38 Questions
What is the primary cause of urinary concentrations of substances?
Tubular function
What is a characteristic of proximal tubular failure?
Low bicarbonate concentration and low pH
What is a consequence of distal tubular failure?
Increased urine volume, even with low renal blood flow
What is a characteristic of proximal and/or distal tubular failure?
Increased volume and inappropriately high pH
What is a characteristic of proximal tubular failure?
Hypokalaemia
What is the name of the process by which the tubules reabsorb water?
Reabsorption
What is the effect of a small impairment of reabsorption on urine volume?
A large increase in urine volume
What is the approximate percentage of filtered water that is normally reabsorbed by the tubules?
99%
What is the primary cause of oliguria in acute kidney injury?
Glomerular damage due to reduced cortical blood flow
What is the primary goal when examining a patient with acute kidney injury?
To exclude reversible causes of pre-renal failure
What is a common symptom of acute kidney injury?
Hyperkalaemia
What is the purpose of renal tract imaging in acute kidney injury?
To identify post-renal urinary tract obstruction
What is an example of a pre-renal cause of acute kidney injury?
Hypovolaemia
What is a complication of acute kidney injury?
Hyperkalaemia
What percentage of filtered water is reabsorbed from the tubular lumen?
70-80%
Which of the following waste products is not reabsorbed from the tubular lumen to a significant degree?
Urea
What is the primary function of the tubular cells in the proximal tubules?
Reabsorption of solutes and water
What is the approximate volume of water that enters the loops of Henle daily?
40-60 L
What happens to most of the filtered urea?
It is passed in the urine
What is the approximate volume of water that is reabsorbed from the proximal tubules?
140-160 L
What is the primary function of the loops of Henle?
Reabsorption of water and dissociation of solute
What ions are actively reabsorbed from the proximal tubules?
Sodium, calcium and magnesium
What is the function of the impermeable layer?
To prevent water from entering the cortex
Which part of the kidney has a higher concentration of solutes?
Medulla
What is the direction of water flow in the cortex?
From the cortex to the blood
What is the role of the renal counter-regulatory system?
To maintain water balance
What is the difference in solute concentration between the cortex and medulla?
The medulla has a higher concentration of solutes
What is the purpose of the cortex in the renal counter-regulatory system?
To filter water from the blood
What happens to the solute concentration as you move from the cortex to the medulla?
It increases
What is the role of the medulla in the renal counter-regulatory system?
To conserve water
What is the effect of a high water intake on the extracellular fluid?
It dilutes the extracellular fluid
Why are osmotic diuretics infused and not absorbed from the gut?
Because they cannot cross cell membranes
What is the effect of maximal water diuresis on the osmolality at the tips of the medullary loops?
It decreases the osmolality to 600 mmol/kg or less
What is the main effect of sodium reabsorption in exchange for hydrogen ions in the proximal tubules?
Reclamation of filtered bicarbonate
What is the effect of aldosterone on the nephron?
It stimulates the secretion of hydrogen ions
What is the result of increasing the circulating volume?
Increased renal blood flow
What is the effect of rapid flow in the vasa recta on the medullary hyperosmolality?
It washes out the medullary hyperosmolality
What is the exchange process in the distal tubules and collecting ducts associated with?
Net generation of bicarbonate
Study Notes
Nephron Function
- Nephrons reabsorb 99% of filtered water, amounting to 140-160 L/day.
- 70-80% of filtered water is reabsorbed in the proximal tubules.
- Tubular cells do not actively deal with waste products like urea and creatinine.
- Most filtered urea is excreted in urine, contributing to its osmolality.
Solute Reabsorption
- Almost all potassium is actively reabsorbed from the proximal tubules.
- Over 70% of filtered sodium, free ionized calcium, and magnesium are reabsorbed from the proximal tubules.
Renal Counter-Regulatory System
- A high water intake dilutes the extracellular fluid, reducing ADH secretion.
- The collecting ducts remain impermeable to water, producing a dilute urine and a high osmolality within the medulla and medullary vessels.
- The countercurrent multiplication process helps correct the fall in systemic osmolality.
Osmotic Diuretics
- Effective osmotic diuretics cannot cross cell membranes to a significant degree.
- Mannitol, a sugar alcohol, is an example of an osmotic diuretic used therapeutically.
Homeostatic Solute Adjustment
- Sodium reabsorption in exchange for hydrogen ions occurs throughout the nephrons.
- In the proximal tubules, this exchange helps reabsorb filtered bicarbonate.
- In the distal tubules and collecting ducts, the exchange process is associated with net generation of bicarbonate.
Clinical Manifestations of Tubular Failure
- Proximal tubular failure: low bicarbonate concentration, low pH, hypokalaemia, hypophosphataemia, hypomagnesaemia, and hypouricaemia.
- Distal tubular failure: pH inappropriately high compared to that in plasma.
- Proximal and/or distal tubular failure: increased volume, pH inappropriately high, generalized amino aciduria, phosphaturia, and glycosuria.
Acute Kidney Injury (AKI)
- Causes of AKI include pre-renal, renal, and post-renal factors.
- Pre-renal factors include hypovolaemia and hypotension.
- Renal factors include tubular damage and glomerular damage.
- Post-renal factors include urinary tract obstruction.
- Clinical history and examination are essential in diagnosing AKI and excluding reversible causes of pre-renal failure.
This quiz covers the role of nephrons in reabsorbing and filtering waste products in the kidney. Learn about the percentage of water reabsorbed and how the tubular cells handle waste products.
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