, how much fluid is there in the intracellular environment?
25L
Normally how much fluid is there in the interstitium?
12L
Normally how much fluid is there in the plasma?
3L
What is the normal cytoplasmic concentration of potassium?
100-120mmol/l
What percentage of total body potassium exists in the extracellular fluid?
1-2%
What is the average UK intake of potassium per day?
80mmol / day
3g
The body can cope with a daily intake of how much potassium if renal function is normal?
20-500mmol/day
What percentage of the glomerular filtrate is reabsorbed by the kidney?
99%
What is the difference between osmolarity and osmolality?
Osmolarity - osmole conc per unit volume
osmolality - osmole conc per unit mass
What is the concentration of solutes in the plasma?
285-295mosm/kg
How long is the PCT?
14mm
What is the only hormone involved in the control of reabsorption in the PCT?
Angiotensin II
Which hormone, in addition to increasing water reabsorption and causing vasoconstriction, creates the sensation of thirst?
ADH
What factor, other than angiotensin II, can induce aldosterone release from the bona glomerulosa of the adrenal cortex?
High potassium
What channel does aldosterone work on in the DCT and collecting ducts to cause sodium reabsorption and potassium secretion?
ENaC
A dysfunction in the Na/K/2Cl channel in the loop of Henle will lead to which hereditary condition?
Bartter’s
A dysfunction in the ENaC channel in the DCT and collecting ducts will lead to which hereditary condition?
Liddle’s
A dysfunction in the transporter channels in the PCT will lead to which hereditary condition?
Fanconi’s syndrome
Measurement of which electrolyte in the urine can differentiate between Barterr’s and Gitelman’s?
Calcium
What are the minimum and maximum rate of daily urine output?
0.4 litres /day
12 litres / day
