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Excretory System
A. The Challenge: Shifts in Extracellular Fluid
1. The volume and composition of extracellular fluid (interstitial plus blood) must be maintained within
tolerable ranges.
2.
3.
4.
Water Gains and Losses
i. Water is gained by two processes:
1. Absorption of water from liquids and solid foods occurs in the gastrointestinal
tract.
2. Metabolism of nutrients yields water as a by-product.
ii. Water is lost by at least four processes:
1. Excretion of water is accomplished by the urinary system.
2. Evaporation occurs from respiratory surfaces and the skin.
3. Sweating occurs on the skin surface.
4. Elimination of small amounts of water in feces is a normal occurrence.
iii. Urinary excretion affords the greatest control over water loss..
b. Solute Gains and Losses
i. Solutes are added to the internal environment by four processes:
1. Nutrients, mineral ions, drugs, and food additives are absorbed by the
gastrointestinal tract.
2. Secretion from endocrine glands adds hormones.
3. Respiration adds oxygen to the blood and metabolizing cells add carbon dioxide.
4. Metabolism reactions contribute waste products.
ii. Extracellular fluid loses mineral ions and metabolic wastes in three ways:
1. Respiratory exhalation rids the body of carbon dioxide.
2. Various mineral ions are lost in sweat.
3. Urinary excretion rids the body of these wastes:
a. Uric acid is formed in reactions that degrade nucleic acids.
b. Ammonia is formed when amino groups are removed from amino acids.
c. Urea is formed by reactions in the liver that unite two ammonia
molecules with carbon dioxide.
d. Phosphoric acid and sulfuric acid are formed during protein breakdown.
iii. The kidneys filter a variety of substances from the blood.
1. Most of the filtrate is returned to the blood; about 1% ends up as urine, a waste
fluid of excess water and solutes.
2. The kidneys regulate the volume and solute concentrations of extracellular fluid.
The Urinary System
a. The kidneys are the central components of the urinary system.
i. Each kidney is a bean-shaped organ about the size of a clenched fist.
ii. Each kidney is composed of two zones--an outer cortex and inner medulla--wrapped with a
renal capsule.
1. Nephrons filter and retain water and solutes, leaving a concentrated urine to pass
through collecting ducts to the central renal pelvis.
2. Urine flows from each kidney through a ureter to a urinary bladder (for storage)
and then out of the body through the urethra.
b. More than a million nephrons are packed inside each kidney.
i. An afferent arteriole delivers blood to the renal corpuscle of each nephron.
1. Filtration occurs in the glomerulus--a ball of capillaries nestled inside the
Bowman's capsule.
2. The Bowman's capsule collects the filtrate and directs it through the continuous
nephron tubules: proximal >>> loop of Henle >>> distal >>> collecting duct.
ii. The capillaries exit the glomerulus, converge into an efferent arteriole, then branch again to
form the peritubular capillaries around the nephron tubules where they participate in
reclaiming water and essential solutes that are carried out of the kidney and back to the
general circulation.
How Urine Forms
a. Filtration, Reabsorption, and Secretion
5.
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i. Urine is a fluid that rids the body of water and solutes that are in excess of the amounts
needed to maintain the extracellular fluid.
ii. Urine forms through a sequence of three processes:
1. In filtration, blood pressure forces filtrate out of the glomerular capillaries into
Bowman's capsule, then into the proximal tubule.
a. Blood cells, proteins, and other large solutes cannot pass the capillary
wall into the capsule.
b. Water, glucose, sodium, and urea are forced out.
2. Reabsorption takes place in the tubular parts of the nephron where water and
solutes move across the tubular wall out of the nephron (by diffusion or active
transport) and into the surrounding capillaries.
3. Secretion moves substances from the capillaries into the nephron walls.
a. Capillaries surrounding the nephrons secrete excess amounts of hydrogen
ions and potassium ions into the nephron tubules.
b. This process also rids the body of drugs, uric acid, hemoglobin
breakdown products, and other wastes.
iii. Urination is a reflex response which empties the bladder.
1. The internal urethral sphincter (involuntary control) regulates flow from the
bladder into the urethra.
2. The external urethral sphincter (voluntary control) opens to void urine from the
body.
3. Kidney stones are deposits of uric acid that collect in the renal pelvis or lodge in
the ureter; they can be removed by surgery or lithotripsy.
b. Factors That Influence Blood Filtration
i. The kidneys can process about 1.5 quarts of blood each minute because of two factors:
1. Blood enters the glomerulus under high pressure in arterioles that have wider
diameters than most arterioles.
2. Glomerular capillaries are highly permeable to water and small solutes.
ii. The rate at which the kidneys filter a given volume of blood depends on the flow of blood
through them and the rate of reabsorption in the tubules; neural and hormonal controls
operate.
Reabsorption of Water and Sodium
a. Reabsorption in the Proximal Tubule
i. Mechanisms within the kidney carefully regulate the excretion and retention of substances
based on intake and bodily need.
ii. Most of the water is reabsorbed in the proximal tubule.
1. Sodium ions are pumped out of the tubule (filtrate) and into the interstitial fluid
surrounding the peritubular capillaries.
2. Significant amounts of water follow passively down the gradient that has been
created.
b. Reabsorption in Other Parts of the Nephron
i. In the descending limb of the loop of Henle, water moves out by osmosis, but in the
ascending portion sodium is actively pumped out.
ii. This interaction of the limbs of the loop produces a very high solute concentration in the
deeper parts of the kidney medulla and delivers a rather dilute urine to the distal tubule.
Hormonal Adjustments of Reabsorption
a. How ADH Influences Water Reabsorption
i. Antidiuretic hormone (ADH) from the posterior pituitary is secreted in response to a
decrease in extracellular fluid; ADH causes the distal tubules and collecting ducts to
become permeable to water, which moves back into the blood capillaries.
ii. When water intake is excessive, ADH secretion is inhibited; less water is reabsorbed, and
thus more is excreted.
iii. Caffeine and alcohol are diuretics, substances that promotes loss of water.
b. How Aldosterone Influences Sodium Reabsorption
i. When sodium levels fall so does the volume of extracellular fluid; this triggers the
juxtaglomerular apparatus to secrete renin, which calls forth angiotensins I and II, which
act on the adrenal cortex to release aldosterone, which promotes sodium reabsorption.
ii. Sodium retention is accompanied by water retention, which can lead to increased blood
pressure--hypertension, which can affect kidney function.
c. Salt-Water Balance and Thirst
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i. When solute concentration in the extracellular fluid rises, the thirst center of the
hypothalamus responds by decreasing saliva production.
ii. The dry sensation in the mouth causes a liquid-seeking behavior.
The Acid-Base Balance
a. Kidneys also regulate the acidity and alkalinity of extracellular fluid.
i. Overall acid-base balance is maintained by controlling hydrogen ions through buffer
systems, respiration, and excretion by the kidneys.
ii. Buffers can neutralize hydrogen ions, and the lungs can eliminate carbon dioxide.
b. Only the urinary system can eliminate excess hydrogen ions, permanently, and restore the
bicarbonate buffering ions to the blood.
i. The HCO that forms in the nephron cells is moved to the capillaries where it neutralizes
excess acid.
ii. The H+ that forms in the cells is secreted into the tubular fluid where it combines with
bicarbonate ions to form carbon dioxide (which is returned to the blood and excreted by the
lungs) and water (which is excreted in the urine).
Maintaining the Body's Core Temperature
a. Many different physiological and behavioral responses help to maintain the body's required internal
core temperature (37°C).
i. If body temperature rises above 41°C, enzymes will be denatured.
ii. As body core temperature drops below 35°C, enzyme activity decreases, shivering stops,
breathing may cease, and consciousness is lost; further decreases usually are fatal.
iii. As endotherms, humans have a body temperature regulated by metabolic activity and
mechanisms that control heat conservation and dissipation.
b. Responses to Cold Stress
i. Mammals respond to cold by constricting the smooth muscles in the blood vessels of the
skin (peripheral vasoconstriction), which retards heat loss.
ii. In the pilomotor response, the hairs become more erect to create a layer of still air that
reduces convective and radiative heat losses.
iii. Rhythmic tremors (shivering) is a common response to cold but is not effective for very
long and comes at high metabolic cost.
iv. Nonshivering heat can be produced by a hormonal stimulation of a special brown adipose
tissue.
v. Hypothermia is a condition in which the core temperature drops below normal; it may lead
to brain damage and death; frostbite is localized cell death due to freezing.
c. Responses to Heat Stress
i. Peripheral vasodilation is the enlargement of the diameters of blood vessels to allow
greater volumes of blood to reach the skin and dissipate the heat.
ii. Evaporative heat loss by sweating is a common and obvious cooling mechanism.
iii. Hyperthermia is a rise in core temperature, with devastating effects such as heat exhaustion
and heat stroke..
iv. During a fever, the hypothalamus resets the body's "thermostat" to a new temporary core
temperature.
1. At the onset of fever, heat loss decreases and heat production increases; the person
feels chilled.
2. When the fever breaks, peripheral vasodilation and sweating increase as the body
attempts to reduce the core temperature to normal.
3. The controlled increase in body temperature (mediated by interleukins, interferons
and prostaglandins) during a fever seems to enhance the body's immune response.