Download Urinary System Pathology Normal Structure and Function Grossly

Urinary System Pathology
Normal Structure and Function
Grossly, kidneys are divided into three parts – cortex, medulla, and pelvis
There are some species differences
 Cattle have kidneys that are composed of multiple lobes
 Horse kidneys have mucus glands in the pelvis, and so there is often a lot of
mucus in this area.
The urine collected in the pelvis moves on down the ureter and into the bladder,
where it remains until there is a conscious decision to relax the sphincter and urinate
 The cortex is where all the action happens - this is the focus of all the filtering
and absorptive functions.
 The nephron is the functional unit of the kidney, consisting of renal corpuscle
(glomerulus and associated blood vessels), proximal convoluted tubule, loop
of Henle, distal tubules, and collecting ducts.
 Blood comes into the kidney and arterioles branch and go directly to the
glomerulus where they are intertwined with the glomerular apparatus.
 The blood and the glomerular cells are very closely attached, and the blood
filtrate gets pushed through the membrane into the urinary space.
 Only water and the smallest molecules can make it through unless there is
something wrong with the glomerulus, in which case much bigger
molecules go through.
From the urinary space of the glomerulus, the filtrate goes on into the proximal tubule
and loop of Henle. The lining cells here are very active, and have a brush border that
greatly increases the surface area. The proximal tubular cells and loop of Henle cells
are active in re-absorbing all the good ions back into the body, including Na+, K+,
and Cl- as well as water. The amount of each of these that is reabsorbed is regulated
by antidiuretic hormone and keeps the general osmolarity of the body in balance.
From here the filtrate moves on into the distal tubules and collecting ducts, which
serve to further concentrate the filtrate, and it becomes urine.
Renal Failure
Renal function is essential for life. When the kidneys fail, that is very bad, because
ionic balance is all screwed up and also excretory wastes are not disposed.
Kidneys can fail because the glomeruli stop working or the tubules stop working.
But a problem in one usually leads to problems in the other.
The body functions fine until more than 75% of renal function is lost (this means 75%
of nephrons aren’t working). At that point, it becomes “end stage kidney disease”
and this can be started by glomerular, tubular, or interstitial problems
RETENTION OF EXCRETORY WASTES
Azotemia is defined as the presence of excessive urea or creatinine in the blood
without clinical manifestation of renal disease. Azotemia may be due to several
causes
Uremia is the presence of urinary constituents in the blood AND the toxic
condition produced by those constituents. Thus uremic animals would be azotemic
AND have clinical signs or systemic lesions caused by the retained waste products.
Nausea and depression are common.
ABNORMAL URINE EXCRETION
Anuria is the absence of urine excretion. Oligouria is reduction in the amount
of urine excreted. There are many causes, including dehydration. Polyuria is the
1
passage of abnormally large amounts of urine. Pollakiuria is increased frequency
of urination (the amount may be normal). Dysuria is painful or difficult urination.
Stranguria is the slow and painful discharge of urine due to spasm of the urethra
or bladder.
EXCRETION OF ABNORMAL SUBSTANCES
Proteinuria is the presence of excess protein in the urine. Glucosuria is the
presence of glucose in the urine. Crystalluria is the presence of crystals in the
urine. Pyuria is neutrophils in the urine Hemoglobinuria is the presence of
hemoglobin in the urine. Myoglobinuria is the presence of myoglobin in the urine,
due to muscle damage. Hematuria is the presence of red blood cells in the urine.
RED URINE
Urine may be red due to: hemoglobin, myoglobin, or red blood cells. In addition,
certain foods (e.g. beets), drugs and even chronic lead or mercury poisoning may
result in red urine.
Circulatory disturbances
There are two big problems that happen in the kidney relative to circulatory
disorders.
 The first big circulatory problem in the kidney is infarction. The arteries
of the kidney are called “end arteries”, which means there is no collateral
circulation. Consequently, if there is a thrombotic or embolic problem in the
animal, these arteries are likely to get infarcted, because with end arteries,
there is no peripheral circulation. Infarcts are red at first, then they become
white, then they get replaced with fibrous tissue which shrinks the area and
causes an indentation.
The second circulatory problem that happens in the kidney is that it is very
susceptible to hypoxia! So anytime there is very severe anemia or hypovolemic
shock, the kidneys might just shut down entirely. The proximal tubule cells will die
if not constantly exposed to enough oxygen, and they never recover. Necrosis of
proximal tubules is called NEPHROSIS and will be covered more later.
Glomerular diseases
When there is glomerular disease, the filtration function is altered. So, instead of just
very small ions and water passing through the glomerulus into the tubules, bigger
molecules that are supposed to stay in the blood (!) get passed through into the
urine. THIS IS BAD!
An additional problem is that when the glomerulus is altered, the blood flow to the
tubules changes. All the blood to the kidney goes through the glomerulus first, so if
there is a problem in the glomerulus, the tubules don’t get enough blood, and they
eventually die too!
Main causes of damage to the glomerulus:
Amyloid is an unusual protein ("beta pleated sheet") formed by cleavage from larger
proteins and then deposited in extracellular areas of tissues, especially glomerulus! It
totally screws up the filtration (there isn’t any) and all the protein in the blood just
spills out into the urine. It is usually seen in old animals.
Immune complex glomerular disease:
If there is chronic antigen circulating and many antibodies produced, these form
complexes that get deposited in the glomerular membranes. The result is similar to
amyloid, filtration is screwed up, basically, there isn’t any, and protein goes right
through into the urine.
2
Acute infectious inflammation due to bacteria, usually part of a bacterial
septicemia, can settle in the glomerulus.
PROTEINURIA is the hallmark of glomerular disease. When the glomerulus is
messed up, there is no filtering, and protein goes directly through the open door of the
glomerulus and into the urine.
As a result there is often hypoproteinemia and edema in many parts of the body.
THROMBOSIS
Thrombosis is a common sequel of severe glomerular amyloidosis and proteinuria in
dogs. This is attributed to the loss of proteins that inhibit coagulation (i.e., antithrombin III) in the urine. Since they are depleted, inhibition of coagulation is less,
and thrombosis may result.
Tubular disease
The tubules are a very important part of the nephron and essential for maintaining the
body’s ionic balance and also ensuring that bad waste products get disposed. When
tubules become necrotic, the term is NEPHROSIS. Nephrosis is really hostile and
life-threatening.
Causes of nephrosis
ISCHEMIA is a big one.
 A sudden severe drop in blood pressure (like SHOCK),
 Massive hemorrhage cause acute tubular necrosis (nephrosis) and the kidneys
will never recover.
HEMOLYSIS will also cause nephrosis. Free hemoglobin in the blood is toxic to the
tubules.
CRYSTALS that settle out in the tubules will damage them mechanically
TOXINS – There are MANY toxins that affect the tubules of the kidney. Certain
plants, heavy metals, excess copper, mycotoxins. Also, too much gentamycin or
kanamycin will cause nephrosis.
BACTERIA – Leptospira grows inside the tubules and kills the tubular cells
(nephrosis).
A characteristic of NEPHROSIS is finding CASTS of protein or cells in the urine
PYELONEPHRITIS
Pyelonephritis happens when bacteria come up from the bladder. There is always
cystitis
(bladder inflammation) associated with pyelonephritis. The organisms from the
bladder can go up the ureter, and the pelvis of the kidney and the collecting ducts
there become inflamed.
Bladder
UROLITHIASIS
Urinary calculi (uroliths) may be found in;
 the renal pelvis (nephroliths),
 ureter (ureterolith)
 urinary bladder (urocystolith)
 urethra (urethrolith).
1. They are most commonly found in the bladder
2. Stones are composed of various minerals and knowing which mineral is present
can help you determine the cause and prevent future occurrences.
3. In males they often lodge in the urethra.
Urinary tract obstruction is common in male cats and is called feline urologic
syndrome (FUS) or BLOCKED CAT. The blockage is due to small stones, almost
like sand.
3
CYSTITIS; Inflammation of the bladder. It is most commonly caused by bacteria
that are present around the anus, specifically coliforms. If these bacteria get the
chance to move into the environment of the urethra and the bladder, they do that.
Some predisposing factors for cystitis include:
 Being a female – females have a shorter urethra so the bacteria don’t have to
travel as far!
 Urine stagnation – animal is not urinating voluntarily and so urine builds up in
the bladder. There is not the flushing action that cleans the urethra regularly.
 Mucosa trauma – usually due to catheterization
 Glucose in the urine – this occurs in diabetes, the bacteria do very well with
extra glucose!
4