1. Osmoregulation and Excretion
Chapter 44
Osmoregulation and Excretion

2. 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

3. Get rid of the bad that was missed the first time
Filtration
- Filter out the bad
Capillary
Excretory
tubule
Filtrate
Reabsorption
- Reclaim the goods
Secretion
Get rid of the bad that was missed
the first time
General schematic view of excretory function
Excretion
- Release the bad
Urine

4. 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

5. 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

6. Interstitial fluid filters through membrane where cap cell and
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

7. 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

8. Coelom Capillary network Bladder Collecting tubule Nephridiopore
Nephrostome has cilia around its opening that brush fluid into the nephrostome. As the fluid passes the tubules water and solutes diffuse between the tubules and the capillaries. Since earthworms inhabit damp soil, they absorb water by osmosis through their skin and must get rid of their excess water. Produce dilute urine
Collecting
tubule
Nephridiopore
Nephrostome
Metanephridium

9. 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

10. Digestive tract
Rectum
Hindgut
Intestine
Midgut
(stomach)
Malpighian
tubules
Salt, water, and
nitrogenous
wastes
Feces and urine
Anus
Malpighian
tubule
Rectum
Reabsorption of H2O,
ions, and valuable
organic molecules
HEMOLYMPH

12. Vertebrate Kidneys
Kidneys, the excretory organs of vertebrates, function in both excretion and osmoregulation

13. 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
1. Human urinary system: 2. Kidney, 3. Renal pelvis, 4. ureter, 5. urinary bladder, 6. urethra 7. adrenal gland
Vessels: 
8. Renal artery and vein, 9. inferior vena cava, 10. abdominal aorta, 11. Common iliac artery and vein With transparency: 12. liver, 13. large intestine, 14. pelvis

14. Posterior vena cava
Renal artery and vein
Kidney
Renal
medulla
Aorta
Renal
cortex
Ureter
Renal
pelvis
Urinary bladder
Urethra
Excretory organs and
major associated blood
vessels
Ureter
Section of kidney from a rat
Kidney structure
Juxta-
medullary
nephron
Cortical
nephron
Afferent
arteriole
from renal
artery
Glomerulus
Bowman’s capsule
Proximal tubule
Renal
cortex
Peritubular capillaries
Collecting
duct
SEM
20 µm
Efferent
arteriole from
glomerulus
Renal
medulla
Distal
tubule To
renal
pelvis
Branch of
renal vein
Collecting
duct
Descending
limb
Loop of
Henle
Nephron
Ascending
limb
Vasa
recta
Filtrate and blood flow

15. 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

17. 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

18. Long tubule after the Bowman’s capsule consists of 4 major regions
Proximal convoluted tubule
Loop of Henle
Distal convoluted tubule
Collecting duct
Convoluted = twisted, coiled

19. 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

20. 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

21. 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

22. 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

24. 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

26. 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

28. Proximal tubule
Distal tubule
NaCl
Nutrients
H2O
HCO3–
H2O K+
NaCl
HCO3– H+
NH3 K+ H+
CORTEX
Descending limb
of loop of
Henle
Thick segment
of ascending
limb
Filtrate
H2O
Salts (NaCl and others)
HCO3– H+
Urea
Glucose; amino acids
Some drugs
NaCl
H2O
OUTER
MEDULLA
NaCl
Thin segment
of ascending
limb
Collecting
duct
Key
Urea
Active transport
Passive transport
NaCl
H2O
INNER
MEDULLA

29. 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

30. STIMULUS osmoreceptor cells in the hypothalamus detect an increase in
Osmoreceptors
in hypothalamus
Thirst
Hypothalamus
Drinking reduces
blood osmolarity
to set point
ADH
Increased
permeability
Pituitary
gland
Distal
tubule
H2O reab-
sorption helps
prevent further
osmolarity
increase
STIMULUS
osmoreceptor cells in
the hypothalamus
detect an increase in
the osmolarity
of the blood
Collecting duct
Homeostasis:
Blood osmolarity

32. 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

33. The renin-angiotensin-aldosterone system (RAAS) is part of a complex feedback circuit that functions in homeostasis
Purpose: Maintain blood volume and pressure

34. apparatus (JGA) responds
Homeostasis:
Blood pressure,
volume
Increased Na+
and H2O reab-
sorption 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

35. 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.