Question

8. Normally urine is hypo-osmotic. What does that mean?

9. What is the counter current mechanism? What is its purpose?

10. How and why does the renin-angiotensin system (RAS) increase BP?

Answer 1

8. Hypo-osmotic urine means the urine has lesser concentration of solute. That is less amount of salts (ions, uricacid, etc) are dissolved in the solvent (urine).

Usually hypo osmotic urine is produced when the subject is having enough fluids intake.

Hyper osmotic urine is when there are more concentrated urine is produced. this occurs in dehydration, low blood pressure.

9. counter current mechanism- It is the process that occurs in the kidneys where there is energy expenditure to maintain an osmotic gradient. this osmotic gradient is the driving force for the reabsorption of  water from the tubular fluid and produces concetriated urine.

This mechanism takes place in the juxtamedullary nephron.

ADH promotes water reabsorption through the walls of the distal convoluted tubule and collecting duct.

Purpose- to reabsorb essential ions, fluids from the tubules. This mechanism is important in dehydration conditions and to maintain homeostatis of the body.

10. RAS pathway is activated when there is low blood pressure in the kidneys or in the systemic circulation. This can occur from the blood loss, or dephydration which can lead to low blood pressure.

The low blood presusre is sensed by the baroreceptors which are in the carotid sinus and in the arch of aorta. This sends signals for the activation of RAS.

The juxta glomerular cells of the kidneys release RENIN. and at the same time, stimulus of low blood pressure causes release of ANGIOTENSIONGEN from the liver into the blood stream.

renin and angiotensinogen increases the levels of ANGIOTENSIN I. This angiotensin I is converted to ANGIOTENSIN II in the presence of ANGIOTENSIN CONVERTING ENZYME. which is present in the lungs, the rise in angiotensin II levels in the blood causes vaso-constriction of the arterioles,, as angiotensin II is a potent vasoconstrictor. this leads to increase in blood presssure.

Angiotensin II also stimulates adrenal gland for the relsease of aldosterone, which causes increase in reabsorption of water and ions from the tubules of kidneys. This causes increase in the blood volme leading to rise in blood pressure.

After the rise in blood pressure, the baroreceptors sense the BP and send a negative feedback to prevent futher release of renin into the blood stream which untimately stops the release of angiotensin aswell and rise in BP any further is inhibited.

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