Exercise 29: Urinary System
The composition of blood and interstitial fluids must remain constant if the body’s 100 trillion cells are to survive. Each cell consumes nutrients and produces waste products which are toxic enough to be deadly if allowed to accumulate. Just as cities have waste management systems to dispose of public wastes, so the body has a waste management system to dispose of cellular wastes: The Urinary System. Without the urinary system the ability to maintain homeostasis would quickly be lost, followed by death.
Components of the Urinary System
The urinary system contains relatively few components, namely the two kidneys, the two ureters, the urinary bladder and the urethra (Figure 29-1). A waste-filled solution called urine is produced by the kidneys, conveyed by the ureters to the urinary bladder for storage and expelled from the body through the urethra.
The kidneys are in effect filters for the blood. Your entire blood volume (~5 liters) filters through your kidneys about 20 times each day. In the process, toxic cellular waste products that were released into the blood are removed while useful nutrients are retained. The wastes are collected as urine and held temporarily in the bladder until willfully expelled from the body at a convenient time.
Each kidney is about 3 inches high, 2 inches wide and 1 inch deep. They are located posteriorly in the abdominal cavity adjacent to the 12th rib. Due to the large amount of space taken up by the liver, the right kidney is usually a bit lower than the left kidney. They are both held in place by fat deposits and the fibrous connective tissues surrounding them. One or both kidneys may “drop” in people having extremely low levels of body fat; a painful and potential deadly condition called ptosis.
The Ureters, Bladder and Urethra
The ureters are tubules that conduct urine from the kidneys to the urinary bladder. Each ureter is ~25 cm (~10 inches) long and gets progressively smaller in diameter toward the bladder. The walls of the ureter are lined with smooth muscle that contract in a peristaltic wave about twice each minute, assisting the flow of urine to the urinary bladder.
Although urine is slowly and continuously produced by the kidneys, most of us would agree that it’d be terribly inconvenient to slowly and continuously relieve urine from the body. Thankfully, God has provided an internal holding tank for urine allowing us to store it inside our bodies until a convenient time arrives for disposal. This holding tank is the urinary bladder, or simply the bladder (Figure 29-2).
The urinary bladder is a flattened sac when empty and expands upward as it fills with urine. A typical bladder holds 600-800 mL of urine; men tend to have slightly larger bladders than women. Both ureters enter the bladder near its base and the urethra (discussed below) also drains the bladder from its base. The two ureters and the urethra mark the points of a triangle called the trigone, a relatively tough and inflexible part of the bladder.
By contrast, the remainder of the bladder is quite flexible and highly muscular. The smooth muscle in the bladder walls (collectively called the detrusor muscle) contract to expel urine from the bladder. During micturition small flap-like projections over the ureteral openings prevent urine from being pushed back up to the kidneys.
As the urinary bladder fills with urine stretch receptors in the walls of the bladder monitor its state of distension. When roughly 250 mL of urine have been collected in the bladder these stretch receptors send a signal to the brain. At this juncture, we can consciously decide to empty the bladder or not. If the bladder is not emptied the signals from the stretch receptors will be ignored and we once again become unaware of the bladder’s condition, until about 500 mL have been collected. The receptors will send increasingly stronger signals to the brain rekindling the desire to micturate (i.e., urinate). Again, the desire can be willfully overcome and the signals ignored until the bladder reaches full capacity, roughly 700-800 mL of urine. Beyond that point urine will be uncontrollably expelled from the body.
During expulsion urine passes from the bladder to the outside world via the urethra. In the male, two sphincters control micturition: The internal urethral sphincter and the external urethral sphincter. The internal urethral sphincter opens reflexively (unconsciously) when the bladder is full; the external urethral sphincter is willfully opened to urinate. Contraction of the detrusor muscle expels urine. Females have no distinct internal sphincter.
Distally, the urethra opens to the external environment at the external urethral orifice. In females, the urethra is short (~4 cm, or 1.5 inches) and is used exclusively by the urinary system. The external urethral orifice lies superior to the vaginal opening. In males, the urethra is considerably longer (~20 cm, or 8 inches) because it passes through the penis (Figure 29-3). It is divided into three sections: 1/ the prostatic urethra which passes through the prostate gland, 2/ the membranous urethra which passes through the muscles of the pelvic floor, and 3/ the penile urethra which passes through the penis. The penile urethra is also called the spongy urethra because it passes through the corpus spongiosum of the penis. The male urethra allows passage of urine and also the passage of sperm during the process of ejaculation. Thus, the male urethra is shared between the urinary and reproductive systems.
Internal Anatomy of the Kidneys
A frontal section of a kidney is shown in the figure 29-4. From this perspective many of the blood-filtering and urine-collecting structures can be seen. Directly beneath the most superficial layer of the kidney (i.e., the renal capsule) lies the renal cortex. This white-ish layer contains numerous microscopic nephrons that filter the blood. Urine generated by this filtration process passes through collecting ducts in the renal pyramids until it is emptied into a minor calyx. Several minor calyces merge to form a major calyx which empties into the renal pelvis. Finally, urine in the renal pelvis drains into the ureter and is carried by peristaltic contractions of the ureter to the bladder.
Blood Flow through the Kidneys
Each minute about ¼ of the body’s total blood volume passes through the kidneys for filtration.
The blood enters the kidney via the renal artery which divides at the hilus into segmental arteries. Segmental arteries divide into lobar arteries which supply blood to individual renal lobes. The lobar arteries divide into interlobar arteries that rise through the renal columns surrounding the pyramids. Interlobar arteries join via arcuate arteries that curve over the base of each pyramid. The arcuate arteries branch into numerous interlobular arteries which rise into the cortex and branch into afferent arterioles, which supply the capillary networks (glomerular and peritubular) that filter the blood. Blood drains from these capillary beds through interlobular veins to the arcuate veins to the interlobar veins and finally into the renal vein that leaves the kidney. The arrangement of these vessels is illustrated in figure 29-5.
The nephron is the basic functional unit of the kidney and is responsible for filtering and processing blood and producing urine (Figure 29-6). Each kidney contains over 1 million nephrons.
The production of urine is a 3-step process. First, the blood is filtered. Second, useful components removed by filtration are put back into the blood by reabsorption. Finally, toxic components not effectively removed by filtration are actively secreted from the blood.
Blood is filtered in the glomerulus, a capillary bed fed by the afferent arteriole and enclosed within a capsule called the glomerular (or Bowman’s) capsule (Figure 29-7). About 10-15% of the blood passing through the glomerular capillary bed is squeezed through the filtration slits of podocytes, specialized cells that wrap around the glomerular capillaries. The blood passing through these slits is called filtrate and contains all the components of blood plasma smaller than proteins (proteins and larger entities are too large to pass through the filtration slits). The 85-90% of the blood which is not filtered leaves the glomerulus via the efferent arteriole.
The newly formed filtrate leaves the glomerulus via the proximal convoluted tubule (PCT) where useful components – like glucose – are selectively reabsorbed back into blood. These components re-enter the bloodstream in capillaries surrounding the tubule called the peritubular capillary bed, which is fed by the efferent arteriole. Once inside the tubules the filtrate is called tubular fluid.
Tubular fluid leaves the PCT and travels down the descending limb and up the ascending limb of the loop of Henle. The loop of Henle absorbs more or less water and NaCl depending on the needs of the body. Thus, this structure plays an important role in regulating the final volume and concentration of urine.
Upon leaving the loop of Henle the tubular fluid enters the distal convoluted tubule (DCT) where further absorption of water, salts and other nutrients takes place. The DCT is also a major site of secretion – the third step in blood processing. Ions, drugs, acids and toxins which escaped filtration in the glomerulus are actively transported (i.e., secreted) from the blood of the peritubular capillaries into the tubular fluid. Tubular fluid leaving the DCT, now called urine, drains into a collecting duct for eventual transport to the bladder. As in the loop of Henle, variable reabsorption of water and ions can be achieved at the collecting ducts to control urine volume and concentration.
Adjustments to urine volume and concentration are achieved by the convoluted tubules, loop of Henle and collecting ducts, but is largely regulated by specialized cells adjacent to the glomeruli. Juxtaglomerular (JG) cells, found within the walls of the arterioles feeding the glomerulus, detect blood pressure. If blood volume is too high, blood pressure increases and these cells cause more water to be moved from the blood to the urine. The macula densa, a specialized group of DCT cells located near the JG cells, contain chemoreceptors that monitor the concentration of ions and nutrients. Together, the macula densa and JG cells form the juxtaglomerular apparatus (JGA), which is largely responsible for regulating urine volume and concentration (Figure 29-7).
Activity 1: Histological Examination of the Urinary System
Obtain a prepared slide of kidney tissue.Before placing the slide on the microscope, hold it up to the light and observe the cortex, renal lobe and pyramid.
Place the slide on a microscope and scan the tissue under low power.You should be able to identify the cortical and medullary areas.Individual glomeruli will be visible as tightly wound tubules – similar to a ball of rubber bands.
Observe a glomerulus under high magnification.You should be able to easily identify the lumen of the glomerular capsule and the contained glomerular capillaries.
JG cells and macula densa cells should be visible where the afferent arteriole feeds the glomerulus.
Tubules belonging to the DCT can be distinguished from those of the PCT by observing the lumen.The lumen of the PCT appears “fuzzy” because PCT epithelial cells project long microvilli into the lumen.These microvilli increase surface area and improve reabsorption efficiency.By contrast, the lumen of the DCT appears clear because those cells do not have microvilli.
Examine prepared slides of the ureter, bladder and urethra.Transitional epithelia should be especially apparent in the urethra sample.
Activity 2: Urinalysis
A urinalysis is a set of tests performed on a urine sample to assess the overall properties of the urine. A typical urinalysis includes assessment of pH, glucose concentration, protein concentration and the presence or absence of blood cells and/or hemoglobin among other things. Observation of the color and smell of the urine may also be included in a urinalysis. Abnormal urine can provide a wealth of information to doctors about the kidneys, liver, blood and many other organs and help diagnose a host of pathologies and illnesses. Specific urine tests can be applied to screen for drugs, e.g., illegal performance-enhancing drugs taken by some athletes, or even pregnancy.
Performing a urinalysis these days is much easier than it used to be. In fact, a complete battery of tests can be performed in a single step in just seconds using Chemstrips® (Figure 29-8). Complete a urinalysis as follows:
Obtain a urine sample from yourself or a lab partner.A small beaker or Dixie cup can be used to collect the urine.At least 50 mL of urine should be collected.Collect the urine “mid-stream” to reduce contamination.
Observe the color and odor of the urine.The color of urine can vary dramatically from clear to dark amber.In general, the more concentrated the urine the darker its color and more pronounced its odor.
Briefly dip a Chemstrip® into the urine.Completely submerge all of the squares on the strip, but only for a few seconds.
Wipe off excess urine from the Chemsrip® and allow 60 seconds for the chemical reactions to occur.Each square on the strip specifically tests for one component or property.
After 1 minute, hold the strip next to the label on the Chemstrip® container.Compare the color of each square with the “decoder” on the label.Record your observations.
Do not allow the reaction to proceed more than 2 minutes. Prolonged reactions will generate false positives.
Abnormal Urine Conditions
Glycosuria – Glucose levels in the urine are normally very low. Elevated levels of glucose in urine is suggestive of diabetes mellitus.
Ketonuria – Ketones may be found in the urine of healthy individuals in the post-absorptive state (i.e., mild starvation). Elevated ketones in the urine may also result from diabetic ketoacidosis.
Leukocytes – Leukocytes, or white blood cells, are usually not found in urine. The presence of leukocytes may indicate a urinary tract infection (UTI).
Most common urinary pathogens are nitrite-producing gram-negative bacteria. The presence of nitrite in the urine is strongly suggestive of UTI.
The protein concentration in normal urine is low, usually less than 30 mg/dL. Elevated urine protein concentration is sometimes observed in pregnant women. High protein levels indicate damage to the gluomerular filtration apparatus of the kidneys.
Blood is normally not found in urine. The presence of blood may indicate serious glomerular damage in the kidneys, or may originate from scratches along the urinary tract (e.g., by a passing kidney stone). In healthy females, blood contamination may be observed if the urine sample is collected during menstruation.
Activity 3: Specific Gravity Determination
Urine specific gravity is a measure of the density of urine as compared to water. By adding solutes to water you create a solution and the more solutes you add the denser the solution will be. The specific gravity (g) of pure water is 1.000 by definition.
Urine contains a number of different types of solutes including urea, sodium, potassium, uric acid, creatine, ammonia, etc. The density of urine is often reflected in its color: the denser the urine the darker its color (Figure 29-9). The specific gravity of human urine varies in density from approximately 1.003 when very dilute to 1.030 when very concentrated. Measure the density of your urine sample as follows:
Pour urine into the provided glass cylinder until it’s about ¾ full.
Carefully place the urinometer into the urine sample with the weighted bulb down.The urinometer will sink and then float with the calibrated scale emerging from the sample surface.After the urinometer has stopped “bobbing” up and down, carefully read the calibrated scale at the lowest part of the meniscus of the urine sample.
Dispose of the urine down the drain and rinse all glassware.Be careful not to break the fragile urinometer.
specific gravity __________________________
Activity 4: Urine Output
Urine production can vary tremendously depending on the needs of the body. Under sympathetic stimulation (“flight or fight”), urine production may stop completely. Conversely, urine can be produced rapidly when the body is overhydrated. In this exercise we will demonstrate how rapidly urine can be produced.
Select a volunteer. It is often fun to select one male and one female volunteer to “compete” for most urine output. If using multiple volunteers, they should all be about the size body size and weight.
Have the volunteer empty their bladder early in the lab period. (One hour will be needed to complete this test).
After voiding their bladder, the volunteer should consume approximately 1.5 liters of water quickly, in about 10 minutes. If the volunteer is small or petite, reduce water intake to 1 liter.
After one hour, the volunteer should void their bladder and collect of the urine in a 1-liter beaker.
1. How much urine did the male volunteer produce in one hour?
2. How much urine did the female volunteer produce in one hour?
3. Divide the volume of urine produced (in mL) by the elapsed time (60 minutes) to
obtain urine output rate (mL/min). What is the urine output for the male and
It is possible to overdose on water with fatal results (a condition called hyponatremia). The amounts of water consumed in this exercise are safe for healthy students of average body weight, but students with low body weight or known medical conditions should not volunteer for this exercise. In addition, students who will be engaging in physical activity (e.g., sporting exercises) following lab should not volunteer.