Osmoregulation

Osmoregulation refers to the regulation of body fluid solute concentrations.

Solute concentrations are measured in osmolarity, usually mOsm/L, which is the number of osmoles within a litre of solution.

Now remember that an osmole refers to the individual ions within a solution. For example, a solution of 1 mmol/L of a salt like NaCl which can split apart in water to become Na+ and Cl- will have both Na+ and Cl- contribute to the osmolarity. So 1 mmol/L of NaCl is 2 mOsm/L.

In a normal body, blood plasma osmolarity is very tightly regulated and kept at around 290 to 300 mOsm/L.

The main components of this osmolarity is made up of ions like sodium, glucose, and urea.

To get the actual osmolarity of the body, a calculation like this one can be used: = 2[Na+] + [Glucose]/18 + [ BUN ]/2.8, where [Glucose] and [BUN] are measured in mg/dL.

Both glucose and BUN can be converted from mg/dl to mOsm/L by dividing them by 18 and 2.8 respectively.

Let’s say that it's a super sunny day out and you forget to bring water with you. Well first, as you walk around, you’re constantly losing water through sweat as well as water vapor from your mouth and nose as you breathe out - these are insensible water losses. Without drinking water, you can quickly get dehydrated.

This causes your plasma osmolarity to increase, because the fluid levels in your blood drop, but the total number of solute particles in remains roughly the same.

Two things now begin to happen simultaneously. First, a region in the brain called the anterior hypothalamus has a cluster of neurons called supraoptic nuclei, which have osmoreceptors that sense even tiny changes in osmolarity, as small as 1 mOsm/L. These neurons are always sampling the blood that passes by.

With increases in plasma osmolarity, water will flow out of the cell causing it to contract.

Increases in osmolarity past the normal set point of 290 to 300 mOsm/L stimulates the hypothalamus to produce antidiuretic hormone or ADH - also called vasopressin.

Second, as the blood volume drops, so does the blood pressure.

Specialized neurons, called baroreceptors, act as pressure sensors in the walls of the cardiovascular system - in a few specific areas. They’re in the posterior wall of the right atria of the heart, in the aortic arch, and in the right and left carotid sinuses, which is where the common carotid splits to become the internal and external carotid.

These baroreceptors detect tiny changes in how stretched out those walls are, so if blood volume decreases and there’s less stretch on those walls, the baroreceptors change their rate of firing a signal up to the hypothalamus.

The hypothalamus senses the change and starts making more antidiuretic hormone.

So ultimately both an increase in osmolarity and a decrease in pressure leads to more ADH.

Antidiuretic hormone is peptide hormone - a small protein that cells use to communicate with one another.

ADH is made within the cell bodies of supraoptic neurons, and then gets packaged into vesicles and gets secreted down long axons which extend into a synaptic cleft in the posterior pituitary.

The ADH gets released into the interstitial fluid, moves through nearby endothelial cells, and gets into the capillary bed of the posterior pituitary gland.

From there, the ADH travels throughout the body and binds to receptors in its two main target tissues, the collecting ducts of the kidney, and the smooth muscle that wraps around arterial walls.

Now, starting with the kidney, the functional unit of the kidney is the nephron, and the collecting ducts connect those nephrons to the minor calyx of the kidney.

This collecting duct part is normally impermeable to water and urea.