Chapter 18: The Urinary System

Summary of Waste Removal

Box 18-1 Pg 446 (table shown in textbook) Excretion: The urinary system's chief function is to regulate the volume and composition of body fluids and excrete unwanted material, but it is not the only system in the body that is able to excrete unnneded substances. The table below compares the excretory functions of several systems. Although all of these systems contribute to the body's effort to remove wastes, only the urinary system can finely adjust the water and electrolyte balance to the degree required for normal homeostasis of body fluids.

tubular filtrate

The glomerular filtrate after it has passed into the proximal convoluted tubule.

Urinary bladder......

When the urinary bladder is empty, it is round and rests on the pubic bones. Structurally it has a thick wall, is lined with many thick folds of transitional epithelium, and has virtually no lumen. In large animals the empty bladder is confined to the pelvic cavity, but in carnivores it extends into the abdominal cavity. As the bladder fills with urine, it becomes pear-shaped, it extends cranially into the abdominal cavity, the folds smooth out, and the wall becomes thinner.

secretion

a process by which substances are produced and discharged from a cell, gland, or organ for a particular function in the organism or for excretion

The kidneys help maintain homeostasis by playing an important role in regulating ______________ _______________.

blood pressure

reagent strip

chemical test strip used to analyze urine or blood on a long, thin plastic strip separated by individual square pads containing a chemically treated paper

The function of the urinary bladder is to:

collect, store, and release urine

In common domestic animals, except the pig, the right kidney is more ___cranial or caudal___ than the left?

cranial

Control of Urination: Urination (micturition or uresis) is the _______________________.

expulsion of urine from the urinary bladder into the urethra for elimination from the body (involves 2-3 steps)

glomerular filtrate

fluid that enters capsular space and is similar to plasma except it contains virtually no proteins

renal pelvis (of kidney)

funnel shaped sac at the superior end of the ureter; a urine collection chamber that forms the beginning of the ureter and is lined with transitional epithelium

The indented area on the medial side of the kidney is called the ____________. This is the area where blood and lymph vessels, nerves, and the ureters enter and leave the kidney.

hilus

distal convoluted tubule (DCT)

is a continuation of the ascending part of the loop of Henle; it follows a twisting path through the cortex (it is not as twisted as the PCT); empty into a series of tubules called collecting ducts

glomerulus

is a tuft of glomerular capillaries

renal corpulse

is located in the cortex of the kidney and is made up of the glomerulus and Bowman's capsule

urinalysis (UA)

is the laboratory analysis of a urine sample; it can be performed by a commercial lab or by vet personnel within a vet clinic; it involves 3 major steps and before beginning it a urine sample must be obtained from the patient, then physical and chemical analysis

Within the afferent glomerular arterioles are specialized ________________ _________ that constantly monitor blood pressure within the arterioles. In addition, there is a group of densely packed cells within the ascending limb of the loop of Henle, called the ___________ __________, that monitors the NaCl concentration of the tubular filtrate.

juxtaglomerular cells; macula densa (If the juxtaglomerular cells detect a decrease in blood pressure or if the macula densa cells come into contact with less NaCl the juxtaglomerular cells will respond by releasing renin.)

The __________ are located in the dorsal part of the abdomen, just ventral to and on either side of the first few lumbar vertebrae.

kidneys

The inner portion of the renal pelvis is the renal ___________.

medulla (sends rays up into the cortex and a pale, gray-red inner area that extends down the renal pelvis)

The kidney's most obvious role is the ______________________.

production of urine to facilitate the elimination of metabolic waste materials from the body; in the process of making urine, the kidney also helps maintain homeostasis in the body by manipulating the composition of blood plasma

In some species such as cattle and pigs, the medulla is made up of numerous, ___________________________ with the apex pointing to the renal pelvis (pigs) or directly to the ureter (cattle). This gives the medulla a scalloped appearance. Kidneys with this structure are called _________________.

pyramid-shaped areas (like candy corn); multipyramidal or multilobar

The outer portion of the kidney is called the _________ _________. It is reddish brown and has a rough granular appearance.

renal cortex

When blood pressure falls, a system called the ___________________________________ responds to bring it back up to a normal level.

renin-angiotensin-aldosterone system

The kidneys are located ______________ to the abdominal cavity; that is, they are outside the parietal peritoneum and so are considered officially outside the abdominal cavity.

retroperitoneal

The _____________ _____________ layer propels the urine through the ureter by peristaltic contractions, much like the contractions of the intestines. This enables urine to move to the urinary bladder regardless of the position of the animal's body.

smooth muscle

glomerular filtrate

the fluid and dissolved materials that filter out of the blood and enter the nephron through the glomerular capsule

capsular space

the space between the visceral and parietal layer; it is continuous with the proximal convoluted tubule

nephrology

the study of the kidney, and the artery and vein that supply and drain blood from the kidney are the renal artery and renal vein

In species such as, dogs, horses, and cats, the medullary pyramids fuse to occupy the entire inner area, and the cortex is pushed to the outside area only. These kidneys are called __________________.

unipyramidal or unilobar

The accumulation of urea in the blood is called ___________, whereas the buildup of creatine is called ___________.

uremia; azotemia

The ____________ continuously move urine from the kidneys to the urinary bladder.

ureters

The inner, transitional epithelium allows the ____________ to stretch as urine is passed through them on its way to the urinary bladder.

ureters

The urinary bladder stores ___________ as it is produced and releases it periodically from the body.

urine

filtration of blood

- occurs in renal corpuscle - high blood pressure in glomerular capillaries forces some plasma into capsular space of Bowman's capsule- fenestrations in capillary endothelium, glomerular filtrate formed (no protein) - glomerular filtration rate (GFR)- depends on rate of blood flow to kidney

renal dysfunction and uremia

- Renal dysfunction is the term used to describe any pathologic condition that results in the inability of the urinary system to remove waste materials adequately from the blood. When this happens, the waste materials, especially nitrogenous wastes from protein breakdown (e.g., BUN), build up in the blood and become toxic to the animal. The resulting condition is called uremia (literally "urine in the blood"). Uremia can be prerenal, renal, or postrenal. - Prerenal uremia is associated with decreased blood flow to the kidneys and may be caused by conditions such as dehydration, congestive heart failure, or shock, if these conditions are left untreated. In these cases, the kidneys are functioning normally, but not enough blood is reaching them, so waste materials can't be adequately removed. - Renal uremia is associated with an inability of the kidney to regulate urine production adequately because of damage to the nephrons. - Postrenal uremia is associated with an obstruction that prevents urine from being expelled from the body. Tumors, blood clots, or uroliths (stones) can cause the obstruction. - Uremia is diagnosed by evaluating a blood sample for the presence of increased amounts of waste materials.

uroliths and urolithiasis

- Uroliths are commonly seen in dogs, cattle, sheep, and goats, but are common in horses. - All uroliths have an organic matrix that stays fairly constant and makes up about 2% - 10% of its structure. The remaining 90% - 98% is made up of one or more of about 20 different minerals. Some of the more common mineral compounds involved are magnesium ammonium phosphate hexahydrate (struvite), calcium oxalate, calcium phosphate, urate, ammonium urate, and cystine. Uroliths form when there are adequate amounts of mineral present in the urine, the transit time of crystals through the urinary system is lengthened, and the pH of the urine favors precipitation. - Urolithiasis (the presence of urinary stones) can occur anywhere in the urinary system (the kidneys, ureters, urinary bladder, or urethra). Clinical signs associated with uroliths depend on where the stones are located. - Urethral obstruction is more common in males in general; in male ruminants, it occurs most often at the urethral sigmoid flexure.

urethra anatomy

- a continuation of the neck of the urinary bladder that runs through the pelvic canal - is lined with transitional epithelium that allows it to expand - the female urethra is shorter and straighter than the long, curved male urethra - in the female, it opens on the floor (ventral portion) or the vestibule of the vulva; in the male, it runs along the ventral aspect of the penis

nephrons

- is within the cortex of the kidney and there are hundreds and thousands of these microscopic filtering, reabsorbing, and secreting units - the basic functional unit of the kidney - it is the smallest part of the kidney that can carry out its basic functions - each one is composed of a renal corpuscle, a proximal convoluted tubule, a loop of Henle, and a distal convoluted tubule

The urinary bladder has 2 parts: _________________________. The bladder looks and acts a lot like a balloon.

- a muscular sac and a neck - The position and size of the bladder sac vary depending on the amount of urine it contains. - The bladder is lined with transitional epithelium that stretches as the bladder becomes filled with urine. - The neck of the bladder extends caudally from the sac into the pelvic canal, and joins the urethra. Around the neck of the urinary bladder are circular sphincter muscles composed of skeletal muscle fibers. The contraction and relaxation of these sphincter muscles (under voluntary control), open and close the passageway for urine to leave the bladder and enter the urethra.

ureter

- a tube that exits the kidney at the hilus and connects to the urinary bladder near the neck of the bladder at its caudal end - the 2 openings from the ureters into the bladder and the opening from the bladder into the urethra, if connected, form an upside-down triangle - referred to as the trigone of the bladder - are a continuation of the renal pelvis, and each ureter leaves its kidney at the hilus

fenestrations (pores)

- allow more fluid to leave the bloodstream - helps with the transfer of plasma out of the glomerular capillaries

The ureters are tubes composed of 3 layers:

- an outer, fibrous layer - a middle, muscular layer made up of smooth muscle - an inner, epithelial layer lined with transitional epithelium

diabetes insipidus

- antidiuretic hormone is not secreted adequately, or the kidney is resistant to its effect - unlike diabetes mellitus (which is sweet) it is tasteless cause there is no glucose present

gross anatomy of the kidneys

- are bean-shaped - covered by a fibrous connective tissue capsule - the right kidney in horses appears compressed end to end, so it becomes somewhat heart-shaped - are reddish brown - all animals except cattle have smooth surface on kidney - cattle's surface is divided into 12 lobes that give a lumpy appearance - hilus - ureters - renal pelvis

detrusor muscle of urinary bladder

- bundles of smooth muscle forming bladder wall - while they contract the bladder is squeezed and urine is expelled - like forcing the water out of the water balloon

renal failure

- can be acute (sudden), and these cases are often caused by toxicity (exposure to toxins such as antifreeze or some pharmaceutical drugs) or decreased renal perfusion due to low a blood pressure event or systemic disease. Acute renal failure is caused by necrosis of the renal tubules resulting either from exposure to the toxic substance or from lack of blood flow - More commonly, however, renal failure is chronic, characterized by a progressive and irreversible destruction of nephrons over months to years causing a gradual loss of renal function. Chronic renal failure is a common disease condition of both cats and dogs. - It is not until two thirds of the nephrons have been lost that clinical signs of renal failure start to appear! When nephron destruction reaches this critical point, the kidneys lose their ability to perform their basic duties: filtration, reabsorption, and secretion.

collecting ducts

- carry tubular filtrate through the medulla into the calyces, which lead to the renal pelvis - play an important role in urine volume, because they are the primary site of action of antidiuretic hormone (ADH) - potassium regulation and acid-base balance control take place here and are important

loop of Henle

- continues from the PCT, descends into the medulla of the kidney, makes a U-turn, and heads back up into the cortex........ ascends later losing brush border

urine volume regulation

- determined by amount of water contained in tubular filtrate when it reaches renal pelvis - two hormones, antidiuretic hormone (ADH) released from the posterior pituitary gland and aldosterone secreted by the adrenal cortex, are responsible for the majority of urine volume regulation

feline urolithiasis

- differ from uroliths in other species in that they are much smaller and resemble sand rather than large stones - can be very irritating - like having sandpaper in the urine; if the sand has a lot of the organic matrix, it clumps together to become a gelatinous plug with a gritty, toothpaste consistency and can cause an obstruction most often in male cats ("plugged tomcats") - struvite and calcium oxalate uroliths are the most common - same as in other species, urine pH, diet, and the presence of a urinary tract infection play important roles in urolith formation

efferent and afferent glomerular arterioles

- efferent has smaller diameter, causing blood (& pressure) to build up within the glomerular capillaries

The 3 main mechanisms of Renal Action (kidneys carry out waste elimination role) are:

- filtration of blood - reabsorption of useful substances back into the bloodstream - secretion of waste products from the blood into the tubules of the nephron

glomerular capillaries

- found between 2 arterioles - have a high blood pressure that is only 30% lower than the blood pressure in the aorta - this high BP is created by a difference in size between the afferent and efferent glomerular arterioles

reabsorption

- gets 9 substances back into the body by way of blood - sodium, potassium, calcium, magnesium, glucose, amino acids, chloride, bicarbonate, and water

Some of the important substances eliminated by secretion are:

- hydrogen - potassium - ammonia

function of urethra male/female

- in both males/females the urethra carries urine from the urinary bladder to the external environment - in females it has a strictly urinary function; it carries only urine - in males it also has a reproductive function

proximal convoluted tubule (PCT)

- is a continuation of the capsular space of Bowman's capsule - is the longest part of the tubular system of the nephron

Bowman's capsule

- is a double-walled capsule that surrounds the glomerulus - its inner layer is the visceral layer and it adheres closely to the surfaces of all glomerular capillaries - its outer layer is the parietal layer

Glomerular Filtration Rate (GFR)

- the term used to describe how fast plasma is filtered as it passes through the glomerulus - it depends on the rate of blood flow to the kidney - it is expressed in milliliters per minute - In a 25-lb dog, approximately 180 ml of plasma (that's about 300 ml of blood by the time you factor in the cells) flows to the kidney every minute. As blood passes through the kidney, about 45 ml of glomerular filtrate is formed every minute. In other words, about 25% of the plasma (45 ml/180 ml) is removed from the circulation each minute. Over a 24-hour period, that amounts to about 64 L of glomerular filtrate being formed. That's more than 16 gallons.

Nerve supply

- to the kidney it is primarily from the sympathetic portion of the autonomic nervous system

The 3 steps of urination are:

- urine accumulation - muscle contraction (are responsible for the sensation of having to urinate: a none house broke dog) - sphincter muscle control (animals that have been trained to control the reflex release of urine by voluntary control of the muscular sphincter around the neck of the bladder: a house broke dog)

podocytes

- what Bowman's capsule inner layer is made of - "foot cells" - have footlike extensions that cover the glomerular capillaries

The ureters enter the urinary bladder at such an oblique angle that, when the bladder is full, it collapses the opening or the ureter, preventing urine from backing up into the ureter.

...................

The 3 major steps of performing a complete urinalysis:

1. A gross examination of the physical properties of the sample. 2. A chemical analysis of substances dissolved in the urine. 3. A microscopic examination of the solid components in the urine.

Urine production can be broken down into 6 basic steps:

1. Blood enters the glomerulus via the afferent glomerular arteriole. 2. High blood pressure in the glomerular capillaries forces some plasma (minus large proteins and blood cells) out of the capillaries and into the capsular space of Bowman's capsule. At this point, the fluid is known as the glomerular filtrate. From there, it moves into the proximal convoluted tubule and is then called tubular filtrate. 3. The balance of the plasma not forced out of the glomerular capillaries leaves the glomerulus via the efferent glomerular capillaries and enters a peritubular capillary network around the rest of the nephron. 4. While the tubular filtrate travels through the tubules of the nephron, some of its constituents, or useful substances, are reabsorbed back into the peritubular capillaries (A). 5. Waste products are secreted from the peritubular capillaries into the tubular filtrate as it travels through the tubules (B). 6. By the time the tubular filtrate reaches the collecting ducts, it has decreased in volume, has changed chemical composition many times, and is now ready to leave the kidney on its way to being eliminated. When the tubular filtrate enters the renal pelvis, it is urine, and nothing more will be done to alter its composition. (One exception here is that mucus may be added to equine urine after it reaches the renal pelvis).

All of the abnormalities seen on physical examination and analyses of the patient's blood and urine can be attributed to a lack of normal kidney function. Here's why....

1. The decreased production of red blood cells, which gives rise to a diminished PCV, is due to the lack of erythropoietin, a hormone normally produced by the kidney. 2. As kidney function fails, the mechanisms that concentrate urine no longer function properly and excessive water is lost in urine, leading to dehydration. Dehydration is manifest as lethargy, decreased blood pressure, and decreased perfusion of tissues. It is also associated with an elevation in PCV and an isosthenuric specific gravity of urine. Both findings are strong indicators that Magic's kidneys are no longer able to concentrate urine. 3. Elevations of physiologic waste products, both BUN and creatinine, occur as the weakened kidneys fail to remove them. The buildup of these compounds in blood is responsible for the symptoms of nausea, vomiting, and anorexia seen commonly in cases of renal failure. 4. The presence of large molecules, such as protein, in the urine is a result of damage to the delicate filtration membranes in the glomerulus.

Up to ______% of the blood pumped by the heart goes to the kidneys.

25

About _______% of all tubular reabsorption takes place in the PCT, where about _______% of the water, sodium, chloride, and bicarbonate and _______% of the glucose and amino acids in the tubular filtrate are reabsorbed. Additional reabsorption also takes place in the loop of Henle, DCT, and collecting ducts.

65; 80; 100

calyx (calyces)

Cup-shaped collecting region in the renal pelvis (central section of the kidney).

The canine urinary tract.

Fig 18-10 Pg 461 (different picture in textbook) A. The urethra of the male dog is curved and longer because it travels the length of the penis. B. The urethra of the bitch is short and straight.

Renal morphology varies with species.

Fig 18-4 Pg 450 (more pictures in textbook) A. Pig B. Cattle kidneys are multipyramidal (multilobar) C. cat and dog kidneys are unipyramidal (unilobar)

Functional anatomy of the nephron.

Fig 18-5 Pg 451 (better picture in textbook) Arrows indicate the direction of fluid flow through the nephron.

Arterial blood supply to renal corpuscle.

Fig 18-6 The arrows show the direction of blood flow.

Reabsorption and Secretion

Fig 18-7 Pg 454 (look at picture in textbook) Reabsorption involves movement of substances out of the tubular lumen, through the tubular epithelium and interstitial fluid, and into the peritubular capillary. Secretion involves movement of substances in the opposite direction—from the peritubular capillary, through the interstitial space, tubular epithelium, and tubular lumen.

trigone of the bladder

Fig 18-8 Pg 459 (shows one simple picture in textbook) triangular region at the base of the urinary bladder between the openings of the two ureters and the urethra

Entrance of the ureters into the urinary bladder.

Fig 18-9 A. When the bladder is empty and while it is filling, its entrances remain open and urine flows freely into bladder. B. When bladder is full, the pressure of the urine present collapses the entrances so that when urine is expelled it does not flow back into the ureters.

refractometer

Instrument that measures specific gravity of urine

renin

Is an enzyme that facilitates the splitting of angiotensin I from angiotensin. ngiotensin I is then converted to angiotensin II by angiotensin-converting enzyme (ACE). Angiotensin II is the active molecule that causes arterial constriction and stimulates the release of aldosterone from the adrenal glands. By increasing sodium and water reabsorption back to the bloodstream, aldosterone will cause an increase in blood volume. As blood volume increases, it will better fill the vascular space and increase blood pressure.

tubular secretion

Moves substances from the blood in the peritubular capillary into the renal tubule.

sodium cotransport

The process by which glucose and amino acids are passively reabsorbed back into circulation in the proximal convoluted tubule. These two substances bind to the same transport protein to which sodium is attached. When the sodium is actively transported into the tubular epithelial cells, the glucose and amino acids follow.

Maintaining homeostasis in the body is the most important overall function of the kidneys. The main processes by which the kidneys help maintain homeostasis include:

• Blood filtration, reabsorption, and secretion. The blood is filtered, useful substances are returned to the circulation, and waste products are secreted from the bloodstream into the fluid that eventually becomes urine. • Fluid balance regulation. The amount of urine produced depends on the amount of water it contains, which helps ensure that the body contains the right amount of water to maintain a healthy internal environment. If the body has excess water and needs to get rid of it, more urine is formed (diuresis). If the body needs to conserve water, less urine will be produced, and the animal will pass little urine (oliguria) or no urine at all (anuria). The amount of water contained in the urine is under the control of the hormones antidiuretic hormone (ADH) and aldosterone. • Acid-base balance regulation. The kidneys help maintain acid-base homeostasis by their ability to remove acidic hydrogen and basic bicarbonate ions from the blood and excrete them in urine. By eliminating these ions in the appropriate amounts, blood pH can be maintained in the proper range. • Hormone production. The kidneys have close associations with the endocrine system—the hormones that help regulate body functions. The kidneys produce hormones, regulate the release of hormones from other organs, and are themselves influenced by hormones. For example, the kidney can influence the rate of release of ADH from the posterior pituitary gland and aldosterone, the mineralocorticoid secreted by the cortex of the adrenal gland. The kidneys are also influenced by both of these hormones. Specialized cells in the kidneys produce erythropoietin, the hormone necessary for red blood cell production, and some prostaglandins. • Blood pressure regulation. The kidneys contain internal receptors that monitor blood pressure. When blood pressure falls, the kidneys secrete a hormone called renin. The release of renin will start a cascade of reactions that will result in vasoconstriction and the retention of sodium and water. By increasing the fluid volume of the blood, blood pressure will also be increased.

Some examples of metabolic waste are:

• Carbon dioxide and water from carbohydrate and fat metabolism • Nitrogenous wastes, primarily urea, from protein metabolism • Bile salts and pigments from red blood cell breakdown • Various salts from tissue breakdown and excessive consumption

Based on her physical examination findings, the veterinary technician makes the following technician evaluations:

• Hypovolemia (tacky mucous membranes) • Decreased perfusion (delayed CRT) • Inappropriate elimination (little urine production) • Vomiting and nausea • Exercise intolerance (lethargy) • Abnormal eating behavior (anorexia) • Underweight (2/5 BCS) • Self-care deficit (matted fur) • Altered mentation (reclusive) • Altered oral health (mucous membrane ulceration)

The main blood vessels responsible for the journey of blood supply through the kidneys each have unique functions:

• The renal artery branches off the abdominal portion of the aorta. It enters the kidney at the hilus. It then divides and subdivides into smaller arteries and arterioles until it becomes a series of afferent glomerular arterioles. • The afferent glomerular arterioles carry blood into the glomerular capillaries of the renal corpuscle. • The glomerular capillaries are a continuation of the afferent arterioles. They filter some of the plasma out of blood, which enters the capsular space of Bowman's capsule, where is it known as the glomerular filtrate. Not all the plasma is filtered out; if it were, blood flow would come to a screeching halt. The blood in the glomerular capillaries leaves the glomerulus and enters the efferent glomerular arterioles. Notice that the blood is still arterial blood at this point, because oxygen exchange hasn't taken place yet. This is the only place in the body where blood entering and leaving the capillaries is oxygenated blood. • The efferent glomerular arterioles divide into a network of capillaries that surround the rest of the nephron. These capillaries are known as the peritubular capillaries (see Figure 18-5). Oxygen transfer to the cells of the nephron takes place here. Also at this level, substances are taken 453out of the tubular filtrate and put back into blood. This is known as tubular reabsorption. Other substances are secreted from the blood into the tubules at this level. This is called tubular secretion. More on these actions later. • The peritubular capillaries that surround the nephron converge to form venules that in turn converge to form larger veins that eventually become the renal vein. • The renal vein leaves the kidney at the hilus and joins the abdominal portion of the caudal vena cava. With regard to waste content, the blood in the renal veins is the purest in the body.

The body has several routes by which waste products are eliminated:

• The respiratory system removes carbon dioxide and water vapor. • The sweat glands eliminate water, salts, and a small amount of urea. • The digestive system removes bile salts and bile pigments. • The urinary system removes urea, salts, water, and other soluble waste products.

The urinary system is made up of:

• Two kidneys that make urine and carry out other vital functions • Two ureters that carry urine to the urinary bladder • One urinary bladder that collects, stores, and releases urine • One urethra that conducts urine from the body