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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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

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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

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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

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5

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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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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