The Urinary System

READING ASSIGNMENT: Chapter 16, Clinical Anatomy and Physiology for Veterinary Technicians
Chapter 7 An Illustrated Guide to Veterinary Medical Terminology (might be helpful)

The urinary system is sometimes thought of as the "plumbing" of the body, but it is so much more than that!

Certainly the major function of the urinary system is excretion of urine, but it is essential in acid / base regulation, water balance, and electrolyte balance in the body.


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Gross anatomy of urinary system

a) The kidneys are paired organs that filter blood and perform the biochemical processes of the urinary tract.

b) The ureters are small tubes connecting each kidney to the bladder.

c) The urinary bladder stores urine and regulates urinary output.

d) The urethra is a  hollow tube connecting the bladder with the external environment.

A) Is the urethra different between sexes and species?


The Kidney (gross anatomy)

cutkidney.gif (15645 bytes)Graphics reprinted with permission by the copyright owner, Hill's Pet Nutrition Inc

a)  The pelvis, located at the base of the medulla, collects urine produced by kidney.

c)  The medulla is the central "middle" portion of the kidney, it is the location of the loops of Henle.

b)  The cortex is the outer  "rim" of the kidney and is located below the capsule, this is where most of the nephrons are located

d)  The capsule is the tough outer most coating of the kidney and is composed of connective tissue

e)  The ureter is the tube that connects kidney to urinary bladder

The kidneys are located in the dorsal lumbar region. They are well protected by the spine, epaxial muscles and fat. 
The right kidney is located cranial to the left. 
The location, size and structure varies slightly between species, i.e. cows have lobed kidneys. 
The kidneys are about 2% of the total body weight, yet receive about 15% of the total blood supply.

Bilateral renal (renal refers to kidney) arteries leave the aorta to supply each kidney. These arteries divide and sub-divide repeatedly finally forming  afferent arterioles. 
Each small arteriole supplies one nephron. 
The nephron is the "functional unit" of the kidney, where the important work of the kidney is performed.

Microscopic anatomy of the kidney

Nephron: The unique structures which are the "functional units" of the kidney.
Each human kidney has about one million nephrons, cows have about 4 million, dogs about 1/2 million and cats have about 200,000.

A very simple comparison can be made between a nephron and a long winding tube with a filter on one end and selective pores located throughout. 
At the "filter end" blood enters and a clear (plasma like) fluid results (the red blood cells and some other blood components are filtered out). 
This clear fluid drains down into a funnel like structure and is routed through a long winding tube. 
Some tiny molecules that get through the filter are essential and are taken back from the tube (via the selective pores).  A few other molecules are passed into the tube and excreted.

Parts of the nephron:  

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a)  afferent arteriole
b)  Bowman's capsule (sometimes the glomerulus and Bowman's capsule are referred to as a renal corpuscle)
c)  loop of Henle
d)  collecting ducts
e)  distal convoluted tubule
f)  proximal convoluted tubule
g)  glomerulus
h) efferent arteriole

NOTE:  There is an extensive capillary network surrounding each nephron, (in this diagram is  represented in red)

Processes of the urinary system

Production of urine involves three processes filtration, reabsorption and secretion.

Blood from the afferent arteriole enters the glomerulus under high pressure. 
The glomerulus is a mass of small capillaries formed from the afferent arteriole it is often described as a "tuft" of capillaries.   This tuft is surrounded by Bowman's capsule, a funnel like structure that continues as the nephron tubule. 
The fluid extracted from the blood is called the ultra filtrate and is similar to plasma. 
The process of blood being "squeezed through the glomerulus" and coming out as a clear fluid, is called filtration. 
When blood enters the glomerulus for filtration specialized cells filter out most large molecules and allow the fluid portion of the blood to seep through. 
Elements that should not pass through the glomerulus and into the ultrafiltrate include red blood cells and protein molecules, if these are found in the ultrafiltrate the glomerulus has been damaged.

Reabsorption is the process of "bringing back" into the capillaries needed elements from the ultrafiltrate. It is a method to recover essential molecules needed by the body, if these substances were not reabsorbed they would be lost in the urine. 
Reabsorption occurs primarily at the first part of the proximal convoluted tubule (about 66% of elements and fluid). Elements that are routinely reabsorbed include amino acids, sodium, vitamins, minerals, urea and glucose. 
There are a variety of ways elements are reabsorbed across the tubular cells into the capillaries including: 
carriers proteins
facilitated diffusion 
active transport mechanisms 
Water is also reabsorbed, it "follows" the other elements.

B)  Explain water reabsorption (hint - osmosis)

Secretion is the process of moving elements from the capillaries surrounding the nephron  into the kidney tubules for expulsion from the body in urine. It occurs primarily at the distal tubules. Relatively few elements are secreted, the list includes hydrogen ions, ammonia, potassium, penicillin and aspirin.

Acid-base balance
The kidney is the "other" organ (remember the lungs?) which regulates the acidity of the body.  Recall the following equation.

H+ + HCO3 >  <H2CO3 >  <  H2O + CO2

Remember that carbon dioxide was excreted from the body by exhalation. At the other end of the equation we see free hydrogen ion (H+).
Hydrogen is excreted from the body via urination regularly. The secretion of both hydrogen ion and ammonia (ammonia is the nitrogen based waste product from tissue metabolism) are essential in the acid regulation of the body. If hydrogen is not excreted normally carbonic acid will form in excess. 

C)  Why is acidosis bad?

Water conservation
Over 99% of the ultrafiltrate is reabsorbed by the kidney and returned to the body. In the adult human approximately 180 liters of ultrafiltrate is produced per day! The reabsorption is controlled primarily by two hormones antidiuretic hormone (ADH) and aldosterone.

d)  What would happen if even 10% of the reabsorption of the ultrafiltrate did not occur? What is a diuretic?

Antidiuretic hormone (ADH):
Antidiuretic hormone is secreted by the posterior portion of the pituitary gland. It is carried in the blood to the kidney where it affects the cells of the proximal tubule. 
ADH causes the proximal tubule to become more permeable to water in the ultrafiltrate.

Thus water is reabsorbed back into the body and diuresis is prevented.

Control of antidiuretic hormone is mediated through a complex feed back loop involving sensory nerves inside the carotid artery which sense blood volume. When the blood volume is low a message (carried through a nerve) is sent to the brain and antidiuretic hormone is released. 
After ADH is released and fluid is reabsorbed back into the body capillaries (from the kidney tubules) the blood pressure increases. 
When the blood pressure is back to normal the secretion of ADH is cut off. This is an example of a negative feed-back loop. (There will be more about feedback loops in the lesson on endocrinology)

There is an inverse relationship between ADH and blood pressure.
Blood pressure up  =   ADH down

e)  Why? What happens if the blood pressure goes down?

Aldosterone secretion is much more complicated, and involves several steps.
1) When the blood pressure coming into the kidney is low, renin (a hormone) from the kidney is released
2) This causes the sequential release of angiotensin I and then angiotensin II from the adrenal glands
3) Lastly aldosterone ( also from the adrenal glands) is released.

Aldosterone affects the kidney tubules by causing the reabsorption of sodium. When sodium is reabsorbed water is also reabsorbed (by osmosis). This causes an increase in the amount of fluid in the blood vessels and an increase in the blood pressure.

4) When the blood pressure increases to normal in the kidney the secretion of renin is stopped and the cycle is "cut off".

f)  What happens to the release of aldosterone after the blood pressure goes up?

Urinary bladder

Parts of the urinary tract:
a)  Prostate gland (the prostate gland is found only in males and is part of the reproductive system.
b)  Urinary bladder. The urinary bladder is a muscular, thin walled sac-like structure that is very expandable.
c)  Urethra

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Graphics reprinted with permission by the copyright owner, Hill's Pet Nutrition Inc.

Urinary bladder

Parts of the urinary tract:
a)  prostate gland
b)  urinary bladder
c)  urethra

VET 111

Textbook Assignment
Clinical Anatomy and Physiology for Veterinary Technicians, by Colville and Bassert

Please answer these questions and return answers by e-mail to Dr. Bidwell @
Assignment will be “spot checked” and logged in and answered e-mailed back to student after receiving assignment. The student is responsible for noting the correct. The questions might be included on exams

The Urinary system – chapter 16

1. What is the most important overall function of the kidney?

2. Which kidney is situated most cranial in all except the pig?

3. How are the kidneys protected within the abdominal cavity?

4. About what % of blood pumped by the heart goes to the kidneys?

5. (from above) Why so much blood to the kidneys?

5.  In 3 words describe the functions of the nephron.

6. Where do reabsorbed substances go (within the kidney)?

7. Where do secreted substances go (within the kidney)?

8. Using the text on p. 379 as a guide (glomerular filtration rate information) calculate the following (the book uses a 25 lb. dog):

How much urine would a 75 lb. dog be expected to produce per day?

9. Define the term uresis.

10. When the urinary bladder is very full, why doesn’t urine back up into the kidney and cause it to swell?

11. What type of epithelial cells line the urinary bladder?

12. Give an example of pre-renal uremia (azotemia), renal uremia (azotemia), post-renal uremia (azotemia.)

13. Urolithiasis occurs only in the urethra of male animals. T or F (please explain your answer.)

Answers to questions:

A) Is the urethra different between sexes and species?  Yes, the female tract is shorter, wider and does not have flexures.  There is also species variation in males, such as the goat has a long narrow penile extension, the bull has a "s" shaped flexure of its penis.

B)  Explain water reabsorbtion (hint - osmosis)  Water is reabsorbed by osmosis, i.e. water moves from an area of high concentration (of water) to an area of lower concentration

C)  Why is acidosis bad?  A high pH interferes with cellular enzyme functions, without enzymes all chemical reactions of the cell cease, thus the cell dies.

d)  What would happen if even 10% of the reabsorbtion of the ultrafiltrate did not occur? What is a diuretic?  The animal would become severely dehydrated and die.  A diuretic is a substance (medication) that causes an increase in urination.

e)  Why? What happens if the blood pressure goes down?    If the blood pressure goes down ADH secretion increases.


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