Water is the main compound of any living creature on earth, in fact, the total body weight of an adult human is composed of about 60% of water. Two third of the amount is intracellular and only one third is extracellular either in the interstitium or in the blood.In order to fully understand water regulation, we need first to discover water movement in the body.
Each cell membrane has a double layer of phospholipids where we can find different ion channels and proteins. It’s generally water-permeable but the flow is low, that’s why each cell develops its own specific water channels called Aquaporins. As their name states, they actually form some kind of pores across the membrane where water passes through easily.
But why we have water channels in the first place? At the beginning that sounds like a dumb question but actually, it’s a pretty good one.Many metabolic reactions that happen in the body require water and most of them are inside the cells, thirst isn’t a lack of water in the blood but rather inside the cells so we need a quicker way to hydrate our cells and that’s through water channels.Besides water channels aren’t always present, their expression can be regulated depending on the cell’s state.
Moving one, if we take two adjacent media with a fully permeable separative membrane (to water and ions) containing water and sodium with different concentrations and wait for a while, what’s going to happen?
Sodium moves from the most concentrated medium(hypertonic) to the least concentrated medium (hypertonic) until both concentrations become equal. Now we replace the fully permeable membrane with a semi-permeable membrane which only allows water movement, what’s going to happen?
This time because they are not allowed to move across the membrane, sodium ions create a difference in water pressure between the two media. Water moves from the least concentrated medium where water pressure is higher in the most concentrated medium where water pressure is lower until the equilibrium, that’s osmosis.
The cell’s membrane is a semi-permeable membrane,it doesn’t let sodium pass passively and the movement of water across its layers is due mainly to two factors: hydrostatic pressure and the concentration of ions with sodium as the most abundant extracellular ion, therefore it becomes osmotically active which means it’s able to create an osmotic state.So controlling sodium levels in the blood is the same as controlling hydration level, that’s why people with high blood pressure decrease salt intake in their diet.
Atrial Natriuretic Factor(ANF):
It’s also called Atrial natriuretic peptide(remember that a peptide is a medium-length chain of amino acids), ANP is a hormone produced by cardiac muscle cells found in the walls of the atria in the heart. In 1956 Henry and Pearse demonstrated that atrial distention increased urinary flow, many researchers were done afterward until the full understanding and discovery of ANF functions.
When the body is exposed to volume overload, mechanoreceptors receptors found in the wall of the atria become stretched. These receptors are called baroreceptors, they induce the production of ANF by cardiac muscle cells. ANF essentially targets the kidney, it has diuretic effects meaning that it increases urine flow thus taking care of the volume excess.ANF has a short longevity in the bloodstream but its effects are very powerful making it a short-term regulation system.
The picture you are looking at is a glomerulus, it’s the functional unit in the kidney where the filtration process takes place.The blood arrives at the kidney via the afferent arteriole to different capillaries inside the glomerulus and return to the bloodstream via the efferent arteriole,what happens is that the blood pressure in the capillaries forces the blood to leak out through pores to the Bowman’s capsule as shown retaining red blood cells and large proteins because of their size.Then the filtrate run through different tubuli where ions and other molecules can be absorbed or secreted depending on the body needs.
In fact, ANF decreases sodium reabsorption in the distal tubule, therefore, increasing urine flow.It also dilates the afferent glomerular arteriole and constricts the efferent glomerular arteriole which increases the pressure in the capillaries leading to an increased filtration rate.Another critical action of ANF is inhibiting the renin–angiotensin–aldosterone system (Read the rest of the article).
Aldosterone is a hormone produced by cells located in the cortex of the adrenal gland also called the suprarenal gland because of its location.It stimulates sodium reabsorption in the distal tubule of the nephron when plasma osmolarity drops and potassium secretion(=elimination in the urine).
Renin on the other side is an enzyme produced by specific juxtaglomerular cells in the kidney in three different situations:
- Low blood flow in the renal artery caused by systemic hypotension, cardiac failure or renal artery stenosis.
- Sympathetic stimulation is some physiological stress conditions such as excessive bleeding.
- Decreased sodium delivery to the distal tubules.
Renin hydrolyzes Angiotensinogen ( produced in the liver ) to Angiotensin, two prohormones with little or no active action in vivo. Angiotensin I is converted to Angiotensin II by angiotensin-converting enzyme(ACE) found in the lungs.
Angiotensin II is the most powerful biological vasopressor known, it stimulates thirst centers within the brain and binds to AT2 receptors and causes a quick rise in the blood pressure, but more importantly, it stimulates the production of Aldosterone.Like Aldosterone, Angiotensin appears also to act directly on the proximal tubule to enhance sodium resorption. This closed system is activated to maintain a proper osmolarity and blood in some conditions such as bleeding, severe dehydration with a salt loss from excessive sweating, vomiting etc..
Also called the Antidiuretic Hormone (ADH), Vasopressin is a hormone produced in the hypothalamus and travels down the axons of the cells to terminates in the posterior pituitary for later uses. It’s produced mainly in response to hypovolemia and it’s partially affected by the circadian rhythm. Besides Angiotensin II stimulates ADH secretion and Atrial natriuretic peptide inhibit it.
ADH increases water permeability in the kidney by up-regulating the transcription of water channels in the collecting duct of the nephrons allowing water absorb. That’s not all, ADH also increases the reabsorption of sodium allowing more and more water absorb.
The sympathetic system is also crucial for regulating blood pressure and water absorption, it can be activated in normal circumstances or in response to many pathological conditions.
All these mechanisms are not separated but rather work in harmony to maintain proper levels of water and ions in the blood.
I hope this post was helpful and I hope you like it ^^ and if you have any question leave a comment in the comment section below and have a nice day!
References & Sources:
Atrial natriuretic factor by George Athanassopoulos and Dennis V.Cokkinos
The rest is from my studies.