11 | Struggling to understand how the kidneys work?

PodcastLiver
Written By Laura Jones

We see renal disease SO commonly in practice.

From crispy CKD cats to acute renal emergencies, toxicities and leptospirosis, we spend a lot of our time nursing renal patients.

 

To provide the best possible care to these patients, we need to understand how our kidneys work, how common renal diseases impact the kidneys, and the signs our patients present with as a result.

And in the first episode of our renal disease podcast series, we’ll be demystifying the anatomy, structure and functions of the kidneys - so you can understand exactly what happens when they start to go wrong.

So let’s talk about the kidneys…

The kidneys are the first organs in the urinary system. The urinary system is made up of the kidneys, ureters, urinary bladder and urethra and its main job is to excrete metabolic waste and regulate fluid balance (and with this, acid-base and electrolyte balance).

This is achieved through the formation of urine in the kidneys, and the passage, storage, and elimination of that urine via the ureters, urinary bladder, and urethra respectively.

The kidneys are paired organs that sit within the retroperitoneal space. The left kidney sits level with the 13th rib, whilst the right kidney sits more cranial to this, underneath the ribs - making it challenging to palpate especially in dogs.

Each kidney is covered by a thick fibrous capsule and is divided into three main sections – the cortex, the medulla, and the pelvis.

The outermost layer is the cortex. Here the blood is filtered, and urine formation begins. The middle layer, the medulla, is where the urine gradually becomes more concentrated, and the pelvis is an open space where urine collects before it exits the kidney. Urine flows from the renal pelvis through the ureter and into the urinary bladder.

Urine is formed and concentrated by filtering blood through hundreds of thousands of nephrons, the functional units of the kidney. 

What is the nephron?

Each nephron is made up of 6 structures - a glomerulus, Bowman’s capsule, proximal tubule, loop of Henle, distal tubule and collecting duct.

The Glomerulus

The glomerulus is a modified capillary bed that filters blood. Blood enters the glomerulus under high pressure through an afferent arteriole and leaves it through an efferent arteriole.

The pressure through which the blood flows through the glomerulus causes water and solutes such as urea, amino acids, glucose, and electrolytes to pass through fenestrations in the capillary membranes. Because the fenestrations are small, larger molecules are normally retained in the bloodstream.

The water and eliminated substances which will go on to become urine - are known as ultrafiltrate. The ultrafiltrate is similar in composition to plasma and interstitial fluid, except it has a much lower protein concentration (in normal patients), since protein does not enter the urine.

Bowman’s Capsule and the Proximal Tubule

The ultrafiltrate enters Bowman’s capsule, which is a cup-shaped structure that surrounds the glomerulus, catching the ultrafiltrate and funnelling it down to the rest of the nephron.

The ultrafiltrate is then directed through the proximal tubule, and here, about 60-65% of the filtered substances are reabsorbed through surrounding capillaries, including amino acids, glucose, and electrolytes like sodium, potassium, calcium, magnesium, and phosphate, as well as water-soluble vitamins and other substances.

The loop of Henle

After the ultrafiltrate is modified in the proximal tubule, it enters the loop of Henle. The loop of Henle is formed of a descending and an ascending limb and begins in the renal cortex before dipping into the medulla, where it makes a sharp U-turn and re-enters the cortex. 

The Distal Tubule and Collecting Duct

After the ultrafiltrate has been concentrated, it leaves the loop of Henle and enters the distal tubule.

Here anti-diuretic hormone (ADH) exerts its effect on the ultrafiltrate. ADH is secreted by the pituitary gland, causing large volumes of water to be reabsorbed - further regulating urine volume and concentration.

From the distal tubule, the now-urine enters the collecting ducts, which carry urine from the renal cortex to the renal pelvis.

So what do our kidneys do, and how do we assess their function?

Filtering the blood and forming urine is only one of the kidney’s important functions.

Our kidneys actually do a LOT more than this, including regulating fluid, electrolyte, and acid-base balance, regulating red blood cell production, and regulating blood pressure, alongside regulating the balance of calcium and phosphate in the body.

This is achieved through the release of hormones like erythropoeitin and renin, alongside calcitriol.

And when it comes to measuring renal function…

We can assess renal function with many different tests - but the ones I want every nurse to confidently understand are:

  • BUN

  • Creatinine

  • SDMA

  • Phosphate

  • FGF-23

  • Electrolytes

  • USG

  • Urine output.

All of these give us an indication as to how well our kidneys are working - either because they are waste products that the kidneys are responsible for eliminating, or because they tell us about urine concentrating ability, and regulation of urine production.

So that’s it for today’s episode on the structure and function of the kidneys. The kidneys are vital organs and have a huge number of important functions - not just in the formation of urine, but also in the maintenance of fluid, acid-base and electrolyte balance, RBC production and blood pressure amongst others. By understanding how the kidneys work, and how to assess their function, we can really start to understand what our patients experience when they have renal disease - and therefore, how we can adapt our treatment, nursing care, and client advice to best meet their needs.

Did you enjoy this episode? If so, I’d love to hear what you thought - screenshot it and tag me on instagram (@vetinternalmedicinenursing) so I can give you a shout out, and share it with a colleague who’d find it helpful!

Thanks for learning with me this week, and I’ll see you next time!

References and Further Reading

Laura Jones
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