anatomy and physiology

Question

The urinary system is precise system that balances all the blood constituents and any change in their values can cause problems to the body. For this part choose one of the following and describe its journey from the small intestine to urinary excretion:

Sodium – Potassium – Proteins – Calcium – Chloride – Magnesium – Water – Lipids – Carbohydrates – urea - creatinine

Your post should also include the steps of urine formation (glomerular filtration, tubular reabsorption, tubular secretion) and pertinent hormones that encourage the movement of these ions on their journey

Answer 1

For this question, I will be picking sodium.

Initially, when sodium is ingested, it will be absorbed into the body via the small intestinal villi by means of facilitated diffusion due to the fact that it is positively charged. Likewise, sodium will just directly dissolve into the bloodstream and not require a carrier protein due to its positive ionic charge. It will arrive at the kidney and thus the nephron by means of the renal artery.

Once at the nephron, sodium will be appropriately filtered by means of the glomerulus, to which after it may be reabsorbed back into the bloodstream by the proximal convoluted tubule. However, if there is an excess of it, then it will proceed to the later parts of the nephron.

The next step the sodium will take is by the descending Loop of Henle, where it will simply just travel down to the bottom to the loop as the descending portion of the Loop of Henle is impermeable to water and to salts. However, once sodium gets the ascending Loop of Henle, which is permeable to water, then it will be able to leave the nephron and as a result take water with it due to the principles of osmosis: "Wherever salt goes (sodium), water follows".

This is why we see animals like camels have very long ascending Loops of Henle: because animals like these that go with very long periods of not getting water need to be able to maximally reabsorb as much water as possible back into the bloodstream. As a result, camels' urine will be 10 times more concentrated than that of humans.

Once the sodium passes by the Loop of Henle, it has the potential to get reabsorbed back into the bloodstream by the distal convoluted tubule and the collecting duct. However, it can only do so in these regions by means of the activation of the hormone aldosterone. This hormone is secreted by the renin-angiotensin system and acts on the distal convoluted tubule and the collecting duct on the nephron by activating the aquaporins in that region which will cause sodium to become reabsorbed back into the bloodstream and take water with it and raise blood pressure, which is the effect of aldosterone.

However, if aldosterone is not secreted, any sodium that has reached that point will simply just go down the collecting duct and be excreted as urine.

I hope this helps you out and if you have any more questions, I would be happy to talk more about it by scheduling a private session!