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Urine formation

Learning how kidneys form urine.


That need for the toilet when you are in the middle of no where and you feel your whole pelvic area is about to explode. In this blog post will explore how does urine get created.


The kidney


The best place to start is at the kidneys. These bean like structures are very busy organs that play a number of very important jobs in the body. They can even hold 22% of your blood at a time. From excretion of unwanted waste, regulation of red blood cells production, regulation of blood pressure to maintaining our optimum pH levels, they are BUSY. They are fascinating organs and they are located just around the 11th and 12th rib. Fun fact is that the right kidney is actually located a bit lower than the left one, due to the liver taking up more space on the right side.


Let's have a look at the anatomy of the kidney from a top view. The kidney is divided in to 2 distinct regions: Renal Cortex (the superficial, light red area) and Renal Medulla (containing these renal pyramids) as shown in the picture below.



The nephron


In order for us to understand how urine gets produced by the kidneys, we have to pay close attention to the star of the show: the nephron. This is the functional unit of the kidney and there are about 1 million of them per kidney. They stretch between both the renal cortex and renal medulla. The anatomy of the nephron can be divided into two parts:


1. The Renal Capsule

  1. Glomerulus

  2. Bowman's Capsule

2. Renal Tubules

  1. Proximal Convoluted Tubule

  2. Loop of Henle

  3. Distal Convoluted tubule

Depending if the nephron (more exactly the Loop of Henle) drops into the renal medulla or not, it can be caller either a juxstamedullary or cortical nephron.



Urine formation


To produce urine, the nephron and collecting ducts go through 3 main processes that touch upon different parts of the nephron each with their individual responsibility.



Glomerular filtration

Oxygenated blood comes to the kidney through the renal artery branching into the renal cortex in arterioles to meet the renal capsule of the nephron, more specifically the glomerulus. The arteriole that enters the glomerulus it is called the afferent arteriole. Once the blood gets filtered through the glomerulus continues its journey through an efferent glomerulus having left behind water, metabolic waste, ions, toxins but also amino acids, glucose, mineral salts and many more substances that are still important for our body. The efferent arterioles will continue through the nephron around the tubules to collect back some of the substances as we will see shortly. This filtered fluid leakes into what encapsulates the glomerulus, the Bowman's Capsule.


The filtration of the blood relies on 1) the leakage and permeability of the layers between the capillary in the glomerular and the capsule of Bowman's, but 2) also the pressures acting on this two sides. The filtration barrier is good to think of it as a kitchen strainer. First of all, the blood vessels are lined with a layer of endothelial cells that are fenestrated, meaning they have passages in them that allow even bigger molecules like proteins to pass through. After that we have the basement membrane which is highly permeable to smaller molecules like ions but not to bigger proteins. A third layer of tubular cells or podocytes wrap around capillaries of the glomerulus. A very good visual representation of this can be found here. After passing through these three layers the remaining fluid is now called the glomerulus filtrate which will be made out of:

  • Water

  • Mineral salts

  • Amino acids and glucose

  • Ketoacids

  • Hormones

  • Creatinine

  • Urea

  • Uric acid

  • Toxins

Things like leukocytes, erythrocytes, platelets and plasma proteins do not pass through filtration and carry on in the blood.


This filtration of blood is also highly dependent the forces that favour filtration and those that oppose it:

  1. Blood pressure within the glomerular capillaries pushing blood through the membrane for filtration (favour filtration)

  2. Blood colloid osmotic pressure - also called oncotic pressure is the pressure that plasma proteins (albumin, globulin, fibrinogen) create on the walls of the container that live in - these are the forces that pull water in the blood from tissue - in this case trying to pull fluid back into the blood (oppose filtration)

  3. Capsular hydrostatic pressure - this is the pressure a fluid exerts on the walls of its container similar to how a water will push against a ballon wall when we fill it. In this case we refer to the pressure created by the glomerulus filtrate against the Bowman's capsules membrane. (oppose filtration)


The NET filtration pressure (NFS) = hydrostatic pressure of the glomerulus (HPgc = 55mmgb) - hydrostatic pressure of the Bownan's capusule (HPcs = 15mmHG) - Oncotic pressure of the glomerulus (30mmg) = 10mmHg


Another important factor in this filtration process is the diameter of the vessels. In the afferent vessels the diameter is big to allow big amounts of molecules in the blood to pass through, whereas the efferent arterial has a smaller diameters to slow down the blood and allow filtration.


Taking in consideration the permeability of the glomeruli, the Net filtration process and the surface are of filtration we can determine the GFR (Glomerular Filtration Rate). This is the amount of filtrate formed in the renal corpuscles. In adults, the average GFR averages 125ml/min of filtrate in males and 105ml/min in females. This is the amount of filtrate formed by both kidneys each minute. A normal GFR should be over 90ml/min. This is a very important measure to determine the health of an individual's kidneys and its results are used to measure the severity of kidney disease.


Tubular reabsorption

Out of the glomerulus fluid only 1% end up to be excreted, meaning we reabsorb 99% of it. This reabsorption happens all throughout the rest of the nephron before descending into the ureters.

  • Proximal Convoluted Tubule

This tubule which can be found straight after the renal capsule, is lined with simple cuboidal epithelial tissue allowing 65% of Na+ to pass through and be reabsorbed into the interstitial fluid. Naturally positive ions attract negative ions so CL- quickly follows Na. In addition 60% of water leaves at this point via osmosis, 65% of K and 90% of HCO3- (bicarbonate ions). Together will all of the above, 100% of glucose and amino acids leave the tubules. Also, not all urea leaves gets excreted into the urine so approx. 50% gets reabsorbed.

  • The Loop of Henle.

The next step for the remaining fluid is the Loop of Henle. This has a descending limb that travels down and an ascending limb that travels up which dips into the medulla of the kidney. The loop is lined with squamous epithelial cells and descends in the Renal Medulla. The environment outside of the Loop of Henle is very salty as 25% of NaCl keeps being pumped out of the glomerular fluid. The water part of the fluid has a very interesting journey, as in the descending part of the loop water is able to leave via osmosis but can't on the ascending loop, hence the fluid becomes very dense by the time it reaches the ascending part.


The fluid continues through to the Distal Convoluted tubule where more NaCL can leave the fluid, K+ can be reabsorbed. The amount of how much of these substances leave the tubules or remain in the fluid is dictated by the influence of 5 hormones which we will look at in close detail later in another article.


Tubular Secretion

After the body has regulated amount of sodium, chloride, calcium, water and potassium that wants to keep in the body, the remaining fluid still filled with other toxins and metabolic wastes makes it way to the Collecting ducts. Here for example, if you are dehydrated the Antidiuretic hormone (ADH) will increase even more the reabsorption of water. Anything else that does't require to be reabsorbed will continue through the Ureters into the bladder. The reabsorption and secretion of these substances and keeping the optimal amount play a key role in maintaining numerous important functions in the body.

In the end, urine will be formed in varying percentages the following substances:

  1. Water (96%)

  2. Urea (2%), Uric acid and Creatinine

  3. Ammonia

  4. Na, K, CL, P, S

  5. Hormones

  6. Oxalates

Once in the urinary bladder, when the urine exceeded 200-400ml, stretch receptors on the bladder wall transmit nerve impulses to the spinal cord at levels S2 and S3 and we will feel the need to go to the bathroom.


Sources:


1. College of Naturopathic Medicine lecture on the Nervous system https://www.naturopathy-uk.com/

2. Khan Academy, The Kidneys and Nephron - Available at: https://www.khanacademy.org/science/biology/human-biology/kidney-nephron/v/the-kidney-and-nephron [Accessed 6th of June 2020)



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