Download Chapter 16—The Urinary System. I. Eliminating waste. a. This is

Chapter 16—The Urinary System.
I.
II.
III.
Eliminating waste.
a. This is accomplished due to the actions of multiple organ systems. Fig. 16.1.
b. There is a constant exchange of ions, gasses, and other solutes between the extracellular fluid
(interstitial fluid and blood plasma) and the intracellular fluid (within the cells).
c. The composition of the extracellular fluid is therefore constantly changing.
d. The urinary system plays a major role in regulating the composition of the extracellular fluid to
maintain homeostasis via the excretion of a variety of wastes that leave the body in urine.
Water gains and losses.
a. Water gains occur through:
i. Absorption from ingested liquids and foods.
ii. Metabolic reactions that yield water as a by-product.
1. Oxygen is the final electron acceptor at the end of the electron transport chain
during aerobic cellular respiration. Oxygen + protons + electrons  Water.
2. Condensation synthesis reactions produce water.
iii. Thirst also affects water gain.
1. Thirst center is located in the hypothalamus.
b. Water loss occurs through:
i. Urinary excretion.
ii. Evaporation from the lungs and through the skin.
iii. Sweating.
iv. Elimination in feces.
v. Hydrolysis reactions consume water.
c. Urinary excretion exerts the most control over water loss, with excess water and excess or
harmful solutes being excreted.
Solute gains and losses.
a. The extracellular fluid gains solutes by:
i. Absorption from ingested foods and liquids.
ii. Secretion from cells.
iii. Respiration.
iv. Metabolism.
b. The extracellular fluid loses solutes through:
i. Urinary excretion.
ii. Respiration.
iii. Sweating.
c. The kidneys regulate water and solute gains and losses, and remove wastes such as:
i. Uric acid (a by-product of nucleic acid breakdown) that can crystallize within joints to
cause gout.
ii. Ammonia (formed by deamination reactions that strip the amino (nitrogen containing)
groups off of amino acids.
iii. Urea—formed in the liver when two ammonia molecules and one molecule of CO2
combine.
iv. Creatine—a by-product of reactions using creatine phosphate as an energy source during
muscular activity.
v. Protein breakdown wastes including:
1. Phosphoric acid.
2. Sulfuric acid.
3. Other nitrogen containing compounds.
IV.
V.
Components of the urinary system. Fig. 16.2.
a. Kidneys—pair of organs that constantly filters water and all solutes (except proteins) from the
blood; they reclaim water and solutes as the body requires and excrete the remainder as urine.
Fig. 16.3.
i. [Kidney anatomy: cortex, medulla, renal capsule, renal pelvis, etc.; contains the
nephrons = individual units that do the actual filtering, reabsorption, and secretion].
b. Ureters—drain urine from the kidneys to the:
c. Urinary bladder—muscular sac-like organ that stores the urine.
d. Urethra—a muscular tube that connects to the bladder, and drains the urine to the external
environment.
e. Anatomy of nephrons and associated blood vessels. Figure 16.4:
i. Nephron. Functional unit of kidney; about 1-2 million per kidney.
1. Renal corpuscle, afferent and efferent arterioles.
a. Glomerulus.
b. Glomerular (Bowman’s) capsule—receives water and solutes filtered from
the blood.
2. Renal (nephron) tubule.
a. Proximal convoluted tubule.
b. Loop of Henle (loop of the nephron).
c. Distal convoluted tubule.
ii. Collecting duct.
f. Peritubular capillaries—reabsorb water and essential solutes from the renal tubules for return to
the blood.
Formation of urine.
a. Each minute, 1.5 quarts of blood pass through the kidneys; this is about 25% of the cardiac
output at rest. All of the blood in the body passes through and is filtered by the kidneys about 30
times per day.
b. Urine—fluid that rids the body of excess water, solutes, and metabolic wastes.
i. Is formed by three processes: glomerular filtration, tubular reabsorption, and tubular
secretion.
c. Glomerular filtration—occurs at the glomerulus. Fig. 16.5.
i. Blood pressure forces water and small solutes (glucose, urea, Na+, etc.) out of the plasma
within the glomerular capillaries and into the glomerular capsule and proximal
convoluted tubule. Fluid = filtrate.
ii. Glomerular capillaries are highly permeable. However, they do not allow large
molecules such as proteins and RBCs to escape, but they do allow 10-100 times more
water and small solutes to leave than normal capillaries would.
iii. Afferent and efferent arterioles:
1. Afferent arterioles have large diameters (less resistance, less of a pressure drop,
less of a velocity decrease).
2. Efferent arterioles have smaller diameter (greater resistance).
3. Therefore, pressure in glomerular capillaries is greater than in normal capillaries,
which allows for the formation of large quantities of filtrate.
iv. Glomerular filtration rate can be altered by the following:
1. Increasing the diameter of the afferent arterioles will increase the pressure in the
glomerular capillaries, increasing filtrate production. A decrease in the diameter
of the afferent arterioles will have the opposite effect.
2. Decreasing the diameter of the efferent arterioles will increase the pressure in the
glomerular capillaries, which will also increase filtrate production. An increase in
the diameter of the efferent arterioles will have the opposite effect.
3. An increase in systemic blood pressure will also increase production of filtrate.
VI.
VII.
v. Neural, endocrine, and local controls (vasodilation and vasoconstriction of afferent &
efferent arterioles) insure that the volume of blood flow to glomeruli remains relatively
constant, even when blood pressure changes.
vi. These factors result in the production of about 45 gallons (180 liters) of filtrate per day.
d. Tubular reabsorption. Fig. 16.7.
i. Occurs along the proximal convoluted tubule (PCT), loop of Henle, distal convoluted
tubule (DCT) and collecting duct. Most of the filtrate’s solutes and water move out of
the nephron by diffusion or active transport, and into the peritubular capillaries (back into
the blood).
ii. The PCT is particularly well suited for reabsorption because it is lined with a simple
cuboidal epithelium with microvilli = very high surface area…Fig. 16.6.
iii. Table 16.1 lists some of the substances present in the filtrate, and how much of each are
reabsorbed (by %).
1. Of the 180 liters of filtrate produced each day, only about 1% (1-2 liters) of the
filtrate is excreted as urine.
e. Tubular secretion—occurs when substances diffuse out of the peritubular capillaries and move
into the cell walls of the nephron, which then secrete the substances (via active transport) into the
filtrate within the nephron tubule.
i. Occurs along the proximal convoluted tubule (PCT), loop of Henle, distal convoluted
tubule (DCT) and collecting duct.
ii. These processes rid the body of hydrogen ions (H+), potassium ions (K+), ammonium
ions (NH4+), uric acid, drugs, some toxins, and other substances.
f. Once the filtrate reaches the end of the collecting ducts, its composition has been modified such
that it is now urine.
i. The urine contains all the materials that were filtered from the blood and not reabsorbed,
plus substances that were secreted from the blood.
ii. The blood leaving the kidney contains most of the water, nutrients, and ions that it
contained when it entered the kidneys, but it has been cleansed of the wastes discussed
above.
iii. From the collecting ducts, urine moves into the hollow interior of the kidney (chambers
called calyces) that all converge on a space called the renal pelvis. The renal pelvis
funnels the urine into the ureter, which transports the urine to the urinary bladder. During
urination, the urine leaves the urinary bladder via the urethra to the external environment.
Acid-base balance.
a. The kidneys also regulate the pH of extracellular fluid and blood by controlling the concentration
of H+ and other ions.
b. Buffers, respiration, and urinary excretion provide the control mechanisms.
c. pH is regulated by:
i. Bicarbonate within the blood: H+ + HCO3- ↔ H2CO3 ↔ H2O + CO2.
ii. Excess H+ is excreted in the urine; the bicarbonate stays in the blood to act as a buffer.
Water conservation via the reabsorption of water and sodium. Fig. 16.8.
a. The kidneys act to keep the water and salt content of the body relatively constant. The amount
of water in the blood directly affects cardiac output and blood pressure.
b. Proximal convoluted tubule:
i. About 2/3 of all water, sodium ions (Na+), and chlorine ions (Cl-) within the filtrate is
reabsorbed through the proximal convoluted tubule. Specifically:
1. The proximal convoluted tubule has transport proteins within its walls that
actively pump Na+ out of the tubule and into the interstitial fluid.
2. Cl- follows Na+; also, 99% of the glucose and amino acids present in the filtrate of
the PCT are reabsorbed by other transport mechanisms.
3. The proximal convoluted tubule is water permeable, so water follows the solutes
out of the proximal convoluted tubule as they are reabsorbed. (Water is moving
down its concentration gradient = osmosis). The water and solutes move into the
peritubular capillaries, returning to the blood.
4. These movements cause the volume of fluid (and dissolved solutes) within the
proximal convoluted tubule to greatly decrease, while the total concentration of
solutes within the tubule stays nearly the same.
c. Descending loop of Henle:
i. The loop of Henle descends into the kidney medulla, and extracellular solute
concentrations progressively increase around it, as it descends. (This is because of what
is occurring in the ascending loop of Henle and in the collecting tubule, discussed below).
ii. The descending loop of Henle is permeable to water, which moves out of the descending
loop by osmosis. This causes solute concentration within the tube to increase.
d. Ascending loop of Henle:
i. The ascending loop is impermeable to water, but Na+ is actively transported out
(followed passively by Cl-).
ii. This causes solute concentrations to:
1. Rise in the extracellular fluid, which favors water movement out of the
descending loop of Henle and the collecting ducts.
2. Fall within the ascending loop of Henle (filtrate is becoming more dilute).
e. Distal convoluted tubule:
i. When the filtrate reaches the distal convoluted tubule, much water and many solutes have
already been reabsorbed, resulting in filtrate volume decreasing greatly.
ii. Filtrate within the distal convoluted tubule is relatively dilute, but water within it must
still be recovered, as well as remaining Na+.
iii. Antidiuretic hormone (ADH) regulates water reabsorption at the distal convoluted tubule
and collecting duct, making them more permeable to water (when additional water
reabsorption is required), which moves (via osmosis) into the interstitial fluid due to the
concentration gradient there.
1. When water intake is excessive, ADH secretion is inhibited so the water can be
excreted.
2. ADH secretion is controlled by the hypothalamus, and is released from the
posterior pituitary gland. Fig. 16.9.
3. ADH secretion (or lack of secretion) occurs in response to extracellular fluid
solute concentration and blood pressure. ADH secretion increases:
a. When the body is low on water.
b. When the extracellular solute concentration is high.
c. When blood pressure falls.
iv. Aldosterone regulates Na+ reabsorption in the DCT and collecting duct.
1. When the body loses more Na+ than is brought in, extracellular fluid volume also
falls (because water follows salt).
a. This causes blood pressure to drop, and the glomerular filtration rate to
decline, both of which cause a decrease in the production of filtrate.
2. Sensors within the blood vessels of the heart and kidneys (and others) detect this,
and juxtaglomerular cells (of the afferent arteriole) within the juxtaglomerular
apparatus (specialized association of cells of the afferent arteriole and the distal
convoluted tubule) begin to secrete the enzyme renin into the blood. Fig. 16.10.
a. Renin converts angiotensinogen (a protein produced by the liver) into
angiotensin I.
b. Angiotensin I is converted by angiotensin converting enzyme (ACE) to
angiotensin II in the lung capillaries.
c. Angiotensin II stimulates the release of aldosterone from the adrenal
cortex.
VIII.
IX.
X.
XI.
3. * Aldosterone stimulates the active transport of Na+ from the distal convoluted
tubules and collecting ducts back into the interstitial fluid. (Less Na+ is excreted
in the urine).
4. * Conversely, when Na+ concentration within the extracellular fluid is too high,
aldosterone secretion is inhibited. (More Na+ is excreted in the urine).
5. In both cases above * water follows Na+ by osmosis.
a. When more Na+ is reabsorbed, so is more water. This increases blood
pressure, glomerular filtration rate, and production of filtrate. As a result,
a more concentrated urine is produced.
b. When less Na+ is reabsorbed, so is less water. This decreases blood
pressure, glomerular filtration rate, and production of filtrate. As a result,
a more dilute urine is produced.
v. Atrial natriuretic peptide (ANP).
1. This hormone is secreted by the cells of the right atrium of the heart due to an
increased stretching of the atria when blood volume and pressure is too high.
2. ANP decreases the reabsorption of Na+ and water at the DCT and collecting duct,
causing the production of a more dilute urine. This decreases blood pressure,
glomerular filtration rate, and production of filtrate.
vi. Table 16.3 provides a summary of hormonal regulation of urine formation at the kidneys.
f. Collecting duct. Fig. 16.8.
i. The distal portion of the collecting duct is permeable to urea, which has until this time
remained within the nephron tubules.
ii. As water reabsorption occurs along the length of the nephron, urea concentration
increases greatly; some urea diffuses out into the deep medulla, further increasing the
extracellular fluid concentration gradient in this region; remaining urea stays in the urine.
1. It is this concentration gradient of urea and salt that drives the movement of water
out of the descending loop of Henle and collecting duct.
Salt and water balance, and thirst.
a. The same stimuli that cause ADH secretion (body low on water, extracellular solute
concentration high, blood pressure falls) also stimulate the thirst center, a cluster of neurons
located in the hypothalamus.
b. Stimulation of the thirst center inhibits saliva production, causing “dry mouth.” The brain
detects the lack of moisture within the mouth and responds by making you feel “thirsty.”
c. Water deprivation, profuse sweating, blood loss, burns, diarrhea, and some medications can
cause intense thirst.
Red blood cells and vitamin D.
a. The kidneys are also responsible for production of the hormone erythropoietin, which stimulates
production of red blood cells in the red bone marrow.
b. In addition, the kidneys transform vitamin D in the blood into its active form, calcitriol, which
stimulates the use and absorption of calcium and phosphorus from the GI tract.
Dialysis and transplant surgery. You will not be tested on this material, but it is interesting to read.
Urination (micturition reflex). Fig. 16.14.
a. Urine is produced constantly, and is transported through the ureters to the urinary bladder. As
the urinary bladder fills, stretch receptors in the smooth muscle in its walls (detrusor muscle) are
triggered when about 200 ml of urine is present.
b. Stretching causes action potentials to travel up sensory neurons to the spinal cord, where they
synapse with:
i. Interneurons within the spinal cord that then synapse with motor neurons that cause the
detrusor muscle of the urinary bladder to contract and the internal urethral sphincter to
relax.
ii. Sensory neurons within the spinal cord that send information to the brain, alerting the
conscious mind of the need and desire to urinate.
XII.
1. If circumstances are appropriate, the brain initiates motor commands to the
external urethral sphincter to relax, allowing urination to occur.
c. Internal urethral sphincter = involuntary. External urethral sphincter = voluntary; we learn how
to control it and how to initiate or delay urination during potty training.
Urinary tract infections.
a. Caused by microorganisms invading the urinary system, usually via the urethra.
b. Diagnosed by examining the urine for the presence of microorganisms and blood cells.
c. Treated with antibiotics.
d. Females are more susceptible than males. Fig. 16.16.
i. Short urethra.
ii. Close proximity of urethral orifice and anus.
1. Proper wiping essential to prevent fecal bacteria from entering urethra.
a. Wipe front to back.
iii. Sexual intercourse can cause microorganisms normally found on the skin to be
transferred to the urethral orifice.
1. Urination is advised shortly after sex to flush these microorganisms out of the
distal urethra.
Study suggestions for this chapter: In the textbook at the end of the chapter, the sections entitled 1)
Highlighting the Concepts, 2) Recognizing Key Terms, and 3) Reviewing the Concepts are all good for you to
gauge your comprehension and focus your study efforts.