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Sodium homeostasis

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High Yield Notes

10 pages

Questions

USMLE® Step 1 style questions USMLE

of complete

There are many factors that influence sodium reabsorption and excretion in the kidneys. Which of the following causes increased excretion of sodium into the urine?  

Transcript

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Content Reviewers

Viviana Popa, MD

Sodium is a positive ion or a cation noted with Na.

Most of the sodium in our body is located outside the cells, in the extracellular fluid, or ECF for short.

In the extracellular fluid, sodium has a concentration of about 135 milliequivalents (mEq) per liter.

And remember that sodium concentration doesn’t necessarily reflect the total amount of sodium in the body, but rather the amount of sodium relative to the amount of water in the body.

So sodium homeostasis refers to the mechanisms employed by the body to maintain a normal sodium concentration in the extracellular fluid.

Sodium is essential in maintaining water balance, as well as for nerve impulse conduction and muscle contraction.

Additionally, sodium is an important determinant of the volume and osmolality of the extracellular fluid, which is made up of plasma and interstitial fluid.

Now, osmolality refers to the total solute concentration in a certain amount of solvent or water.

By affecting plasma osmolality, sodium determines plasma and blood volume.

So, at the end of the day, it’s important to maintain the sodium concentration in order to keep enough blood inside our arteries.

This blood is called the effective arterial blood volume or EABV, and it’s what ends up perfusing our various organs and tissues.

Okay, now, sodium comes from our diet.

The daily recommended sodium intake is about 2.3 grams per day which is the equivalent of a teaspoon of salt per day.

Once ingested, sodium is absorbed in the blood by the GI tract, and travels through the bloodstream unbound to plasma proteins.

At the other end, some sodium is eliminated from the body through sweat and through feces, but most of it comes out, along with water, as pee.

So the kidneys are the cornerstone of sodium homeostasis.

See, the kidneys are made up of lots and lots of nephrons, and each nephron is made up of a renal corpuscle and a renal tubule.

The renal corpuscle, in turn, is made up of the glomerulus, which is a tiny clump of capillaries, and Bowman’s capsule surrounding it.

So, blood gets to the glomerulus through the afferent arteriole, which is a branch of the renal artery, and leaves the glomerulus through the efferent arterioles.

Summary

Sodium homeostasis refers to the regulation of sodium levels in the body. Sodium is an important electrolyte mainly found in extracellular fluid, which helps maintain fluid balance, blood pressure, nerve impulse conduction, and muscle contraction. The body regulates sodium levels through hormones that control the reabsorption of sodium in the kidneys, as well as through thirst mechanisms. Factors that stimulate sodium reabsorption include the renin-angiotensin-aldosterone system, ADH, and the sympathetic nervous system. Factors that stimulate sodium excretion include PTH, and peptides like ANP. An imbalance in sodium levels can lead to health problems such as edema, hyponatremia, and hypernatremia.

Sources

  1. "Medical Physiology" Elsevier (2016)
  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2017)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "Sodium and Potassium in the Pathogenesis of Hypertension" New England Journal of Medicine (2007)
  6. "Potassium Homeostasis: The Knowns, the Unknowns, and the Health Benefits" Physiology (2017)
  7. "Sodium balance is not just a renal affair" Current Opinion in Nephrology and Hypertension (2014)