Your kidneys regulate your blood pressure through a hormone called Renin.

 

Did you know?  Your kidneys regulate your blood pressure through a hormone called Renin.

Our body has many hormones that adjusts different body systems for us to function every single day. Everyone is familiar with the sex hormones: oestrogen, progesterone and testosterone but have you heard about Renin?

 

Renin is a hormone with the primary function, to increase blood pressure by the control of two other hormones angiotensin and aldosterone. This process is called the Renin-Angiotensin-Aldosterone System. (RAAS)

 

The RAAS activates when there is a fall in sodium chloride (NaCl), extracellular fluid volume (ECF), or arterial blood pressure releasing renin from the kidney initiating a series of events to help correct one or all of these major health issues.

 

Low renin hypertension may be related with high aldosterone levels, mineralocorticoid excess and fluctuating glucocorticoid levels.

 

High renin hypertension may be related with a decreased sodium level or low blood volume which in turn may elevate blood pressure.

 

The kidneys are the filtration hub for homeostasis within the human body, cleaning the blood through specialised vascular and tubular network within nephrons resulting in excreting the excess solutes and waste products called urine out of the body.

 

The process of urine formation is a highly structured process allowing substances to be filtered, nutrients and electrolytes with water absorbed or excreted depending on the body’s requirements resulting liquid waste removed from the body through micturition.

 

The Nerdy Information

The Formation of Urine

The kidneys are the filtration hub for homeostasis within the human body, cleaning the blood through specialised vascular and tubular network within nephrons resulting in excreting the excess solutes and waste products called urine out of the body.

The functional unit of the kidney is called a nephron.  There are approximately one million nephrons in each kidney.  A nephron consists of vascular and tubular components as seen in figure 1.

Loop of Henle.

The first step towards urine formation begins with glomerular filtration.  Glomerular filtration as it states occurs in the glomerulus where protein-free plasma is filtered through the glomerulus capillaries into Bowmans capsule passing through the proximal tubule leading to the descending loop of Henle.
Let’s go back a step and discuss how the plasma arrives at the glomerulus. Blood enters the kidney through arteries that branches into smaller afferent arterioles. The afferent arterioles carries the blood into the glomerulus where filtration of electrolytes such as sodium, potassium, chlorine, nitrogen, glucose, amino acids, water, hormones and waste occurs due to passively forced filtration. (Sherwood, 2013) Cells and plasma proteins are too large to filter through the sieve like membranes leave via the efferent arteriole.  The outer membrane of the capillary contains podocytes, feet like projections that are able to flatten (relax) or narrow (contract) depending on the filtration rate requirement. Podocytes regulate permeability allowing blood and fluid adjustment being filtered into the tubules for either absorption or secretion.  The passively forced filtration is called glomerular filtration rate (GFR) due to capillary blood pressure that forces the blood through the glomerulus and into Bowman’s capsule.

 

There are three (3) physical forces involved that contribute to the net pressure resulting in filtration.

 

1. Glomerular capillary blood pressure Favours filtration 55 mmHg
2. Bowman’s capsule hydrostatic pressure Opposes filtration 15 mmHg
3. Plasma-colloid osmotic pressure Opposes filtration 30 mmHg
 
 Table 1.1

            Glomerular filtration rate. (Classes midland stech.edu, 2014)

 

The GFR net pressure remains at a constant due to the afferent arterioles ability to slow down the blood flow by vasoconstriction (reducing the diameter of the arteriole) or increase blood flood by vasodilation (stretching/ increasing the diameter of the arteriole).

Amazingly 180 litres of filtrate occurs through both kidneys each day which equates to approx. 125mL/min.  Out of the 180 litres, 178.5 litres is reabsorbed into the circulatory system with the remaining 1.5 litres excreted.

Tubular Reabsorption

Reabsorption varies, depending on the bodies needs such as water, sodium, glucose even urea is reabsorbed in different quantities however it is a very selective process though 99% of filtered plasma is reabsorbed. See Table 2.1

 

Substance Reabsorbed Excreted
Water 99% 1%
Sodium 99.5% .5%
Glucose 100% 0%
Urea (waste product) 50% 50%
Phenol (waste product) 0% 100%

Table 2.1

 

The process of reabsorption occurs within nephron via the tubular components.  Filtrate moves from the glomerulus through forced filtration into the Bowman’s capsule moving passively into proximal tubules.  From the proximal tubules glucose, amino acids, sodium and water are absorbed with the remaining filtrate flowing down the descending, then up the ascending loop of Henle developing a concentration gradient. Further absorption of sodium and water depending on concentration gradients may be absorbed across the loop of Henle. Filtrate continues flowing through distal tubules concluding at collecting ducts where urea may passively diffuse with remaining waste accumulating in the renal pelvis.

 

There are five (5) barriers that substances must cross to be reabsorbed.  This is known as transepithelial transport.

1. Leaving the tubular lumen, passing through the luminal cell membrane,
2. Entering the cytosol,
3. Passing from the cytosol across the basolateral membrane, (contains Na-K+pump)
4. Entering and navigating through the interstitial fluid,
5. Making way into the capillary wall of the plasma.

 

Filtrate leaves the Bowman’s capsule entering the proximal tubule where substances are reabsorbed. Sodium is an example. Sodium crosses the luminal membrane by a symport carrier and with it taking nutrients such as glucose. Once sodium is within the cell, the sodium/potassium pump with the use of ATP (active transport) allows passage across the basolateral membrane. The sodium is now in the interstitial fluid where transport occurs down the concentration gradient whereby entering the capillary wall.  It must be noted that sodium does not cross the descending loop of Henle (impermeable) due to countercurrent multiplication.  Sodium does play a critical role in the reabsorption of chloride, water and urea. Water is passively reabsorbed via osmosis however the ascending loop of Henle is impermeable to water. Urea passively leaks through the tight junctions of the proximal tubules. Glucose, amino acids and water soluble vitamins is absorbed actively and regulated by the endocrine system not kidney regulation.

 

Tubular Secretion

 

Secretion is the reverse action of reabsorption with substances moving from the peritubular capillaries into the tubular lumen involving the five (5) transepithelial transport resulting in urine.  This also is a second way for substances to be secreted.  Secretion of hydrogen ions, potassium ions and organic cations are three (3) major substances regulated by the kidney through proximal, distal and collecting tubules.

 

The secretion process involves the following system:

Peritubular capillaries secrete substances into interstitial fluid making way to the basolateral cell membrane and into the cell of the distal or collecting tubule.  Substances entering the collecting duct or lumen passing into the renal pelvis accumulating before released down the ureters into the bladder followed through urethra then emptied outside of the body.

 

An example of secretion is one of the major substances – Hydrogen ion (H+)

H+ are secreted through the proximal, distal and collecting ducts. H+ is critical in the regulation of the body’s acid base balance known as pH levels.  When the body’s fluid is too acidic secretion occurs however when the levels are too basic H+ will be conserved.

 

 

N2ClH2O

(Koeppin & Stanton., 2008)

Countercurrent multiplication.

This is an important factor with absorption and secretion.  In regards to secretion the concentration of substances will increase or decrease due to the body’s requirements.  When there is too much water and low solutes it’s called hypotonic contrary too little water and high solutes is called hypertonic.  The result will be either hyper or hypo urine concentration.  There is always a concentration gradient of 200 mOsm between the ascending loop and interstitial fluid.

 

Renin-angiotensin-aldosterone system. (RAAS)

 

The RAAS activates when there is a fall in sodium chloride (NaCl), extracellular fluid volume (ECF), or arterial blood pressure releasing renin from the kidney initiating a series of events to help correct one or all of these major health issues.  This is a five (5) step process.

 

  1. The kidney secretes renin in response to a fall in NaCl, ECF or arterial blood pressure.
  2. Renin activates angiotensinogen found in the circulatory system produced by the liver.
  3. Angiotensinogen is converted to angiotensin 1.
  4. Angiotensin 1 is converted to angiotensin 2 by the angiotensin-converting enzyme (ACE) produced in the lungs.  The circulatory system plays a key role in flow of RAAS.
  5. Angiotensin 2 passes through the adrenal cortex stimulating the hormone aldosterone which stimulates sodium reabsorption in the distal and collecting ducts.

 

This entire process that takes place without any effort on our conscience body to helps return sodium, extra-cellular fluid and/or arterial blood pressure back to a normal level.

Angiotensin 2 releases vasopressin a hormone that regulates osmosis – water retention or water excretion.

Vasopressin will cause the water to be reabsorbed within the distal tubules and collecting ducts.

Angiotensin 2 also will drive thirst causing more fluid intake.

Angiotensin 2 will cause vasoconstriction.

Janelle Tanzer

 

 

 

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