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Chapter 35. The Urinary System. Basic Functions. Urinary systems help maintain homeostasis —the relatively constant internal environment Composition of blood and extracellular fluid - PowerPoint PPT Presentation
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Chapter 35
The Urinary System
Basic Functions Urinary systems help maintain homeostasis—
the relatively constant internal environment Composition of blood and extracellular fluid Control the concentration or osmolarity, of dissolved
substances in cells and their extracellular environment
Excretion - removal of unwanted substances Produces urine - contains waste products of cellular
metabolism
Urinary System Functions Three basic processes
Blood or extracellular fluid is filtered, removing water and small dissolved substances
Nutrients are selectively reabsorbed back into the filtered fluid
Excess water, excess nutrients, and dissolved wastes are excreted from the body in urine
Flatworm Urine Systems The earliest excretory system served to maintain
water balance, the primary function of the simple excretory system of flatworms
This system consists of protonephridia - tubules that branch
throughout extracellular fluid surrounding flatworm tissues
Collects excess water from the extracellular fluid using ciliated “flame cells” and forces the fluid out through excretory pores
The large body surface of flatworms also serves as an excretory structure through which cellular wastes diffuse
tubule
excretorypore
extracellularfluid
flamecell
cilia
nucleus
(a) Flatworms use protonephridia
excretory pore
eye spot
Flatworms Use Protonephridia
Insect Urinary Systems Insects have an open circulatory system where hemolymph fills the
hemocoel and bathes internal tissues directly
Insect excretory systems are Malpighian tubules that extend outward from the intestine and end blindly within hemolymph
Wastes and nutrients move from the hemolymph into the tubules by diffusion and active transport, water follows by osmosis
Urine is conducted into the intestine, solutes are secreted into the hemolymph by active transport
Produces concentrated urine, which is excreted along with feces
abdomen
Malpighian tubules
intestine
rectumhemocoel(filled withhemolymph)
cellular anddigestive wastes
(b) Insects use Malpighian tubules
Insects Use Malphigian Tubules
Earthworm Excretory Systems In earthworms, mollusks, and other invertebrates, excretion is performed by
tubular structures called nephridia
The body cavity is filled with extracellular fluid into which wastes and nutrients diffuse
Each nephridium begins with a funnel-like opening, the nephrostome, ringed with cilia that direct extracellular fluid into a narrow, twisted tubule surrounded by capillaries
As the fluid traverses the tubule, salts and nutrients are reabsorbed back into the capillary blood, leaving the wastes and water behind
Urine is excreted through a nephridiopore
Each segment in an earthworm’s body contains a pair of nephridia
coelom (filled withextracellular fluid)
nephrostome
nephridium
nephridiopore
(c) Earthworms use nephridia
capillarybed
Earthworms Use Nephridia
Vertebrate Urinary Systems Kidneys - organs of the vertebrate urinary system,
where blood is filtered and urine is produced
Because vertebrates live in a wide variety of habitats, vertebrate kidneys face different challenges in maintaining constant conditions within their bodies
Homeostatic Kidney Functions The mammalian urinary system consists of kidneys, ureters,
bladder, urethra
These organs filter the blood, collecting, then excreting dissolved waste products in urine
During filtration, water and dissolved molecules are forced out of the blood
The kidneys return nearly all of the water and nutrients required by the body to the blood
The urine retains wastes, which are expelled
Mammalian Urinary Systems Helps maintain homeostasis in several ways:
Regulate blood levels of ions - sodium, potassium, chloride, calcium
Maintain proper pH of the blood by regulating hydrogen and bicarbonate ion concentrations
Regulate water content of the blood
Retain important nutrients - glucose and amino acids in the blood
Eliminate cellular waste products as urea
Secrete substances that help regulate blood pressure and oxygen levels
Urea A waste product of protein digestion
Eliminate nitrogenous wastes that are formed when cells break down amino acids
Nitrogenous wastes from cells enter the blood as ammonia (NH3), toxic
The livers of humans and other mammals convert ammonia into urea, which is less toxic
Urea is filtered from the blood by kidneys and excreted in urine
Urea Formation and Excretion
ammonia NH3
amino acid
Proteins in food are digested
Amino acids are carried inthe blood to body cells
The cells convert the amino groups (-NH2) toammonia, which is carriedin the blood to the liver
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urea
The liver converts ammoniato urea, which is less toxic
In kidney nephrons, ureais filtered into the urine
Urea is carried in the bloodto the kidneys
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5
Human Urinary System Kidneys - paired organs located on either side of the spinal column,
just above the waist
Blood enters each kidney through the renal artery, after blood has been filtered, it exits through the renal vein
Urine leaves the kidney through the ureter - a narrow, muscular tube
Rhythmic contractions of the ureter transports urine to the bladder, a hollow, muscular chamber that collects and stores urine
The bladder wall is lined with smooth muscle and is capable of considerable expansion, accommodating up to a pint of urine
left renal artery
left kidneyleft renal vein
aorta
left ureter
urinarybladder
urethra(in penis)
vena cava
The Human Urinary System
Bladder Sphincters Urine is contained within the bladder by two sphincter muscles
The internal sphincter, where the bladder joins the urethra, opens automatically during the reflexive contractions of the smooth muscle
The external sphincter, located slightly below the internal sphincter, is under voluntary control, allowing the brain to suppress urination unless the bladder becomes overly full
When open, the sphincters allow urine to flow into the urethra, a single narrow tube that conducts urine outside the body
Animation: Kidney Overview
Kidney Structure The structure of the kidney supports its function
of producing urine
Each kidney contains a solid outer layer - the renal cortex and the inner layer - the renal medulla
The renal medulla surrounds a branched, funnel-like chamber - the renal pelvis, which collects urine and funnels it into the ureter
Cross-Section of a Kidney
renalartery
renalvein
ureter
renal pelvis(cut away toshow thepath of urine)
renalcortex
enlargement of a single nephron andcollecting duct
renal medulla
renalcortex
renalpelvis
nephron urineto thebladderrenal
medulla
collectingduct
Nephrons The renal cortex is made up of more than 1
million microscopic filters or nephrons
Two major parts –
The glomerulus, a dense knot of capillaries where fluid is filtered out of the blood through the porous capillary walls
A long, twisted tubule, where urine formation occurs
Nephron Bowman’s capsule, a cup-like chamber that surrounds the glomerulus and
receives fluid filtered out of the blood from the glomerular capillaries
Collected fluid is conducted to the proximal tubule
The loop of Henle carries the filtered fluid from the cortex, deep into the medulla and back to the cortex
The distal tubule - in the cortex - collects the filtrate from the loop of Henle and passes it to the collecting duct
The collecting duct is not part of the nephron, but collects fluid from many nephrons and deposits it in the renal pelvis
collecting duct
distal tubule
proximal tubule
glomerulus
Bowman’scapsule
arterioles
venule
branch of therenal vein
branch ofthe renalartery
loop of Henle
capillaries
Individual Nephron and Blood Supply
Animation: Parts of the Nephron
The kidney’s blood supply The kidneys have an enormous blood supply, receiving more than
one quart of blood every minute
Blood flows to each kidney from the renal artery, which branches into arterioles that each supply nephrons with blood for filtration
The arterioles branch into capillaries and form the glomerulus of each nephron
The capillaries empty into an outgoing arteriole that branches into capillaries that surround the tubule
The capillaries carry blood into a venule that takes the blood to the renal vein and then the inferior vena cava
collecting duct
distal tubule
proximal tubule
glomerulus
Bowman’scapsule
arterioles
venule
branch of therenal vein
branch ofthe renalartery
loop of Henle
capillaries
Individual Nephron and Blood Supply
Urine Production – 3 Stages1. Filtration - water and small dissolved molecules are
filtered out of the blood
2. Tubular reabsorption - water and necessary nutrients are restored to the blood
3. Tubular secretion - wastes and excess ions remaining in the blood are secreted into the urine
Urine Formation and Concentration1
Tubular secretion:Additional wastes areactively transported into the proximal and distaltubules from the blood
3
4
Filtration: Water, nutrients, and wastes are filtered from theglomerular capillaries into theBowman’s capsule of the nephron
Tubular reabsorption: In theproximal tubule, most water and nutrientsare reabsorbed into the blood
2
Concentration: The loop ofHenle produces a salt concentrationgradient in the extracellular fluid;in the collecting duct, urine maybecome more concentrated than theblood as water leaves by osmosis
bloodleaving theglomerulus
loop ofHenle
blood enteringthe glomerulus
Bowman’scapsule
collectingduct
distal tubule
proximaltubule
Filtration Small organic nutrients - amino acids and glucose - are
filtered out and returned to the blood Large quantities of water and ions are filtered out, but the return rate is
adjusted to meet changing needs Ions include sodium (Na+), chloride (Cl–), potassium (K+), calcium (Ca++),
hydrogen (H+), and bicarbonate
Urine is formed in the glomerulus and tubule of the nephron Filtration - when water carrying small dissolved molecules and ions is
forced through the walls of the capillaries that form the glomerulus Blood cells and large proteins are too large to leave the capillaries, so
remain in the blood The fluid filtered out of the glomerular capillaries – the filtrate –
collected in Bowman’s capsule and continues through the tubule
Tubular Reaborption Occurs primarily in the proximal tubule, water and other
nutrients are reabsorbed in other tubule areas Returns organic nutrients - glucose, amino acids, vitamins, ions (Na+,
Cl–, K+, Ca2+, H+ and HCO3–) to the blood
Restores most of the water, water follows the nutrients and ions by osmosis through aquaporins - proteins that form water pores
Remaining wastes and excess ions move from the blood into the proximal and distal tubules Excess K+ and H+, small quantities of ammonia, drugs, food additives,
pesticides, and toxins (ie: nicotine)
Tubular secretion occurs primarily by active transport and takes place in both the proximal and distal tubules
When the filtrate leaves the distal tubule, it is urine
The Loop of Henle Creates an extracellular concentration gradient in the
renal medulla
The functions of the loop of Henle Some water and salt is reabsorbed from the filtrate as it passes
through the loop Most importantly, it creates a high salt and urea concentration in
the extracellular fluid within the medulla
Water Regulation Why is a high salt concentration is important? Water
regulation
The kidneys help maintain water content in body tissues by producing….
dilute, watery urine when fluid intake is high concentrated urine when fluid intake is low
Water is conserved by allowing it to move out of the collecting duct by osmosis and down its concentration gradient
The more concentrated the extracellular fluid, the more water leaves the urine as it moves through the collecting duct
Summary The loop of Henle produces and maintains a high salt
concentration gradient in the extracellular fluid of the medulla by transporting salt out of the filtrate
The salt and urea gradient causes an osmotic gradient between the filtrate and the surrounding extracellular fluid
The most concentrated fluid surrounds the bottom of the loop
The collecting duct passes through this gradient as it conducts urine from the distal tubule in the renal cortex into the renal pelvis
Details of Urine FormationFILTRATION
TUBULAR REABSORPTION& TUBULAR SECRETION
URINECONCENTRATION
renal cortex
renal medulla
osmosis
diffusion
active transport
Bowman’scapsule
loop of Henle
proximaltubule
distaltubule
1
2
3
4
5
67
8
H2O*H2O
H2O*
H2O*H2O
H2O
H2O
H2O
NaCIurea
NaCI
NaCI
NaCI
H+
NH3
somedrugs
Na+
nutrients
HCO3–
Ca2+
Cl–
K+
collecting duct
H+
K+
somedrugs
(extracellular fluid)
NaClCa2+
Concentration As the filtrate descends into the loop of Henle and
collecting duct… It is exposed to the osmotic gradient surrounding the
nephron Water leaves the filtrate by osmosis and enters the
surrounding capillaries
Filtrate becomes urine once it enters the collecting duct and can be more than four times as concentrated as blood
Kidneys Regulate Osmolarity of Blood
Kidneys regulate the water content of the blood Human kidneys filter out 1/2 cup of fluid from the
blood each minute Fine-tuning the composition of blood and helping maintain
homeostasis
If the kidneys did not reabsorb this water, the rate of filtration would require that we drink 50 gallons of water a day
The urinary system needs to restore nearly all of the water that is initially filtered out of the glomeruli
Antidiuretic Hormone Antidiuretic hormone (ADH) – Regulates reabsorption
and influences the ability of kidneys to reabsorb water
Secreted by the posterior pituitary gland, carried in the blood It stimulates cells of the distal tubule and collecting ducts to insert more
aquaporin proteins into their membranes The abundance of aquaporin membranes determines the permeability of
the membranes to water
Normally some ADH is always present in the blood
Within the hypothalamus, receptors monitor blood osmolarity, which increases when water is lost
Animation: Urine Formation
An Example When water is lost during dehydration:
If blood osmolarity exceeds optimal level, the hypothalamus stimulates the pituitary gland to release ADH into the bloodstream
Cells of the distal tubule and collecting duct insert more aquaporins into their membranes, increasing permeability to water
The more concentrated extracellular fluid draws water out by osmosis, restoring water to the blood through nearby capillaries
Dehydration Stimulates ADH Release and Water Retention
Receptors in the hypothalamus detect the increased blood osmolarity and signal the pituitary gland
ADH increases the permeability of the distal tubule and the collecting duct, allowing more water to be reabsorbed into the blood
Water is retained in the bodyand concentrated urine isproduced
The pituitary gland releases ADH into the bloodstream
Heat causes water loss and dehydration through sweating
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5
4
1
Kidneys regulate BP and Oxygen Kidneys release substances that help regulate
blood pressure and oxygen levels
When blood pressure falls, kidneys release renin
Renin catalyzes the formation of the hormone angiotensin in the blood
Angiotensin Combats low blood pressure in three ways
It stimulates the proximal tubules of nephrons to reabsorb more Na+ into the blood, causing water to follow by osmosis (increase blood volume)
It stimulates ADH release, causing more water to be reclaimed from the distal tubule and collecting duct
It causes arterioles throughout the body to constrict, which directly increases blood pressure
Erythropoietin When blood oxygen levels are low, kidneys release
erythropoietin
Stimulates the bone marrow to make more red blood cells
The higher number of red blood cells increases the oxygen carrying capacity of the blood
Vertebrate kidneys are adapted to diverse environments Mammals have structurally different nephrons,
depending upon the availability of water in their natural habitat
Mammals adapted to dry climates have long loops of Henle
Longer loops allow a higher concentration of salt to be produced in the extracellular fluid of the medulla, so more water is reclaimed from the collecting duct A mammal with very long-looped nephrons is the desert
kangaroo rat
A Well-Adapted Desert Dweller
More Mammal Adaptations Mammals adapted to habitats with an abundance of
fresh water have short loops of Henle Beavers, which live along streams, can only concentrate their
urine to about twice their blood osmolarity
Humans have a mixture of long- and short-looped nephrons, and can concentrate urine up to four times the osmolarity of blood
Freshwater Fish Animals have evolved homeostatic mechanisms,
including kidney adaptations, to maintain water and salt within their bodies – osmoregulation
Freshwater fish live in a hypotonic environment Water continuously moves into their bodies by
osmosis, salts diffuse out Freshwater fish acquire salt from food and through
their gills but never drink Their kidneys retain salt and excrete large quantities
of extremely dilute urine
(a) Freshwater fish
The kidneys conserve saltand excrete large amounts of dilute urine
Salt is pumped in by active transport
Salt and somewater entersin food
watersalt
Water moves in byosmosis; salt diffuses out
fresh water
Osmoregulation in Fish
Saltwater Fish Saltwater fish live in a hypertonic environment; seawater
has a solute concentration of two to three times that of their body fluids
Water is constantly leaving their tissues by osmosis, and salt is constantly diffusing in and being taken in with food
To compensate, saltwater fish drink to restore their lost water, and excess salt they take in is excreted by active transport through their gills
(b) Saltwater fish
Salt and water enterin food and by drinkingseawater
Some salt is excreted insmall quantities of urine
Water moves out byosmosis; salt diffuses in
Salt is pumped out by active transport
watersalt
salt water
Osmoregulation in Fish
Fish nephrons completely lack loops of Henle, and so they cannot produce concentrated urine
To conserve water, the kidneys of saltwater fish excrete small quantities of urine containing salts not eliminated by their gills