Physiology quiz #2

What parts of a nephron does ADH influence?

late distal renal tubule collecting ducts

Ingested fluids are initially absorbed into the blood, hence the ECF. What determines whether any of the fluid gets transferred into the ICF? Note ECF refers to extracellular fluid and ICF refers to intracellular fluid.

osmolarity difference between the ECF and ICF

Results in renin secretion

renal sympathetic nerve stimulation decreased distal tubule NaCl concentration

ECF volume decreases

thirst and ADH retains water Reabsorb more sodium

Low plasma osmolarity

water is excreted thirst decreases cessation of water intake decreased ADH release decreased water absorption

ECF volume increases

water reabsorption increases reabsorb less sodium

Define the term specific gravity

- is a measure of the concentration of particles in urine. The more dehydrated you are, the higher the specific gravity measurement will be. If you drink too much fluid, the specific gravity of your urine will be decreased.

Circulating angiotensin II is increased in response to a decrease in blood pressure. Order the following statements to describe the effect of angiotensin II beginning with "mean arterial blood pressure (MABP) decreases, etc".

1. MABP decreases. This results in activation of the RAAS system 2. Vasoconstriction decreases blood flow through the glomerulus 3. Decreased amount of salt and water are filtered 4. Less salt and water excreted from the body 5. More salt and water remains in the blood 6. MABP increases

Excessive water intake can be intoxicating and even lead to death. Which of the following water volumes approach that maximum water intake per hour? Visit the background information to answer this question.

1000 ml

What fraction of the total body water is in the ICF?

2/3

What hormone is involved in regulating renal water excretion?

ADH

What is an aquaretic agent and how does it work? (Explain what hormone receptor it targets and how that changes water retention)

An aquaretic agent can increase urinary volume resulting from a decreased urine osmolarity. Additionally, plasma sodium is increased. It works by blocking vasopressin that would normally bind to its vasopressin receptor, specifically in the distal tubules and collecting ducts, this impairs the kidneys from concentrating any urine, leading to solute-free water excretion.

For the volunteer who performed protocol 1 (no fluid intake), describe the changes (if any) in urine flow rates you saw during the lab.

Given that the protocol 1 volunteer does not drink during the lab, the flow rate should fall below 1 mL/min (and the specific gravity should rise) during the 2-hour lab. The rate of urine production and hence the flow rate should remain low throughout the lab, often less than 1 mL/min. The specific gravity should be approximately 1.030, which would be expected for a state of mild dehydration.

Describe what hyponatremia is and which hormones are important in water and sodium regulation. Describe the hormone effects and the receptor mechanisms that change water retention.

Hyponatremia occurs when the concentration of sodium in your blood is abnormally low and is commonly observed in hospitalized patients and can be associated with unfavorable outcomes. High serum arginine vasopressin (AVP) levels are associated with the pathogenesis of the hyponatremia in hypervolemic and euvolemic states such as congestive heart failure, cirrhosis, and the syndrome of inappropriate antidiuretic hormone secretion (SIADH). These disorders are associated with free water retention resulting in hyponatremia. Angiotensin II and Aldosterone hormones are important in water and sodium regulation. A decrease in plasma osmolarity with water intake would suppress AVP secretion and lead to urinary extrecion of the excess water Aquaporin channels are water channels that allow water to move passively into the cell guided by the osmotic gradient estabilisbed by NaCl and Urea and they promote the reabsorption of water in the kidney which creates more of a concentrated or a hyperosmotic urine and increase water retention.

Describe how the RAAS pathway works and how it may contribute to sodium balance.

Na+ homeostasis is regulated by changes in ECF volume. An decrease in ECF volume will activate the renin-angiotensin II-aldosterone system for Na+ reabsorption back to the blood, which causes water to be reabsorbed back to blood as well. ANP, however, increases excretion of Na+ and water. For isotonic saline drinkers, the RAAS system will be inhibited.

Which of these is the most important determinant of the sensation of thirst and the release of antidiuretic hormone (ADH)?

Osmolarity of the plasma

Which of these would be seen if the effects of angiotensin II were inhibited?

RAAS is stimulated in response to low MABP. Angiotensin II acts to increase vasopressin release, which stimulates water reabsorption by the kidney. Therefore, inhibition of angiotensin II would decrease vasopressin release and cause Peripheral vasodilation.

Our bodies monitor body water content through measuring plasma osmolarity. Why don't we monitor body sodium content by measuring plasma sodium concentration?

The amount of sodium in the ECF is already regulated by the ADH levels, hence this is not the signal for sodium regulation. As a result the concentration of sodium is not dependent on the body's sodium levels.

Protocol 3: Isosmotic solution (Result)

The rate of urine output remained fairly similar throughout the lab. It may have increased a little following the ingestion of the isosmotic salt solution. However, as the ingested sodium was confined to the extracellular compartment, all of the ingested water would also have remained there too. So the extracellular osmolarity would have been unchanged. Hence, no osmotic stimulus would have suppressed ADH secretion. The expansion of plasma volume may have had a small inhibitory effect on ADH release but this would have been relatively unimportant over the course of this lab. Over the two hours, a relatively small fraction of the fluid ingested would have been excreted in the urine. But over the next day or so, all of the additional fluid will be lost. (We do regulate our sodium content, and consequently our extracellular volume, but this occurs over a much longer time course than the regulation of plasma osmolarity.) As in the other protocols, the results would have been influenced by the volunteer's initial state of hydration.

For the volunteer who performed protocol 2 (which illustrates a normal water diuresis), can you describe how the kidneys handle a water load?

The rate of urine production should be low at the beginning of the lab but should increase as the ingested water is absorbed. Depending on the volunteer's previous state of hydration, it can reach levels as high as 10-15 mL/min. As the rate increases, the specific gravity will fall and can become as low as 1.003-1.005. As for protocol 1, the rate at which the volunteer begins to excrete the water load and the pattern of the subsequent diuresis is determined by the volunteer's initial state of hydration.

Protocol 2: Hyposmotic solution (Result)

The rate of urine production should have been low at the start of the lab but should have increased as the ingested "water" was absorbed. Depending on the volunteer's previous state of hydration, flow rates may have reached levels as high as 10-15 mL/min or even a little more. As the rate increased, the specific gravity should have decreased, becoming as low as 1.003-1.005. As in Protocol 1, the rate at which the volunteer excreted the water load and the pattern of the subsequent diuresis would be influenced by the volunteer's initial state of hydration.

Protocol 1: Control (Result)

The rate of urine production should have remained low throughout the lab, at around 1 mL/min or less, with the specific gravity around 1.020 or so. If the volunteer had drunk a significant quantity of fluid in the hours before coming to the lab, the initial rates of urine production would have been higher (and specific gravity lower). Given that these volunteers did not drink during the lab, this rate should have decreased below 1 mL/min (and the specific gravity increased) as the lab progressed.

Describe how the kidneys handle a water load

Water homeostasis is regulated by ECF osmolarity. An increase in ECF osmolarity will trigger ADH release from the hypothalamus, which mainly stimulates the distal tubules and collecting ducts for water reabsorption. For the hypoosmotic group, ADH release is inhibited.

Angiotensin II

acts to increase vasopressin release, which stimulates water reabsorption by the kidney.

When an individual is normally hydrated and drinks isotonic saline you would expect which of the following?

all of that ingested isotonic fluid goes to ECF

ADH

antidiuretic hormone helps to maintain balance in water content of the body. The main function of ADH is to reabsorb water from kidney.

Explain how ADH contributes to water balance

antidiuretic hormone helps to maintain balance in water content of the body. The main function of ADH is to reabsorb water from kidney. If the water level increases above normal range, the solute concentration decreases and that is sensed by the hypothalamus which stimulate pituitary to secrete less ADH so that less amount of water can be reabsorbed. If the water level of the blood decreases, solute concentration of the blood increases. The change in solute concentration is detected by the hypothalamus and stimulate the pituitary to secrete more ADH. This results in more reabsorption of water from filtrate and the water level of the blood increases.

Effects Angiotensin II only

arteriolar vasoconstriction -stimulates vasopressin release -increases thirst

What would urine look like from an individual with high levels of ADH? Assume the individual is a healthy young person.

dark yellow and have higher specific gravity

Angiotensin II and Aldosterone

decrease salt excretion by kidneys

urine osmolarity

determined by the need to get rid of excess water, or to retain water. - When plasma osmolarity is reduced and circulating levels of ADH are low, the urine will be dilute compared to plasma. - When plasma osmolarity is increased and circulating levels of ADH are high, the urine will be more concentrated than plasma.

High plasma osmolarity

increase in water reabsorption water is ingested increased ADH release water is retained by the body thirst increases

Does not result in renin secretion

increased circulating aldosterone