The vet nurse’s guide to renal and urinary tract disease
Renal and lower urinary tract disorders are some of the most common conditions we see, particularly in our feline patients. From triage and emergency stabilisation, to sampling, imaging and diagnostics, inpatient care and beyond, the role of the veterinary nurse is vast when it comes to managing these patients.
In order to plan the best possible nursing care for our renal and urinary patients, we first need to:
Understand the common renal and urinary diseases we see
Understand the effects they have on our patients and why we see the signs we do
Understand the diagnostic tests we perform, and what they tell us
To do this, we first need to understand what organs make up the urinary tract, and how each of them works.
In today’s post, I’m going to talk you through the 4 main structures of the urinary tract, and discuss their structure and function - so that you can apply this knowledge to the patients in your hospital.
The urinary tract can be divided into upper and lower portions. The upper urinary tract is made up of the kidneys and ureters, and the lower urinary tract consists of the urinary bladder and urethra.
The Upper Urinary Tract
The Kidneys
The kidneys are the first organs in the urinary tract. They sit within the retroperitoneal space in the dorsocaudal abdomen, at around the level of the 12-13th rib, and the right kidney sits a little more cranial than the left.
Each kidney is covered with a fibrous capsule made up mostly from collagen fibres alongside smooth muscle and blood vessels. The kidneys themselves are made up of three main components: the cortex, the medulla and the pelvis.
The cortex is the outermost layer, and here the blood filtration process begins
The medulla is located beneath the cortex, and here the urine is concentrated
The pelvis is located centrally within the kidney, and here urine collects before excretion through the ureters and into the urinary bladder
The Nephron
The kidneys are made up of many nephrons. Each nephron consists of the:
Glomerulus
Bowman’s capsule
Proximal convoluted tubule
Loop of Henle
Distal convoluted tubule
Collecting duct
The nephrons are responsible for filtering the blood, re-absorbing key solutes and electrolytes, and forming urine.
The Glomerulus
The glomerulus is a capillary bed which filters blood. Blood enters the glomerulus under high pressure via an afferent arteriole; here, water and solutes such as electrolytes, glucose and amino acids are forced through the glomerulus, where they are collected by Bowman’s capsule. Larger molecules such as proteins cannot move through the glomerular membrane, so remain in the bloodstream. The water and solutes collected by Bowman’s capsule is not yet urine as they have not been filtered, so the fluid is known as ‘ultrafiltrate’ at this point.
The Renal Tubules and Loop of Henle
The ultrafiltrate then enters the proximal tubule. The renal tubules and loop of Henle are responsible for maintaining fluid, acid/base and electrolyte balance, as well as excreting waste products. Amino acids, glucose and electrolytes are reabsorbed into the bloodstream in the proximal tubule.
The ultrafiltrate then enters the loop of Henle, where it is concentrated. From here the ultrafiltrate enters the distal tubule, where sodium and water are reabsorbed.
The now-urine then enters the collecting duct, where it flows to the renal pelvis for excretion.
Renal Functions
In addition to urine production and concentration, the kidneys have many other key roles within the body, including red blood cell production, regulating blood pressure and maintaining calcium and phosphorus balance.
Red Blood Cell Production
The kidneys secrete erythropoietin from renal interstitial cells, in response to tissue hypoxia. Erythropoietin works on the bone marrow to increase the production and survival of red blood cells, improving tissue oxygenation and perfusion.
Blood Pressure Regulation
The kidneys secrete renin from juxtaglomerular cells in response to hypotension or reduced renal sodium delivery.
Renin plays a vital role in the RAAS - the Renin, Angiotensin, Aldosterone System. This is the method by which aldosterone is released from the adrenal gland. The system works like this:
Renin is released from the kidney
Renin converts angiotensinogen to angiotensin I in the liver
Angiotensin converting enzyme (ACE) then converts angiotensin I to angiotensin II
Angiotensin II acts on the adrenal cortex, stimulating it to produce aldosterone
Angiotensin II also acts on the pituitary gland, stimulating it to produce antidiuretic hormone (ADH).
Maintaining Calcium and Phosphorus Balance
The Kidneys are also responsible for maintaining calcium and phosphorus balance, through the formation of calcitriol in the renal tubules. Calcitriol is bioactive Vitamin D3, and after vitamin D is ingested in the patient’s diet, the kidneys convert it to calcitriol.
Vitamin D plays a key role in maintaining a normal calcium and phosphorus balance. It does this by:
Stimulating absorption of calcium, phosphorus and magnesium in the GI tract
Controlling renal calcium loss
It also plays a vital role in developing and maintaining the skeleton.
The Ureters
The ureters are paired fibromuscular structures which transport urine from the kidney to the urinary bladder. The urine flows from the renal pelvis through the ureter via peristalsis, and collects in the urinary bladder prior to elimination. The ureters insert into the bladder in the trigone region.
The length and diameter of the ureters varies widely depending on breed - it has been reported to be as narrow as 0.4mm in diameter in cats, and up to 2.5mm in medium-large breed dogs.
This is particularly important to note in cats, who are at increased risk of ureteral obstructions with uroliths, resulting in post-renal azotaemia and acute kidney injury.
The ureters are formed of three layers:
A fibroelastic outer layer known as the adventitia
A muscular layer known as the tunica muscularis, subdivided into inner and outer longitudinal layers and a middle circular layer
An inner mucosal layer, subdivided into lamina propria and a transitional epithelial layer. The lamina propria contains blood vessels, nerves and collagen fibres, and the transitional epithelial layer allows the ureters to dilate and stretch to some degree.
The Lower Urinary Tract
The Urinary Bladder
The urinary bladder acts as a reservoir, holding urine until it is expelled. It can be divided into three regions:
The apex (the cranial, blind-ended end of the bladder)
The neck (the funnel-shaped region between the ureteral openings and the urethra)
The body (the portion of bladder between the apex and neck)
The bladder wall is made up of several layers, which allows it to stretch and expand, protect itself, and void urine. These are:
The outer, muscular layer (the detrusor muscle) - which allows voiding
The middle, submucosal layer - which is formed of connective tissue
The inner, mucosal layer - which is lined with urothelium. These are the transitional epithelial cells which stretch as the bladder fills with urine
The GAG (glycosaminoglycan) layer - this is the innermost layer and covers the bladder wall. The GAG layer is a protective layer, and functions to prevent bacteria from adhering to the bladder wall. It also prevents the urine from damaging the urothelial layer, causing inflammation.
Beneath the GAG layer is the bladder lumen, which fills with urine. The bladder is supplied by sensory neurons, which sense when the bladder fills, as well as sensing pain.
The Urethra
The urethra arises from the bladder neck, and functions as the route for urine to leave the body.
The proximal (or internal) urethral sphincter covers the bladder neck; this is formed from smooth muscle, and creates urinary tone and resistance.
The external urethral sphincter encircles the caudal urethra and is made up of skeletal muscle. This muscle is responsible for voluntary urination and continence.
In males, the urethra can be divided into preprostatic, periprostatic and postprostatic portions. The urethra ultimately enters the penis and leaves the body. The prepuce envelops and protects the penis.
In females, the urethra continues caudally until it enters the vaginal vestibule and vulva.
So that’s a recap of the structure and function of the canine and feline urinary tract. As you can see, it’s really important to bear this in mind as it helps explain many of the changes we see in patients with renal and urinary tract disease - allowing us to plan and deliver the best possible treatment and care.
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References & Further Reading
Matthews, K. 2016. Ureters. Available from: https://veteriankey.com/ureters/
Merrill, L. 2012. Small Animal Internal Medicine for Veterinary Technicians and Nurses. Iowa: Wiley-Blackwell.
Minnesota University Veterinary Anatomy, undated. Lower Urinary Tract. Available from: http://vanat.cvm.umn.edu/lut/Anatomy.html