Transcript
Water Homeostasis
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Module: Balancing Fluids
Activity: Animations
Title: Water Homeostasis
Introduction
1. a. Water homeostasis is crucial to life. Define blood osmolarity.
The maintenance of fluid volume and composition despite changes in water input.
b. What is the nephron’s role regarding osmolarity?
The nephron functions in the regulation of the blood’s osmolarity solute concentration.
c. What two factors regulate body fluid osmolarity?
- formation of a medullary osmotic gradient.
- the amount of Antidiuretic hormone available at the collecting ducts.
d. Describe the collecting duct role relative to urine concentration.
The collecting ducts absorb water and form concentrated urine. If the duct is permeable to water, water will pass through the collecting ducts to form dilute urine.
Medullary Osmotic Gradient
2. Identify the three factors in the nephron determine the osmotic gradient.
The three factors are
• Differences in water and solute permeability and the reabsorption in different sections of the loop of Henle.
• urea cycling in the medulla.
• countercurrent exchange in the vasa recta.
3. Countercurrent Multiplier
a. Active transport of solutes is the primary cause of medullary osmotic gradient. Describe, for example, ion activities along the ascending loop of Henle in the kidney medulla.
Here the active transport of sodium drives the passive reabsorption of chloride ions. Addition of these ions to the interstitial fluid of the medulla increases its osmolarity.
b. Explain how/why filtrate osmolarity increases along the descending loop of Henle.
Squamous cells in the descending limb are permeable to water but impermeable to most solutes. Water leaves the filtrate in the descending limb of the loop, but the solute cannot enter, thus increasing the filtrate osmolarity. Due to the water movement new filtrate entering the descending limb becomes more and more concentrated as it flows to the bottom of the loop.
c. Explain how/why filtrate osmolarity decreases along the ascending loop of Henle.
The cuboidal epithelial cells of the ascending limb provide for active reabsorption of sodium and chloride ions, but are impermeable to water. Due to the active reabsorption of solutes along the ascending limb, the filtrate being concentrated becomes more and more diluted towards the distal convoluted tubule.
d. Describe how the ascending and descending limbs of the loop of Henle affect each other consequently increasing the work being done by each.
Water moves out of the descending limb and produces a more hypertonic filtrate towards the bottom of the loop. In the ascending limb the solutes pumped out of the concentrated filtrate increases the medullary osmotic gradient. More solutes leaving he ascending limb cause more water to leave the descending limb. These processes multiply each other until there is dynamic equilibrium between the osmolarity of the fluids in the different limbs of the loop and the surrounding medullary space.
e. Define the countercurrent multiplier.
A mechanism that constantly establishes the osmolarity gradient throughout the renal medulla.
4. Urea Recycling
a. Describe filtrate osmolarity as it enters the medullary portion of the collecting duct.
When the filtrate enters the medullary part of the collecting duct most of the water has been reabsorbed leaving urea relatively concentrated.
b. Explain the urea recycling process.
Collecting duct cells are highly permeable to urea so urea diffuses into the medulla and increases the interstitial osmolarity. The effect of urea cycling greatly increases the medullary osmotic gradient values to the final amounts. The rest of the The nephron is mostly impermeable to urea, therefore the urea is recycled back to the collecting duct of the medulla.
5. Vasa recta Countercurrent Exchange
a. Explain why the shape of the vasa recta plays an important role as blood travels through these capillaries.
Because the shape of the vasa recta follows the limbs of the loop of Henle and provides a mechanism to maintain the medullary osmotic gradient.
b. Describe blood osmolarity changes as it descends into the kidney medulla in the vasa recta.
Blood enters the medulla of the kidney with normal osmolarity. As blood moves into the medulla highly permeable vasa recta capillaries exchange solutes with the interstitial fluid and blood osmolarity increases.
c. Describe blood osmolarity changes as it ascends back out of the medulla toward the cortex.
As blood moves out of the medulla up the cortex it loses solutes and the blood osmolarity decreases to nearly normal values.
d. What causes the small increase in blood osmolarity that is leaving the vasa recta. What does this osmolarity increase indicate?
The small increase in osmolarity in the blood leaving the vasa recta is a result of the blood colloid osmolarity and it indicates that some water is lost from the body.
e. Why is this countercurrent exchange essential?
Because it restores the osmolarity of the medulla so that tubular filtrate can be concentrated..
Forming Urine: Dilute and Concentrated
6. What conditions cause urine to become concentrated?
The conditions are: body fluid volume and body fluid composition.
7. Describe osmolarity changes as filtrate moves through the loop of Henle.
Dilute urine is formed when the body is normally hydrated. The medullary osmotic gradient determines the osmolarity of the filtrate. Filtrate osmolarity increases as it flows down the descending limb of the loop. The ascending limb is highly permeable to solutes but impermeable to water and filtrate osmolarity decreases as it flows up the ascending limb.
8. How do the distal convoluted tubules and collecting ducts affect filtrate osmolarity en route to producing dilute urine?
In the DCT the osmolarity of the filtrate is much lower than blood osmolarity. Both the DCT and collecting duct are not very permeable to water, but permeable to solutes, and filtrate osmolarity continues to decrease. Upon exiting the collecting duct most of its solutes are lost and has a very low osmolarity.
9. Concentrated urine is produced when the body is dehydrated or large amounts of fluids are lost. Filtration occurs as usual even under these circumstances, however, Antidiuretic hormone is stimulated from the posterior pituitary gland.
Describe the ADH affect on collecting ducts.
ADH promotes the production of water pores in the cells of the collecting duct of the medulla. As more water is reabsorbed from the collecting ducts the filtrate osmolarity becomes higher. The filtrate becomes more and more concentrated conserving water.
b. Describe the consequential affect on urine concentration and blood volume.
The water eventually moves via osmosis into the blood increasing blood volume and decreasing blood osmolarity.