Excretory System

Purpose: Maintain homeostasis by

regulating water balance and by

removing harmful substances

(nitrogenous waste from protein

digestion)

Nitrogenous Waste Removal

1. Aquatic: excrete NH3 – ammonia –

highly toxic but very soluble – FISH

2. Land: Urea – less toxic but less water

soluble – requires more water for

removal

3. Uric Acid: insoluble in water – forms a solidand requires little water for removal – less likely to seep into tissues since its insoluble – forms a pastelike substance which requires less water for excretion – good for BIRDS and REPTILES

– keeps waste from poisoning eggs during development and keeps birds from having extra water waste which would weigh them down for flying

Proteins Nucleic acids

Amino acids Nitrogenous bases

–NH2

Amino groups

Most aquatic

animals, including

most bony fishes

Mammals, most

amphibians, sharks,

some bony fishes

Many reptiles

(including

birds), insects,

land snails

Ammonia Urea Uric acid

NH3NH2

NH2

O C

C

CN

C

ON

H H

C O

NC

HN

O

H

Osmoregulation: absorption and secretion of

water and solutes to maintain proper water balance

of an organism and its surroundings

1. Marine Fish:

- Hypotonic tissues: more solute outside of

the body

- Flow of water out of the fish dehydrate

Responses:

- constant drinking

- infrequent urination

- excretion of salt from gills

Gain of water and

salt ions from food

and by drinking

seawater

Osmotic water loss

through gills and other parts

of body surface

Excretion of

salt ions

from gills

Excretion of salt ions

and small amounts

of water in scanty

urine from kidneys

2. Fresh Water Fish:

- Hypertonic tissues: more solute in the body

- flow of water into the tissues

Responses:

- little drinking

- frequent urination

- absorption of salt by gills

Uptake of

water and some

ions in food

Osmotic water gain

through gills and other parts

of body surface

Uptake of

salt ions

by gills

Excretion of

large amounts of

water in dilute

urine from kidneys

Excretory Mechanisms

1. Contractile Vacuole: Protists

- vacuoles that accumulate water and

then merge with the plasma

membrane and release the water

outside of the cell

Video #2

2. Flame Cells:

Platyhelminthes (Planaria)

- Clusters of cilia moving body fluids

through a system of tubes

- Wastes in tube exit out of the body by

pores (nephridiopores)

Nucleus

of cap cell

Cilia

Interstitial fluid

filters through

membrane where

cap cell and tubule

cell interdigitate

(interlock)

Tubule cell

Flame

bulb

Nephridiopore

in body wall

Tubule

Protonephridia

(tubules)

3. Nephridia (metanephridia):

Annelids

- pairs of tube systems in each segment

- Takes in interstitial fluid through the nephrostome (ciliated intake)

- Selective secretion of materials as fluid passes through collecting tube

- Retained materials pass into coelom and into capillaries for circulation

- Concentrated wastes exit through the excretory pore (nephridiopore)

Nephrostome Metanephridia

Nephridio-

pore

Collecting

tubule

Bladder

Capillary

network

Coelom

4. Malpighian Tubules: Insects

- System of tubes attached to the mid-

gut of the digestive system

- Collect materials from the hemolymph

and deposit them into the digestive

system – open circulatory system

- Digestive system absorbs materials

while wastes are excreted

Digestive tract

Midgut

(stomach)

Malpighian

tubules

Rectum

IntestineHindgut

Salt, water, and

nitrogenous

wastes

Feces and urineAnus

Malpighian

tubule

Rectum

Reabsorption of H2O,

ions, and valuable

organic molecules

HEMOLYMPH

5. Kidney: absorbs water and nutrients from the

blood and concentrates urea for excretion

Kidney Structure:

Cortex: lowest osmolarity in interstitial fluid (allows for the absorption of materials) –outermost layer of kidney

Medulla: highest osmolarity – allows for the retention of water – inner portion of kidney

Renal Pelvis: collects urea and wastes

Contains NEPHRONS: tube for absorption and secretion – functional unit of the kidney

(b) Kidney structure

UreterSection of kidney from a rat

Renal

medulla

Renal

cortex

Renal

pelvis

Figure 44.13b

Juxta-

medullary

nephron

Cortical

nephron

Collecting

duct

To

renal

pelvis

Renal

cortex

Renal

medulla

20 µm

Afferent

arteriole

from renal

arteryGlomerulus

Bowman’s capsule

Proximal tubule

Peritubular

capillaries

SEM

Efferent

arteriole from

glomerulus

Branch of

renal vein

Descending

limb

Ascending

limb

Loop

of

Henle

Distal

tubule

Collecting

duct

(c) Nephron

Vasa

recta(d) Filtrate and

blood flow

Kidney Filtration Facts

Each Minute the pair of kidneys

Filters 1200 cc (mL) of blood

Which makes 125 cc of filtrate

Of which 124 cc are reclaimed

Making 1 cc of urine

“The majority of sources that I found reported that the

adult bladder could contain about 600 to 800 cm3 (ml).

However, they also noted that the Micturition point is

between 150 and 300 cm3 (ml).” - Daniel Shaw – 2001

http://hypertextbook.com/facts/2001/DanielShaw.shtml

Structure of Nephron/Path of Filtrate

Cortex:

1. Bowman’s Capsule

2. Proximal Tubule

Medulla

3. Descending Loop of Henle

4. Ascending Loop of Henle

Cortex

5. Distal Tubule

6. Collecting Duct

Juxta-

medullary

nephron

Cortical

nephron

Collecting

duct

To

renal

pelvis

Renal

cortex

Renal

medulla

20 µm

Afferent

arteriole

from renal

arteryGlomerulus

Bowman’s capsule

Proximal tubule

Peritubular

capillaries

SEM

Efferent

arteriole from

glomerulus

Branch of

renal vein

Descending

limb

Ascending

limb

Loop

of

Henle

Distal

tubule

Collecting

duct

(c) Nephron

Vasa

recta(d) Filtrate and

blood flow

Path of Blood Vessels

1. Renal Artery

2. Bowman’s Capsule and Glomerulus

(cluster of capillaries)

3. Proximal and Distal Tubules

4. Ascending Loop of Henle

5. Descending Loop of Henle

6. Renal Vein

Juxta-

medullary

nephron

Cortical

nephron

Collecting

duct

To

renal

pelvis

Renal

cortex

Renal

medulla

20 µm

Afferent

arteriole

from renal

arteryGlomerulus

Bowman’s capsule

Proximal tubule

Peritubular

capillaries

SEM

Efferent

arteriole from

glomerulus

Branch of

renal vein

Descending

limb

Ascending

limb

Loop

of

Henle

Distal

tubule

Collecting

duct

(c) Nephron

Vasa

recta(d) Filtrate and

blood flow

Counter Current Flow

Cortex:

1. Bowman’s Capsule

2. Proximal Tubule

Medulla

3. Descending Loop of Henle

4. Ascending Loop of Henle

Cortex

5. Distal Tubule

6. Collecting Duct

1. Renal Artery

2. Bowman’s Capsule and Glomerulus (cluster of capillaries)

3. Proximal and Distal Tubules

4. Ascending Loop of Henle

5. Descending Loop of Henle

6. Renal Vein

NOTE: Around the Loop of Henle:

Blood flows in the opposite direction of

the filtrate inside the loop

Excretion Process

1. Bowman’s Capsule:- End of the nephron surrounding a cluster of

capillaries (glomerulus)

- Afferent arteriole enters the Bowman’s capsule to form the glomerulus and leaves as the efferent arteriole

- Bowman’s capsule absorbs water, sugar, salts and wastes from the glomerulus due to the pressure forcing the materials out

- Larger components of blood (RBC’s, platelets, large proteins) remain in the blood vessel

2. Proximal Convoluted Tubule (PCT):

- Secretion and absorption of materials by the kidney

Kidney secretes H+ ions, ammonia into the PCT – maintains pH

- the materials become part of the filtrate (solution inside of the nephron)

Capillaries around the PCT reclaim salt, bicarbonate, sugars and water

3. Descending Loop of Henle

- Permeable to water but not to salt

- Site of water reclamation

- Water is absorbed into the capillaries

- Filtrate becomes more concentrated

as the salt remains in the filtrate

4. Ascending Loop of Henle

- permeable to salt but not to water

- salt is reclaimed by the capillaries and

filtrate becomes less concentrated

IMPORTANCE: Counter current flow of

capillaries to filtrate path in the nephron

First, the blood in the capillary picks up salt by

active transport as it travels past the

ascending loop

This raises the osmolarity of the blood allowing

for the reabsorption of water as it flows up

the descending loop.

5. Distal Convoluted Tubule

- Regulates the K+ and the NaCl levels

6. Collecting Duct

- Allows for the reclamation of water and

salt to maintain balance in kidney

- carries the filtrate to the renal pelvis

which connects to the ureter which

empties into the bladder

- The bladder empties through the

urethra

The Urinary System Crash Course

Osmolarity Control of Urine Content:

1. Antidiuretic Hormone (ADH)

- produced in the hypothalamus

- stored and released in the pituitary

gland

- Loss of water (sweating, lack of intake)

increases the blood osmolarity

- Chemosensors in hypothalamus sense the

high levels and secrete ADH

- ADH travels to the distal tube and the

collecting duct making them MORE

permeable to water increasing the

absorption of water and concentrating the

urine (pee is darker)

Excess water in the blood (from drinking 3

nalgenes of water a day) causes little ADH

to be released so the uptake of water is

limited causing the urine to be dilute and

increased urination (diuresis)

ADH counters diuresis

Substances that increase diuresis are called

diuretics (caffeine and alcohol)

Osmoreceptors

in hypothalamus

Drinking reduces

blood osmolarity

to set point

H2O reab-

sorption helps

prevent further

osmolarity

increaseSTIMULUS:

The release of ADH is

triggered when osmo-

receptor cells in the

hypothalamus detect an

increase in the osmolarity

of the blood

Homeostasis:

Blood osmolarity

Hypothalamus

ADH

Pituitary

gland

Increased

permeability

Thirst

Collecting duct

Distal

tubule

2. Renin-Angiotensin-Aldosterone System

(RAAS): response to a drop in blood

pressure or volume

- Juxtaglomerular Apparatus responds by

releasing Renin

- Renin causes the blood protein

Angiotensin to be converted into

Angiotensin II which stimulates the

constriction of blood arterioles

- Blood vessel constriction narrows the diameter and increases pressure- WHY? Keep blood pumping to brain

- Angiotensin also stimulates the secretion of Aldosterone from the adrenal gland (located on top of the kidney)

- Aldosterone increases the absorption of NaCl and water which raises the blood volume and pressure

Increased Na+

and H2O reab-

sorption in

distal tubules

Homeostasis:

Blood pressure,

volume

STIMULUS:

The juxtaglomerular

apparatus (JGA) responds

to low blood volume or

blood pressure (such as due

to dehydration or loss of

blood)

Aldosterone

Adrenal gland

Angiotensin II

Angiotensinogen

Renin

production

Renin

Arteriole

constriction

Distal

tubule

JGA

RAAS is countered by the secretion of

Atrial Natriuretic Factor from the atrial

wall – this is a response to blood

pressure that is too high (pressure on

the atrial wall is the signal)

- ANF blocks the production of renin

- CRASH COURSE