Physiology-Urinary Concentration Flashcards
What is the normal plasma osmolality?
290 mOsm/L
You are running a 10k because your wife made you. You didn’t train for it and she rolled you out of bed 10 minutes before it started. The whistle blows and you start running. About halfway through the race you start to feel light-headed and thirsty, but you don’t have to pee. What is causing you to feel this way?
The body assesses its water content in three ways: the hypothalamus senses increased plasma osmolarity and triggers thirst, the CV baroreceptors sense decreased effective circulating volume (ECV) and induces sympathetic tone, the juxtaglomerular apparatus secretes renin, triggering aldosterone & ADH release and you won’t have to pee.
You are driving through the countryside when you swerve away from fat cow in the middle of the dirt road. You crash, your car is broken down and you have no cell phone service so you have to sit and wait for someone to come. What will your plasma osmolality be by the time you are so thirsty that you decide to milk to the cow for a drink?
295 mOsm/L. This is when the hypothalamus is on full throttle thirst stimulation.
You are driving through the countryside when you swerve away from fat cow in the middle of the dirt road. You crash, your car is broken down and you have no cell phone service so you have to sit and wait for someone to come. You get so thirsty that you decide to milk the cow and drink it. How does your brain ensure you retain that fluid for the time being?
The hypothalamus senses the slight increase in osmolality caused by dehydration and triggers ADH release from the posterior pituitary. ADH acts on the principal cells of the collecting ducts to reabsorb water so you stay hydrated.
You are a sick scientist/dog owner. You decide that you are tired of your old dog peeing on the carpet and give him daily ADH injections. How will this result in the dog peeing on your carpet less?
ADH binds the V2 receptors on cells in the collecting ducts. This triggers activation of adenylyl cyclase -> cAMP -> movement of aquaporin channels to the lumenal side of the collecting duct.
You are a sick scientist/dog owner. You decide that you are tired of your old dog peeing on the carpet and give him daily ADH injections. What side effects will your dog have as a result of this?
ADH will cause fluid retention and V1-stimulated vasoconstriction. Your dog will have a massive increase in blood pressure.
What is the “knob” the body can adjust to control effective circulatory volume? What is the “knob” for controlling osmolality?
ECV is controlled by salt regulation. Osmolality is controlled by water regulation.
You just learned about kidney stones in pathology lab and decided that you probably don’t want one. Consequently you have been drinking water all day. When you pee, you notice that your pee isn’t as yellow as it normally is. Changes in what portions of the nephron are causing your pee to appear more clear?
The distal tubules are the main players when it comes to concentrating urine.
Your wife is pulling her hair out because you spend so much money on diapers. She determines that if you only changed your baby when she is poopy, you would save 15 diapers a day. Why is this baby peeing at least 15 times a day?
Infants have a smaller loop of Henle going through the renal medulla. You need a long loop of Henle in order to concentrate urine. Since they can’t concentrate urine, they pee a lot.
What is responsible for water reabsorption through the thin descending loop of Henle? Where does water reabsorption stop?
The thin descending loop travels through the hyperosmotic medulla. This causes water to leave the tubule, and ultimately the descending loop of Henle also becomes hyperosmotic. Water will stop flowing out of it around the start of the thick ascending loop of Henle.
What portion of the nephron is always hypertonic? What portion is sometimes hypertonic?
The descending loop of Henle. The collecting duct is sometimes hypertonic depending on the actions of ADH and aldosterone.
Your family doctor takes your blood pressure and it is 150/95. He says you have hypertension. He is also Chinese, so he’s kind of hard to understand and you think he tells you to never eat salt. So you don’t eat any salt for a week and get really sick. How would this new diet of yours affect the countercurrent multiplier mechanism in the nephron?
Salt is responsible for establishing the medullary osmotic gradient. Since the thick ascending limb is impermeable to water, Na+ and Cl- are actively reabsorbed as you go up the thick ascending limb into the interstitium. This increases the osmalarity of the interstitium, causing reabsorption of H20 from the thin descending loop of Henle. If this man did not eat any salt, this gradient would be disrupted.
A 22 year old soldier comes to you after a gunshot wound to the stomach. His blood pressure is 80/20, but you get him stabilized and he survives. 2 days later he goes into renal failure. What portions of the nephron are most likely causing him to go into renal failure?
The thick ascending loop of Henle and the proximal convoluted tubule are heavily involved in reabsorption of electrolytes, which requires ATPase. Ischemia almost instantly damages these cells that have such high energy demands and causes tubular necrosis and acute renal failure.
Your family doctor takes your blood pressure and it is 150/95. He says you have hypertension. He is also Chinese, so he’s kind of hard to understand and you think he tells you to never eat salt. So you don’t eat any salt for a week and get really sick. How would this new diet affect movement of water at the collecting duct when ADH is present?
ADH will cause aquaporins to move to the lumenal side of the tubule; however, water will not be reabsorbed as well because the medullary osmotic gradient is decreased due to low sodium levels.
You are lost in the wilderness with nothing to hunt with for 2 months. You have a stream and stay hydrated; however, you rarely have anything to eat. How will absence of protein in your diet affect reabsorption of water in your nephrons?
The medullary osmotic gradient will decrease from 1200 to about 800. This is because urea, which comes from protein, reabsorption in the thin descending loop increases the osmolality of the medulla by about 500. Without it Na+ being pumped into the medulla is the only factor contributing to the osmolality of the medulla.
Where is most urea reabsorbed? Where is it secreted?
30-40% is passively reabsorbed along the proximal tubule. Some of the urea gets back into the loop of Henle and is secreted in the urine.
How does ADH affect urea concentration in the medulla?
ADH stimulates UTA-1, a urea transporter that is very permeable to urea. This allows urea to pass into the medulla and increase H20 reabsorption due to increased medullary osmotic gradient.
How does urea get passively transported back into the vasa recta?
UT-B transporter
You just learned about kidney stones in pathology lab and decided that you probably don’t want one. Consequently you have been drinking water all day. How might this affect your kidney’s ability to reabsorb water?
Over hydrating decreases the efficacy of the countercurrent exchanger. High water concentration in the vasa recta inhibits passive movement of water from the renal interstitium into the vasa recta. This dilutes the renal interstitium because less water is taken up and dissipates the gradient created by the countercurrent multiplier.
What can cause decrease water retention due to defects with the ADH pathway?
*
