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CHAPTER 44
Osmoregulation and
Excretion
CONCEPT 44.3: DIVERSE
EXCRETORY SYSTEMS ARE
VARIATIONS ON A TUBULAR THEME

Excretory systems regulate solute movement between
internal fluids and the external environment
Excretory Processes
 Most excretory systems produce urine by refining a filtrate
derived from body fluids
 Key functions of most excretory systems:
Filtration: pressure-filtering of body fluids
 Reabsorption: reclaiming valuable solutes
 Secretion: adding toxins and other solutes from the body fluids to
the filtrate
 Excretion: removing the filtrate from the system

Capillary
Filtrate
Excretory
tubule
Filtration
- Filter out the bad
Reabsorption
- Reclaim the goods
Secretion
-Get rid of the bad that was missed
the first time
Urine
Excretion
- Release the bad
SURVEY OF EXCRETORY SYSTEMS
Systems that perform basic excretory functions vary widely
among animal groups
 They usually involve a complex network of tubules
 Invertebrates:

Planarians have flame cells
 Earthworm/Annelids have nephridia
 Insects have Malpighian tubules

PROTONEPHRIDIA: FLAME-BULB SYSTEMS
A protonephridium is a network of dead-end tubules lacking
internal openings
 The smallest branches of the network are capped by a
cellular unit called a flame bulb/cell
 These tubules excrete a dilute fluid and function in
osmoregulation

Cilia
Interstitial fluid filters through
membrane where cap cell and
tubule cell interdigitate
(interlock)
Tubule cell
Flame
bulb
Protonephridia
(tubules)
Tubule
Nephridiopore
in body wall
METANEPHRIDIA
Each segment of an earthworm has a pair of open-ended
metanephridia
 Metanephridia consist of tubules that collect coelomic fluid
and produce dilute urine for excretion

Coelom
Capillary
network
Bladder
Collecting
tubule
Nephridiopore
Nephrostome
Metanephridium
MALPIGHIAN TUBULES
In insects and other terrestrial arthropods, Malpighian
tubules remove nitrogenous wastes from hemolymph and
function in osmoregulation
 Insects produce a relatively dry waste matter, an important
adaptation to terrestrial life

Digestive tract
Rectum
Intestine Hindgut
Midgut
(stomach)
Malpighian
tubules
Feces and urine Anus
Salt, water, and
nitrogenous
wastes
Malpighian
tubule
Rectum
Reabsorption of H2O,
ions, and valuable
organic molecules
HEMOLYMPH
VERTEBRATE KIDNEYS

Kidneys, the excretory organs of vertebrates, function in
both excretion and osmoregulation
CONCEPT 44.4: NEPHRONS AND ASSOCIATED BLOOD VESSELS ARE
THE FUNCTIONAL UNIT OF THE MAMMALIAN KIDNEY
The mammalian excretory system centers on paired kidneys,
which are also the principal site of water balance and salt
regulation
 Each kidney is supplied with blood by
a renal artery and drained by a
renal vein
 Urine exits each kidney through a duct
called the ureter
 Both ureters drain into a common
urinary bladder where it is stored
 Urine leaves the bladder through the
urethra

Posterior vena cava
Renal artery and vein
Kidney
Renal
medulla
Renal
cortex
Renal
pelvis
Aorta
Ureter
Urinary bladder
Urethra
Ureter
Excretory organs and
major associated blood
vessels
JuxtaCortical
medullary nephron
nephron
Afferent
arteriole Glomerulus
from renal
Bowman’s capsule
artery
Proximal tubule
Peritubular capillaries
Renal
cortex
Collecting
duct
20 µm
Renal
medulla
To
renal
pelvis
Nephron
Section of kidney from a rat
Kidney structure
SEM
Efferent
arteriole from
glomerulus
Distal
tubule
Collecting
duct
Branch of
renal vein
Descending
Loop
limb
of
Henle Ascending
limb
Vasa
recta
Filtrate and blood flow
STRUCTURE AND FUNCTION OF THE NEPHRON AND ASSOCIATED
STRUCTURES

The mammalian kidney has two distinct regions: an outer
renal cortex and an inner renal medulla

The nephron, the functional unit of the vertebrate kidney,
consists of a single long tubule and a ball of capillaries called
the glomerulus which is cupped by the Bowman’s capsule
LONG
TUBULE AFTER THE BOWMAN’S CAPSULE
CONSISTS OF 4 MAJOR REGIONS

Proximal convoluted
tubule

Loop of Henle

Distal convoluted
tubule

Collecting duct
BLOOD VESSELS ASSOCIATED WITH THE
NEPHRONS
Each nephron is supplied with blood
by an afferent arteriole, a branch of
the renal artery that divides into the
capillaries
 The capillaries converge as they
leave the glomerulus, forming an
efferent arteriole
 The vessels divide again, forming
the peritubular capillaries, which
surround the proximal and distal
tubules

FILTRATION OF THE BLOOD

Filtration occurs as blood pressure
forces fluid from the blood in the
glomerulus into the lumen of
Bowman’s capsule

glomerulus & Bowman’s capsule
make up the renal corpuscle
Fluid that enters the Bowman’s
capsule in now called the filtrate
 Rest of the blood enters the
efferent arteriole and into the
peritubular capillaries

PATHWAY OF THE FILTRATE
From Bowman’s capsule, the filtrate
passes through three regions of the
nephron: the proximal tubule, the
loop of Henle, and the distal tubule
 Fluid from several nephrons flows
into a collecting duct

FROM BLOOD FILTRATE TO URINE: A CLOSER LOOK
Filtrate becomes urine as it flows through the mammalian
nephron and collecting duct
 Secretion and reabsorption in the proximal tubule greatly
alter the filtrate’s volume and composition

FROM BLOOD FILTRATE TO URINE: A CLOSER LOOK
Filtrate becomes urine as it flows through the mammalian
nephron and collecting duct
 Secretion and reabsorption in the proximal tubule greatly
alter the filtrate’s volume and composition
 Reabsorption of water continues as filtrate moves into the
descending limb of the loop of Henle
 In the ascending limb of the loop of Henle, salt diffuses from
the permeable tubule into the interstitial fluid

The distal tubule regulates the K+ and NaCl concentrations
of body fluids
 The collecting duct carries filtrate through the medulla to the
renal pelvis and reabsorbs NaCl

Proximal tubule
Distal tubule
NaCl Nutrients
HCO3– H2O
K+
H2 O
H+
NH3
NaCl
K+
HCO3–
H+
CORTEX
Descending limb
of loop of
Henle
Filtrate
H2O
Salts (NaCl and others)
HCO3–
H+
Urea
Glucose; amino acids
Some drugs
OUTER
MEDULLA
H2 O
Thick segment
of ascending
limb
NaCl
NaCl
Thin segment
of ascending
limb
Key
Active transport
Passive transport
Collecting
duct
Urea
NaCl
INNER
MEDULLA
H2 O
REGULATION OF KIDNEY FUNCTION
The concentration of the urine is regulated by nervous and
hormonal control of water and salt reabsorption in the
kidneys
 Antidiuretic hormone (ADH) increases water reabsorption in
the distal tubules and collecting ducts of the kidney


An example of an ADH hormone is vasopressin
Osmoreceptors
in hypothalamus
Hypothalamus
Thirst
Drinking reduces
blood osmolarity
to set point
ADH
Pituitary
gland
Increased
permeability
Distal
tubule
STIMULUS
osmoreceptor cells in
the hypothalamus
detect an increase in
the osmolarity
of the blood
H2O reabsorption helps
prevent further
osmolarity
increase
Collecting duct
Homeostasis:
Blood osmolarity
REGULATION OF KIDNEY FUNCTION
The osmolarity of the urine is regulated by nervous and
hormonal control of water and salt reabsorption in the
kidneys
 Antidiuretic hormone (ADH) increases water reabsorption in
the distal tubules and collecting ducts of the kidney



An example of an ADH hormone is vasopressin
Lack of vasopressin is diabetes insipidus
Lots of dilute urine is produced
 Drinking alcohol inhibits the release of vasopressin and mimics
diabetes insipidus  frequent urination

The renin-angiotensin-aldosterone system (RAAS) is part of a
complex feedback circuit that functions in homeostasis
 Purpose: Maintain blood volume and pressure

Homeostasis:
Blood pressure,
volume
Increased Na+
and H2O reabsorption in
distal tubules
STIMULUS:
The juxtaglomerular
apparatus (JGA) responds
to low blood volume or
blood pressure (due to
dehydration or
loss of blood)
Aldosterone
Arteriole
constriction
Adrenal gland
Angiotensin
Distal
tubule
Angiotensinogen
JGA
Renin
production
Renin

Another hormone, atrial natriuretic factor (ANF), opposes
the RAAS
Inhibits renin secretion and thus the production of angiotensin,
and stimulates aldosterone release.
 Its effect is increased excretion of water and sodium and a
lowering of blood pressure, which reduces the workload of the
heart.
