Section 6

Question 2

What is plasma clearance?

Answer 2

Plasma clearance is the volume of plasma cleared of a substance by the kidneys per minute. It expresses the effectiveness of the kidneys in removing a substance from the internal fluids. The unit of plasma clearance is volume of plasma, not the amount of the substance.

Plasma clearance is calculated using the equation: Clearance rate (ml/min) = (Urine concentration (quantity/ml) x Urine flow rate (ml/min)) / Plasma concentration (quantity/ml)

Question 3

The plasma clearance rate varies for different substances, depending on how the kidneys handle each substance.

What are the the three variations in plasma clearance?

Answer 3

• Substances that are filtered, not reabsorbed
• Substances that are filtered AND reabsorbed
• Substances that are filtered and secreted, not reabsorbed

Question 4

What type of substances are filtered but not reabsorbed by the kidneys?

Answer 4

Substances that are filtered but not reabsorbed include inulin, an exogenous carbohydrate found in onions and garlic.

Since all glomerular filtrate is cleared of these substances, the volume of plasma cleared/min = the volume of plasma filtered/min

Question 5

How does the plasma clearance rate differ for substances that are filtered and reabsorbed?

Answer 5

For substances like glucose, which are completely reabsorbed, the plasma clearance rate is zero.

However, for substances like urea, which are only partially reabsorbed, the plasma clearance rate is less than the glomerular filtration rate.

Question 6

What happens to the plasma clearance rate for substances that are filtered, secreted, but not reabsorbed?

Answer 6

The plasma clearance rate for substances that are filtered, secreted, but not reabsorbed is greater than the glomerular filtration rate. An example is hydrogen ions, where the plasma clearance rate is calculated to be 150 ml/min.

Question 7

Select which of the following Hydrogen ions are:
- reabsorbed
- secreted
- neither

Answer 7

• secreted

Question 8

Select which of the following Glucose ions are:
- reabsorbed
- secreted
- neither

Answer 8

• reabsorbed

Question 9

Select which of the following urea ions are:
- reabsorbed
- secreted
- neither

Answer 9

• reabsorbed

Question 10

Select which of the following Inulin ions are:
- reabsorbed
- secreted
- neither

Answer 10

neither

Question 11

What is the fundamental principle behind concentrating urine?

Answer 11

Osmosis

Question 12

How do the kidneys produce concentrated urine despite the surrounding tissues having a higher osmolarity?

Answer 12

The kidneys produce concentrated urine due to a vertical osmotic gradient in the interstitial fluid of the medulla.

Question 13

How does the osmolarity change as you move from the cortex to the renal pelvis in the medulla?

Answer 13

In the medulla, the osmolarity gradually increases from 300 mOsm/L in the cortex to 1200 mOsm/L in the renal pelvis.

Question 14

What are the structural differences/similarities of the two different types of nephrons, specifically with reference to the Loop of Henle?

Answer 14

Cortical: The loop of Henle only dips slightly into the medulla.

Juxtamedullary: The loop of Henle dips all the way down to the renal pelvis. The vasa recta of these nephrons also goes all the way to the renal pelvis. Flow in the loop of Henle and the vasa recta goes in opposite directions in what is called countercurrent flow.

In both types of nephrons, the descending collecting ducts that go all the way to the renal pelvis. These anatomical arrangements, coupled with the permeability and transport properties of the different sections of the tubule, are what allow the kidneys to make urine of different concentrations.

The loops of Henle establish the vertical osmotic gradient, the vasa recta preserve the gradient, and the collecting ducts use the gradient, along with vasopressin, to produce urine of varying concentrations. Collectively, this is known as the medullary countercurrent system.

Question 15

What is the first step in establishing the medullary vertical osmotic gradient?

Answer 15

The first step involves strong osmotic reabsorption of water in the proximal tubule due to the active reabsorption of Na+.

Question 16

How much of the filtrate volume has been reabsorbed by the end of the proximal tubule, and what is the osmolarity?

Answer 16

By the end of the proximal tubule, 65% of the filtrate volume has been reabsorbed.

The osmolarity of the tubular fluid at the end of the proximal tubule is 300 mOsm/L, which is isotonic to other bodily fluids.

Question 17

What happens in the loop of Henle to contribute to the establishment of the vertical osmotic gradient?

Answer 17

In the loop of Henle, an additional 15% of filtered water is reabsorbed.

The ascending limb reabsorbs Na+ without water, while the descending limb is highly permeable to water but does NOT reabsorb Na+.

Question 18

What is the primary mechanism by which water is reabsorbed in the descending limb of the loop of Henle?

Answer 18

The descending limb of the loop of Henle is highly permeable to water, allowing passive reabsorption of water.

Question 19

Describe the mechanism of countercurrent multiplication

Answer 19

The process of countercurrent multiplication in the loop of Henle establishes a vertical osmotic gradient in the kidney’s medulla. This gradient is crucial for concentrating urine. In this process:

• Fluid entering the descending limb is isotonic to the interstitial space.
• Na+ actively reabsorbed in the ascending limb increases the interstitial osmolarity.
• Water moves out of the descending limb, increasing its osmolarity to match the increased osmolarity of the interstitial fluid.
• This concentration gradient is maintained as fresh fluid enters, leading to a highly concentrated tubular fluid in the medulla and a diluted fluid in the ascending limb.
• Eventually, equilibrium is reached, resulting in highly concentrated tubular fluid entering the ascending limb and diluted fluid entering the distal tubule.

Once this increment medullary gradient is established, it remains constant due to the continuous flow of fluid and solute transport.

Question 20

Arrange these in order of occurence:

• 200 mOsm/L gradient is first established between the interstitial fluid and the ascending
  limb
• Fluid flows forward several frames again
• 200 mOsm/L gradient is established once again at each horizontal level
• Ascending and descending limbs reestablish the 200 mOsm/L gradient
• Fluid flows forward several frames
• Vertical osmotic gradient is established and maintained in an ongoing fashion

Answer 20

1. 200 m O s m /L gradient is first established between the interstitial fluid and the ascending limb
2. Fluid flows forward several frames
3. Ascending and descending limbs reestablish the 200 m O s m /L gradient
4. Fluid flows forward several frames again
5. 200 m O s m /L gradient is established once again at each horizontal level
6. Vertical osmotic gradient is established and maintained in an ongoing fashion

Question 21

What are the two purposes of countercurrent multiplication?

Answer 21

Establishing an osmotic gradient: It creates a difference in concentration between the upper and lower parts of the kidney’s medulla. This gradient helps the collecting ducts to produce both concentrated and diluted urine, depending on the body’s needs.

Reducing urine volume: By concentrating the urine in the kidneys, countercurrent multiplication allows the body to conserve water and salt, leading to a significant reduction in the overall volume of urine produced.

Question 22

What is vasopressin?

Answer 22

Vasopressin, also known as antidiuretic hormone, is a hormone released from the posterior pituitary gland.

Question 23

When is vasopressin released relating to the kidney?

Answer 23

Vasopressin is released in response to a water deficit, when the extracellular fluid (ECF) is hypertonic. Its release is inhibited when the ECF is hypotonic.

Question 24

What is the action of vasopressin in the kidneys?

Answer 24

Vasopressin acts on distal tubular cells to increase the number of aquaporin molecules in the luminal membrane, facilitating water reabsorption into the epithelial cells.

Question 25

How does water move after being reabsorbed into the epithelial cells?

Answer 25

Once inside the epithelial cells, water passively moves into the interstitial fluid and plasma.

Question 26

In which part of the nephron does vasopressin increase water reabsorption? Which part does it not have an effect on?

Answer 26

Vasopressin increases water reabsorption only in the distal and collecting tubules, as it has no effect on the proximal tubule or the loop of Henle where 80% of water is reabsorbed.

Question 27

What are the osmolarity levels of the tubular fluid entering the distal tubule and the interstitial fluid in the renal cortex?

Answer 27

The tubular fluid entering the distal tubule is around 100 mOsm/L, while the interstitial fluid in the renal cortex is 300 mOsm/L gets even higher, approaching 1200 mOsm/L around the collecting tubules, as they plunge towards the renal pelvis.
These gradients mean that water wants to leave the tubular fluid due to osmosis, but it can only do so in the presence of vasopressin

Question 28

In a state of dehydration, describe what happens in the tubules

Answer 28

When someone is very dehydrated, the release of vasopressin will increase the number of aquaporin channels in the distal and collecting tubules.

Question 29

Describe the maximum influence of vasopressin

Answer 29

Under the maximum influence of vasopressin, the osmolarity of the tubular fluid at the end of the collecting ducts can be up to 1200 mOsm/L, isotonic to the interstitial fluid.

This is the maximum concentration of urine that can be achieved by the body. Urine production can be reduced to as little as 0.3 m l/min.

Question 30

Describe what would happen in the tubules when someone has excess of water

Answer 30

In this case, the body fluid osmolarity is below 300 mOsm/L. The tubular fluid entering the distal tubule is still 100 mOsm/L.

When the body fluids are so hypotonic that vasopressin secretion is completely suppressed, this prevents the insertion of aquaporins in the luminal membrane of the distal and collecting tubules so NO water is reabsorbed.

In this manner, urine with an osmolarity of 100 mOsm/L can be produced with a volume of up to 25 ml/min

Question 31

How does the vertical osmotic gradient allow for the production of both more concentrated and more dilute urine than normal bodily fluids?

Answer 31

The ascending loop of Henle always makes the tubular fluid very hypotonic by reabsorbing Na+ but not water. This ensures that the tubular fluid is always hypotonic to the interstitial fluid, allowing the kidneys to excrete more dilute urine than normal bodily fluids when vasopressin release is suppressed.

When water needs to be conserved, vasopressin release can cause the tubular fluid to become more concentrated before excretion. Therefore, the loop of Henle plays a key role in allowing the kidney to excrete urine that ranges in concentration from 100 to 1200 mOsm/L.

Question 32

What are the characteristics of the vasa recta that support the countercurrent multiplier mechanism?

Answer 32

The vasa recta, the blood supply to the renal medulla, is closely associated with the descending and ascending loops of Henle due to its hairpin shape. It is highly permeable to NaCl and water, and the blood flow through the vasa recta is countercurrent to fluid flow through the loop of Henle.

Question 33

What effect does the interstitial fluid osmolarity of 1200 mOsm/ml in the medulla have on the composition of the blood in the vasa recta?

Answer 33

The high osmolarity of the interstitial fluid in the medulla results in passive solute and water exchange between the vasa recta and the interstitial fluid. This process, known as countercurrent exchange, preserves the hypertonic gradient of the medulla in the vasa recta blood.

Question 34

What are the changes in the osmolarity of the blood in the vasa recta as it travels through the renal medulla?

Answer 34

1. As efferent arteriolar blood leaves the renal cortex, its osmolarity is 300 mOsm/L, isotonic to the interstitial fluid.
2. As the blood moves through the descending loop, it remains isotonic to the interstitial fluid by reabsorbing Na+ and water.
3. At the bottom of the loop, the plasma osmolarity reaches 1200 mOsm/L.
4. During the ascent in the ascending limb, water is reabsorbed and Na+ leaves to maintain isotonicity with the medullary levels.
5. Upon re-entering the cortex, the osmolarity of the blood in the vasa recta returns to 300 mOsm/L, again isotonic to the interstitial fluid.

Question 35

What is the importance of distinguishing between different types of water reabsorption?

Answer 35

It’s essential to differentiate between water reabsorption that follows solute reabsorption and water reabsorption independent of solute reabsorption.

Question 36

What is osmotic diuresis?

Answer 36

Osmotic diuresis involves increased excretion of both water and excess un-reabsorbed solute. It occurs in conditions such as diabetes where high glucose levels prevent complete reabsorption, leading to excess glucose in the tubules attracting water and increasing urine production.

Question 37

What is water diuresis?

Answer 37

Water diuresis refers to increased excretion of water with little or no change in solute excretion. It often occurs after alcohol consumption due to suppressed vasopressin secretion, resulting in the excretion of dilute urine in volumes greater than the alcohol consumed, leading to dehydration despite drinking.

Question 38

Through what body part –> what part, and how is the urine transmitted after being formed in the kidneys?

Answer 38

Urine is transmitted through the ureters by peristaltic contractions to the bladder

Urine doe not normally flow backwards, but it can if enough pressure is generated

Question 39

What is the location of urine storage, and through what is the exit through the urtethra (connected to it) guarded by?

Answer 39

The bladder is the main storage.

The internal urethral sphincter and the external urethral sphincter control the exit through the urethra.

Question 40

What is the function of the bladder?

Answer 40

The bladder, composed of smooth muscle with a specialized epithelial lining, expands to increase storage capacity. It is innervated by the parasympathetic nervous system, and its stimulation causes bladder contraction.

Question 41

Describe the internal urethral sphincter.

Answer 41

The internal urethral sphincter is under involuntary control and closes the outlet to the urethra when the bladder is relaxed. Although it’s part of the bladder wall and not a true sphincter, it helps regulate urine flow.

Question 42

What is the role of the external urethral sphincter?

Answer 42

The external urethral sphincter encircles the urethra and is supported by the pelvic diaphragm. The urethra is kept closed by a constant, tonic firing of motor neurons.

Comprised of skeletal muscle, it’s under voluntary control, and therefore can be deliberately tightened to prevent urination, even when the bladder contracts.

Question 43

What is micturition, and through what general mechanisms does it occur?

Answer 43

Micturition, or urination, is the process of bladder emptying, governed by two mechanisms: the micturition reflex and voluntary control.

Question 44

Describe the micturition reflex.

Answer 44

When bladder filling reaches a certain threshold, internal pressure stretches the bladder wall activating afferent fibers of the spinal cord where interneurons activate the parasympathetic system to stimulate bladder contraction and relaxation of the external sphincter.

This reflex can be overridden by voluntary signals from the cerebral cortex, but eventually, the reflex becomes stronger than voluntary control, leading to uncontrollable bladder emptying.

Question 45

How does voluntary control affect micturition?

Answer 45

Voluntary control involves the perception of bladder filling before the reflex is activated.

Signals from the cerebral cortex can override the micturition reflex temporarily. However, as urine production continues, the pressure-activation of the reflex eventually overrides voluntary control, leading to bladder emptying.