Biology 242 – Lab
LAB #13
(13th/19 Lab Sessions for Winter Quarter, 2008)
TOPICS TO BE COVERED:
»Gross anatomy of the Kidneys
»Microscopic anatomy of a Kidney (its stroma and parenchyma)
»Gross anatomy of other Urinary System Structures
»Examination of various models including: Urinary Apparatus, Kidney & Nephron
»Demo dissection of Sheep’s Kidney
»Dissection of urinary system structures in the Cat
»NOTE: Urinalysis will be performed next Lab session
DESIRED OUTCOMES:
After completing the activities described for this lab session, students should:
»Identify and describe the gross anatomical structures of the Urinary System.
»Identify microscopic structures of the kidney on prepared histological slides.
»Trace the blood flow through the kidney.
»Explain the function of the kidney.
»Identify the basic components of the nephron.
»Describe the differences between the male and female urinary tracts.
MATERIALS NEEDED:
»Anatomical models and charts: Urinary System/Apparatus, Kidney, Nephron, Vascular System
»Microscopic slides: renal cortex (where renal corpuscles are located), renal medulla, transitional epi.
»Several sheep’s kidneys »241 Histology Slide Boxes »Cats »Dissection trays/tools
»Photograpahic Atlas, Ch. 16
Activity #1: Gross Anatomy of the Kidneys
By way of introduction, remember that all cells produce waste products through their metabolic activities. Wastes such as urea, creatinine, carbon dioxide, nitrogen wastes, and excess electrolytes MUST be eliminated from the body to maintain homeostasis. Several organ systems eliminate wastes from the body: the lungs
remove CO2 during exhalation; the digestive tract eliminates undigested solid wastes and the skin removes some
salts and some urea in sweat. The primary function of the Urinary System is to maintain the composition,
volume, and pressure of blood by removing and restoring selected amounts of water and solutes. These eliminated products are collectively called urine. The Urinary System is composed of a pair of kidneys, a pair of
ureters, a single urinary bladder, and a single urethra.
While the primary function of the kidneys is maintenance of the volume, constituency, and pressure of
the blood (and they perform this function by removing nitrogenous wastes from the body and maintaining fluid, electrolyte, and acid/base balance), the kidneys also perform the following functions: production of the hormone erythropoietin (which regulates hematopoiesis); assisting the liver detoxify certain compounds; and making glucose during times of starvation. The ureters, urinary bladder, and urethra transport and temporarily store urine until it
can be eliminated from the body during a process known as micturition (aka: urination).
The kidneys are situated against the posterior body wall between the waist and the 11th and 12th pairs of
ribs (approx. vertebral levels T12 – L3); however, the right kidney lies one vertebral level lower than the left kidney
because of the position and mass of the liver. The kidneys, adrenal glands, and most of the ureters are technically NOT in the peritoneal (abdominal) cavity; rather they are said to be retroperitoneal because they lie behind
the parietal reflection of peritoneal serosal membrane that lines the abdominal cavity (and the visceral reflection
of peritoneal serosal membrane that covers most of the digestive organs). The kidneys are encased within three
layers of connective tissue: (1) the outer renal fascia, (2) the middle adipose capsule, and (3) the inner renal
Biology 251 – LAB #13 – continued
Page Two
capsule. The middle adipose capsule is the thickest of the three layers and consists of adipose connective tissue.
This adipose c.t. layer helps to secure the kidneys in place, padding them and keeping them from slipping inferiorly. (A potential consequence of extreme weight loss is the resulting disappearance of this adipose c.t.
layer, causing the kidneys to slip downward and the ureters to kink, preventing normal flow of urine to the
urinary bladder. This is a condition known as nephroptosis.)
Each kidney is about 13 cm (5.12 inches) long and 2.5 cm (1 inch) thick. The medial aspect of each
kidney contains a notch called the hilus. Blood vessels, nerves, lymphatics, and the ureter all enter and/or exit
the kidney through the hilus. Internally, the kidney is divided into three regions (Refer to Fig. 16.2, P 151):
1. Renal cortex: the outer region, which consists of many blood vessels that serve the functional units of the
kidney, the nephrons. Most of each nephron is located within this cortex (cortical) region.
2. Renal medulla: the middle region, consisting of a variable number (8 – 18) of triangular medullary pyramids
separated by structures called renal columns (these resemble the cortex in appearance). The pyramids have a
striated (“striped”) appearance, because of the preponderance of small tubules, the loops of Henle, and the
collecting ducts.
3. Renal pelvis: this inner region, which houses a basin for collecting urine drained from the pyramids. At the
apex of each renal pyramid is a renal papilla which empties urine into a small cuplike space called the minor
calyx. Several minor calyces empty into a common space, the major calyx. (Whether there are only 8 or as
many as 18 minor calyces, there are usually only 2 – 3 major calyces; and these major calyces merge to form the
renal pelvis. The cavity surrounding the renal pelvis is the renal sinus.) Urine drains from the renal pelvis into
the ureter.
The blood supply of the kidneys is quite unique. Every minute approximately 25% of the total blood
volume travels through the kidneys (at the rate of 1200 milliliters/min) to be filtered. The blood is delivered to
the kidneys by the renal arteries which branch off the abdominal aorta. Once each renal artery enters the
hilus of the kidney, it divides into five segmental arteries which branch into interlobar arteries that pass
through the renal columns. The interlobar arteries divide into arcuate arteries which course laterally along the
bases of the pyramids and enter the cortex as interlobular arteries. These arteries branch into afferent arterioles that enter the nephron; specifically that supply the glomerulus (a tuft of specialized blood vessels referred
to as “capillaries” – although they contain arteriolar blood).
As above, when an afferent arteriole enters a nephron, it forms a “capillary tuft” called the glomerulus.
A smaller-diameter efferent arteriole exits the glomerulus and only now forms not one, but two, true capillary
beds around the tubular portions of the nephrons. One capillary bed, known as the peritubular capillaries, surrounds the cortical nephrons as well as the proximal and distal convoluted tubules of the juxtamedullary nephrons. However, the second true capillary bed, known as the vasa recta capillaries, surround the loop of Henle
in juxtamedullary nephrons. Both capillary beds, or networks, are involved in the reabsorption of materials
from the filtrate of the renal tubules back into the blood. The vasa recta and peritubular capillaries drain into
interlobular veins which then drain into arcuate veins along the base of the renal pyramids. The arcuate veins
drain into interlobar veins which pass through the renal columns and empty into segmental veins. Blood from
these segmental veins drains into the renal vein which exits the hilus and drains into the inferior vena cava.
Student exercises:
1. Using Figs. 16.1, 16.2, and 16.4 – 16.7 in the Photographic Atlas, identify the structures outlined below and
label them on the Handout Sheet given in this Lab:
A. Surrounding connective tissue layers: (1) Renal fascia (2) Adipose capsule (3) renal capsule
B. Hilum
Biology 251 – LAB #13 – continued
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C. Regions: (1) Renal cortex (2) Renal medulla containing renal columns, renal pyramids + renal papillae
(3) Minor calyces
(4) Major calyces
(5) Renal pelvis
D. Blood supply: (1) Vessels delivering blood to each kidney: R + L Renal arteries and subsequent branches:
segmental arteries, interlobar arteries, arcuate arteries, interlobular arteries, afferent arterioles, GLOMERULUS, efferent arterioles, peritubular capillaries and vasa recta capillaries.
(2) Vessels draining blood away from each kidney: interlobular veins, arcuate veins, interlobar veins, segmental veins, and R + L Renal veins which drain independently into the IVC.
Activity #2: Microscopic Anatomy of a Kidney: Observations of the Nephron
Microscopically, each kidney is composed of approximately 1.25 million tiny, functional units called
nephrons. Each nephron consists of two portions: (1) a renal corpuscle and (2) a renal tubule.
The first part of each nephron (where blood is filtered) is called the renal corpuscle, and it consists of two parts:
the glomerulus and the surrounding glomerular (aka Bowman’s) capsule. Fluid, filtered from the blood,
called filtrate, first enters a space known as the capsular space, which is found within the structure called the
glomerular capsule (aka Bowman’s capsule). The glomerular capsule consists of two layers: (a) the parietal
layer, which is simple squamous epithelium, and the (b) visceral layer, which consists of specialized/modified
squamous epithelial cells called podocytes that surround the capillaries of the glomerulus. The podocytes have
cytoplasmic extensions called pedicels (“foot-like processes”). As these podocytes wrap around the endothelium of the glomerulus, small, slit-like gaps between the pedicels are formed. These gaps are called filtration
slits or slit pores. To be filtered out of the glomerulus, a substance must be small enough to pass through the
capillary endothelium and its basements membrane and squeeze through the filtration slits to enter the capsular
space. ( Any substance that can pass through these layers is removed from the blood as part of the filtrate. The
filtrate, therefore, contains both essential materials and wastes.) Together, the (1) glomerular endothelium, (2)
basement membrane, (3) podocytes of the visceral layer of Bowman’s capsule are called the filtration membrane.
The second part of each nephron (where the filtrate circulates through a series of convoluting or twisting tubules and a
U-shaped section to reclaim essential materials and remove wastes and excess ions ) is called the renal tubule, and it
consists of three parts: the proximal convoluted tubule (PCT) is the first segment after Bowman’s capsule and
it is a coiled tubule; the next portion is the loop of Henle which consists of a thin descending limb, a loop of the
limb, and a thick ascending limb; and finally, another convoluted section, the distal convoluted tubule (DCT).
Surrounding the convoluted tubules are the peritubular capillaries, while the loops of Henle are covered with
the vasa recta capillary bed. The nephron ends where the distal convoluted tubule empties into a collecting
duct. Several nephrons drain into the same collecting duct. These ducts merge with other collecting ducts into
a common papillary duct which drains into a minor calyx. There are between 25 and 35 papillary ducts per
renal pyramid.
Two types of nephrons occur in the kidney: cortical and juxtamedullary. Approximately 85% of the
nephrons are cortical nephrons and have their glomeruli and most of their tubules in the renal cortex region.
Juxtamedullary nephrons have their glomeruli at the junction of the cortex and medulla. These nephrons have
long loops of Henle that extend deep into the medulla before turning back toward the cortex.
The ascending limb of the loop of Henle twists back and comes into contact with the afferent arteriole
that supplies its glomerulus. This point of contact is called the juxtagomerular apparatus. The cells of the renal
tubule become tall and crowded together and form the macula densa. The macula densa acts as a chemoreceptor monitoring [NaCl] in this area of the renal tubule. Certain smooth muscle cells of the afferent arteriole in
this same area, called juxtaglomerular cells (JG cells) act as a baroreceptor monitoring blood pressure. These
JG cells develop round nuclei and can release an enzyme called renin, which puts into play the renin-angiotensin pathway of renal autoregulation. Angiotensin II is a hormone that causes vasodilation of blood vessels
and can restore blood flow to ischemic glomeruli. Also, through the action of the hormone aldosterone, an
increase in sodium and water reabsorption in the distal convoluted tubules takes place, and blood pressure is
additionally increased thus restoring GFR.
BIOLOGY 251 – LAB #13 – continued
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Student exercises:
1. Examine the kidney models and locate all the structures of a nephron. Complete the labeling of the parts of a
nephron on the handout sheet given during this lab.
2. Obtain a microscope and a slide of the kidney.
a) Use scanning and low powers (10X and 40X objectives respectively) and locate the renal cortex and renal
medulla. Examine several renal tubules, visible as ovals on the kidney slide. Be specific, what type of
tissue lines these tubules? Is the renal capsule visible on the outer margin of the kidney tissue?
b) Locate a renal corpuscle in the cortex region which appears as a small, round structure with a “knot”
inside of it. Distinguish among the parietal layer of Bowman’s capsule, the capsular space, and the
glomerulus. Can you “see” the podocytes of the visceral layer of Bowman’s capsule? Why or why not?
c) Draw a section of the kidney slide in the space below. Label the cortex, a renal corpuscle, and a renal
tubule. Note: The renal medulla may or may not be present on your kidney slide.)
Activity #3: Discussion/Observation of Other Structures of the Urinary System
We remember that from the glomerular capsule, the filtrate enters the renal tubule, which consists of a PCT,
loop of Henle (with its descending limb, U-shaped loop, and ascending limb) and DCT. Several distal convoluted
tubules drain into one collecting duct, and many collecting ducts merge into a common papillary duct. There are
approximately 25 – 35 papillary ducts per renal pyramid, and there are anywhere from 8 to 18 renal pyramids
per kidney. As the filtrate flows through the renal tubule and collecting ducts, it is modified and most of the
water and solutes are reclaimed. After the filtrate drains from the collecting ducts into larger tubules called papillary ducts, each of which empties into a space called a minor calyx, it is called urine. The minor calyces
drain into 2 – 3 major calyces, which empty into the renal pelvis. The renal pelvis is the dilated, upper end of
the ureter.) From the pelvis, urine enters the muscular tubes called the ureters, which “massage” the urine via
peristalsis down to the posteroinferior aspect of the urinary bladder. (Since urine does NOT simply passively fall
down each ureter by the force of gravity, but rather it is actively conveyed down each ureter by muscular peristalsis, you
now know the answer to the question: “Does urine reach the bladder if you are standing on your head?!?” ) Also, the
ureters are lined with a mucus-producing epithelium to protect the ureteral walls from acidic urine.
At the inferior aspect of the urinary bladder (aka: the floor of the bladder), there is a smooth, triangle-shaped
area called the trigone, which is delineated by three openings: the two, lateral-lying, in-coming ureteric orifices
and the one, midline, out-going internal urethral orifice. The internal urethral orifice is composed of smooth
muscle under autonomic nervous system control continues into the urethra, via which urine exits the body.
The urinary bladder is a hollow muscular organ that functions in the temporary storage of urine. In males, it
lies between the pubic symphysis and the rectum. In females, it is posterior to the pubic symphysis, inferior to
the uterus, and superior to the vagina. The inner wall of the bladder is lined by a mucous membrane which is
thrown into folds called rugae (except in the trigone region where it is smooth). These rugae allow expansion
and shrinkage of the bladder as it fills with and empties urine. And, of course, as we already have studied, the
epithelial component on this mucous membrane is transitional epithelium.
As was already mentioned, a single duct, the urethra, drains urine from the bladder out of the body. Around
the opening to the urethra are two sphincter muscles, the internal and external urethral sphincters. The internal urethral sphincter is composed of smooth muscle and is therefore under involuntary nerve control. The
external urethral sphincter is composed of skeletal muscle, is part of the skeletal muscle group known as the
urogenital diaphragm, and is under voluntary control. This sphincter enables you to control the release of urine
from the urinary bladder. As the bladder fills and expands with urine, stretch receptors signal motor neurons in
the spinal cord to relax the internal urethral sphincter and to contract the detrusor muscle of the bladder wall.
This reflex automatically causes closing of the external urethral sphincter. Only when appropriate and convenient, the external urethral sphincter should be voluntarily relaxed to allow urination (aka: micturition) to occur.
Student exercise: Label the following structures on the diagram given as a handout:
1. R + L Ureters
2. Urinary bladder – detrusor muscle, R + L ureteral orifices, internal urethral orifice, and trigone region
3. Urethra – internal urethral sphincter, external urethral sphincter, external urethral orifice
BIOLOGY 251 – LAB #13 – continued
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Activity #4: Demonstration Dissection of a Sheep’s Kidney
A fresh and/or preserved kidney specimen will be available, and we will note the relative thicknesses of the
surrounding connective tissue coverings if they are intact. Scissors will be used to cut through the renal capsule
to remove the kidney – note how “close-fitting” this innermost connective tissue covering is to this organ. To
review the gross anatomy of the kidney, we will list surface structures that are able to be identified.
Next, we will distinguish between the ureter, the renal artery, and the renal vein. The following are some
hints to aid in this process:
a) The renal artery typically has the thickest and most muscular wall, and prior to entering the kidney, it branches into five segmental arteries.
b) The renal vein is thinner-walled and looks flimsier; often it is larger in diameter than the renal artery. Remember, veins DO NOT BRANCH; so once the renal vein has left the kidney, it joins the inferior vena cava.
c) The ureter has a thick, muscular wall, too; but it does not branch once it leaves the kidney. Also, its diameter
is smaller than either the renal artery or the renal vein.
Keeping these points in mind, we will try to determine the location of the renal artery, the renal vein, and the
ureter on the specimen. Let’s also see if we can determine if we have a Right or a Left kidney. (Remember, in
anatomical position, the renal vein is anterior to the renal artery which is anterior to the renal pelvis (the dilated
upper-end of the ureter.)
A coronal (aka: frontal) section of the kidney will be made and the internal structures identified. How many
renal pyramids can be identified in the medullary region? How many minor calyces are there? How many major calyces can be identified?
Activity #5: Dissection of Urinary System Structures in the Cat
Refering to Pages 52 – 54 of the Gilbert Cat Atlas, identify the structures as shown in Figures 39, 40, and 41.
Depending on whether your cat specimen is male or female, you will be instructed differently about the relationship of the ureters to the uterine horns (in the female) and to the ductus deferens (in the male).
Figure 42 shows the kidneys and other urinary system structures in relation to the bicornuate uterus in a female; and Figure 43 shows a lateral view of the pelvic portion of the male urogenital system.