Homeostatic Mechanisms 2
Reminder: Evolution is a Tinkerer
Evolution is limited by historical constraints.
It should be expected that the evolution of
physiological systems demonstrates common
ancestry between different groups of organisms.
Example 1: Circulatory Systems
The Evolutionary Point
Circulatory systems are the major way that
material are exchanged between the cells of the
body and the environment (by way of organs).
Capillaries are the major vessel where material
is exchanged.
Overall, there is an evolution in complexity to
supply organisms with more efficient circulation,
as required by increasing metabolic needs.
Exchange of materials
• Animal cells exchange material across their
cell membrane
– fuels for energy
– nutrients
– oxygen
– waste (urea, CO2)
• If you are a 1-cell organism that’s easy!
– diffusion
• If you are many-celled that’s harder
In circulation…
• What needs to be
transported
– nutrients & fuels
• from digestive system
– respiratory gases
• O2 & CO2 from & to gas
exchange systems
– intracellular waste
• waste products from
cells: water, salts,
nitrogenous wastes
– protective agents
• immune defenses
– regulatory molecules
• hormones
Circulatory systems
• All animals have:
– circulatory fluid = “blood”
– tubes = blood vessels
– muscular pump = heart
open
hemolymph
closed
blood
Open circulatory system
• Taxonomy
– invertebrates
• insects,
arthropods,
mollusks
• Structure
– no separation
between blood &
interstitial fluid
• hemolymph
Closed circulatory system
• Taxonomy
– invertebrates
• earthworms, squid,
octopuses
– vertebrates
• Structure
– blood confined to vessels
& separate from
interstitial fluid
• 1 or more hearts
• large vessels to smaller
vessels
• material diffuses
between blood vessels
& interstitial fluid
closed system = higher pressures
Vertebrate circulatory system
• Adaptations in closed system
– number of heart chambers differs
2
low
pressure
to body
3
4
low O2
to
body
high
pressure
& high O2
to body
What’s the adaptive value of a 4 chamber heart?
4 chamber heart is double pump = separates oxygen-rich &
oxygen-poor blood; maintains high pressure
Evolution of vertebrate circulatory system
AMPHIBIANS
REPTILES (EXCEPT BIRDS)
MAMMALS AND BIRDS
Lung and skin capillaries
Lung capillaries
Lung capillaries
FISHES
Gill capillaries
Artery
Pulmocutaneous
circuit
Gill
circulation
Heart:
ventricle (V)
A
Atrium (A)
Systemic
circulation
Vein
Systemic capillaries
A
V
Left
Right
Systemic
circuit
Systemic capillaries
Right
systemic
aorta
Pulmonary
circuit
Pulmonary
circuit
Left
Systemic
Birds AND
aorta
V
V
Right mammals!
Left
Wassssup?!
A
A
Systemic capillaries
A
V
Right
A
V
Left
Systemic
circuit
Systemic capillaries
Evolution of 4-chambered heart
• Selective forces
– increase body size
• protection from predation
• bigger body = bigger stomach
– endothermy
• can colonize more habitats
– flight
• decrease predation & increase hunting
• Effect of higher metabolic rate
– greater need for energy, fuels, O2, waste
removal
• endothermic animals need 10x energy
• need to deliver 10x fuel & O2 to cells
convergent
evolution
Blood vessels
arteries
veins
artery
venules
arterioles
arterioles
capillaries
venules
veins
Exchange across capillary walls
Fluid & solutes flows out
of capillaries to tissues
due to blood pressure
Lymphatic
capillary
Interstitial fluid flows
back into capillaries
due to osmosis
 plasma proteins  osmotic
• “bulk flow”
pressure in capillary
BP > OP
BP < OP
Interstitial
fluid
What about
edema?
Blood
flow
85% fluid returns
to capillaries
Capillary
Arteriole
15% fluid returns
via lymph
Venule
Mammalian
circulation
systemic
pulmonary
systemic
What do blue vs. red areas represent?
Mammalian heart
to neck & head
& arms
Coronary arteries
Coronary arteries
bypass surgery
Heart valves
• 4 valves in the heart
– flaps of connective tissue
– prevent backflow
SL
• Atrioventricular (AV) valve
– between atrium & ventricle
– keeps blood from flowing back
into atria when ventricles contract
• “lub”
• Semilunar valves
– between ventricle & arteries
– prevent backflow from arteries into
ventricles while they are relaxing
• “dub”
AV
AV
Lub-dub, lub-dub
• Heart sounds
– closing of valves
– “Lub”
• recoil of blood against
closed AV valves
– “Dub”
• recoil of blood against
semilunar valves
SL
AV
AV
• Heart murmur
– defect in valves causes hissing sound when stream of
blood squirts backward through valve
Cardiac cycle
• 1 complete sequence of pumping
– heart contracts & pumps
– heart relaxes & chambers fill
– contraction phase
• systole
• ventricles pumps blood out
– relaxation phase
• diastole
• atria refill with blood
systolic
________
diastolic
pump
(peak pressure)
_________________
fill (minimum pressure)
110
____
70
The control of heart rhythm
1 Pacemaker generates
2 Signals are delayed
wave of signals
to contract.
SA node
(pacemaker)
3 Signals pass
to heart apex.
at AV node.
AV node
throughout
ventricles.
Bundle
branches
Heart
apex
ECG
4 Signals spread
Purkinje
fibers
Measurement of blood pressure
Pressure
in cuff
above120
Rubber cuff
inflated
with air
Artery
120
Pressure
in cuff
below 120
Blood pressure
Reading: 120/170
Pressure
in cuff
below 70
120
70
Sounds
audible in
stethoscope
Artery
closed
• High Blood Pressure (hypertension)
– if top number (systolic pumping) > 150
– if bottom number (diastolic filling) > 90
Sounds
stop
Differentiation of blood cells
Pluripotent stem cells
(in bone marrow)
Lymphoid
stem cells
Myeloid
stem cells
Basophils
B cells
T cells
Lymphocytes
Eosinophils
Neutrophils
Erythrocytes
Platelets
Monocytes
Blood clotting
2 The platelets form a
1 The clotting process begins
plug that provides
emergency protection
against blood loss.
when the endothelium of a
vessel is damaged, exposing
connective tissue in the
vessel wall to blood. Platelets
adhere to collagen fibers in
the connective tissue and
release a substance that
makes nearby platelets sticky.
3
This seal is reinforced by a clot of fibrin when
vessel damage is severe. Fibrin is formed via a
multistep process: Clotting factors released from
the clumped platelets or damaged cells mix with
clotting factors in the plasma, forming an
activation cascade that converts a plasma protein
called prothrombin to its active form, thrombin.
Thrombin itself is an enzyme that catalyzes the
final step of the clotting process, the conversion of
fibrinogen to fibrin. The threads of fibrin become
interwoven into a patch (see colorized SEM).
Collagen fibers
Platelet releases chemicals
that make nearby platelets sticky
Platelet
plug
Fibrin clot
Clotting factors from:
Platelets
Damaged cells
Plasma (factors include calcium, vitamin K)
Prothrombin
Thrombin
Fibrinogen
Fibrin
5 µm
Red blood cell
Atherosclerosis
Connective
tissue
Smooth muscle
Endothelium
(a) Normal artery
50 µm
Plaque
(b) Partly clogged artery
250 µm
Correlated to high fat diets.
Evolutionary pressures drive these food preferences.
Coronary Embolism
A blockage of blood flow
through the arteries that
supply cardiac muscle
with blood.
Damage is irreparable.
A function of the serial
blood circulation through
the heart (thanks
evolution!).
Aneurysm (cerebral pictured)
Due to a weakening of
the walls of particular
blood vessels.
Can occur anywhere.
Quick Check: Make Sure You Can
1. Explain the functions of the circulatory
system in animal physiology.
2. Describe evolutionary trends in circulatory
systems, and the reasons for those trends.
3. Label all parts of the mammalian heart and
diagram blood flow through it.
4. Explain the causes of circulatory system
disruptions and how disruptions of the
circulatory system can lead to disruptions of
homeostasis.
Example 2: Excretory Systems
The Evolutionary Point
Excretory systems pass water and waste solutes
into tubules from interstitial fluids and blood.
Evolution of excretory systems demonstrates
increasing ability to control filtration &
reabsorption of water and waste products, as
well as adaptations to the environment.
Flatworms: Nephridia
Waste (and water) is
pumped from the
(open)circulatory system
into tubules that line to
body
The waste is then
excreted.
Flatworms excrete very
dilute waste solutions
(they live in freshwater).
Earthworms: Metanephridia
Waste material and water
move from capillaries into
collecting tubules.
Water and solutes are
selectively reabsorbed by
the epithelial cells that line
the tubules.
The concentrated filtrate is
then excreted through
pores.
Arthropods: Malpighian Tubules
Malpighian tubules pump nitrogenous waste and ions
into the digestive tract of arthropods.
The uric acid precipitates as a solid (excreted through
the rectum, and most of the water is reabsorbed.
Vertebrates: Kidneys
Vertebrates have paired kidneys
Blood is filtered into nephrons.
Solutes are then reabsorbed
(along with water).
Urine of varying concentrations
collects in the bladder and is
then excreted through the
urethra
Quick Check: Make Sure You Can
1. Describe evolutionary trends in circulatory
systems, and the reasons for those trends.
Example 3: Osmoregulation in Plants
The Evolutionary Point
The transport of water from roots to shoots is
necessary for continuing photosynthetic activity.
The transport of sugars from leaves to the rest
of the plant is necessary for continuing
respiration.
Osmoregulation in roots and shoots is crucial.
The evolution of vascular tissue was necessary
for transport in all plants larger than bryophytes
(mosses, liverworts, etc.)
Bryophytes Don’t Have Vascular Tissues
Why? Huh?
Transport in plants
• H2O & minerals
– transport in xylem
– Transpiration
• Adhesion, cohesion &
Evaporation
• Sugars
– transport in phloem
– bulk flow
• Gas exchange
– photosynthesis
• CO2 in; O2 out
• stomates
– respiration
• O2 in; CO2 out
• roots exchange gases
within air spaces in soil
Why does
over-watering
kill a plant?
Ascent of xylem fluid
Transpiration pull generated by leaf
Water & mineral absorption
• Water absorption from soil
– osmosis
– aquaporins
• Mineral absorption
– active transport
– proton pumps
• active transport of H+
aquaporin
root hair
proton pumps
H2O
Mineral absorption
• Proton pumps
– active transport of H+ ions out of cell
• chemiosmosis
• H+ gradient
– creates membrane
potential
• difference in charge
• drives cation uptake
– creates gradient
• cotransport of other
solutes against their
gradient
Water flow through root
• Porous cell wall
– water can flow through cell wall route (apoplastic)
& not enter cells (symplastic)
– plant needs to force water into cells
Casparian strip
Controlling the route of water in root
• Endodermis
– cell layer surrounding vascular cylinder of root
– lined with impermeable Casparian strip
– forces fluid through selective cell membrane
• filtered & forced into xylem cells
Aaaah…
Structure–Function
yet again!
Root anatomy
dicot
monocot
Mycorrhizae increase absorption
• Symbiotic relationship between fungi & plant
– symbiotic fungi greatly increases surface area for
absorption of water & minerals
– increases volume of soil reached by plant
– increases transport to host plant
Mycorrhizae
Transport of sugars in phloem
• Loading of sucrose into phloem
– flow through cells via plasmodesmata
– proton pumps
• cotransport of sucrose into cells down
proton gradient
Pressure flow in phloem
• Mass flow hypothesis
– “source to sink” flow
• direction of transport in phloem is
dependent on plant’s needs
– phloem loading
• active transport of sucrose
into phloem
• increased sucrose concentration
decreases H2O potential
– water flows in from xylem cells
• increase in pressure due to increase in
H2O causes flow
On a plant…
What’s a source…What’s a sink?
can flow
1m/hr
Experimentation
• Testing pressure flow hypothesis
– using aphids to measure sap flow &
sugar concentration along plant
stem
Don’t get mad…
Get answers!!
Ask Questions!
Quick Check: Make Sure You Can
1. Explain the functions of the transport system
of plants.
2. Describe evolutionary trends in plant
transport systems and the reasons for those
trends
3. Explain the mechanics of transpiration and
pressure-flow.