Principles of fluid flow in vertebrates I:

Comparative circulations(BMS107 L13 2010)

Sarah Etheringtons.etherington@murdoch.edu.au

Tel: 9360 6708VB 2.042

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Learning Objectives

1. Explain the advantages of a closed circulatory system in vertebrates

2. Understand and briefly describe the evolutionary process of the separation of the pulmonary circulation from the systemic circulation.

3. Provide examples of the different types of circulations (fishes, reptiles and mammals) including the varied anatomy of the heart

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Lecture Outline:Comparative circulations

Introduction to circulatory systemsIntroduction to circulatory systems• Circulatory systems transport nutrients, waste and

signalling molecules between body tissues

• Some very small organisms lack circulatory systems, relying on diffusion to transport molecules

• Diffusion is rapid over small distances, but very slow over large distances (no good unless you’re small, flat or porous…)

• Large animals move fluid through their bodies by bulk flow

BULK FLOW = MOVEMENT OF FLUID AS A RESULT OF A PRESSURE GRADIENT

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(1) Components of circulatory systems

Components of Circulatory SystemsComponents of Circulatory Systems

• Circulatory systems move fluids by generating pressure in one part of the circuit

• Fluid flows through the body from high pressure to low pressure

• Circulatory systems need three main components• Pump

• System of tubes

• Fluid

Types of pumpsTypes of pumps

• Muscle contracts, increases pressure inside vessel

• Muscle can be either inside or outside of the chamber

• Valves required to ensure unidirectional blood flow

• Rhythmic waves of contraction move along the blood vessel

• Only seen at embryonically in vertebrates

CONTRACTILE CHAMBER

EXTERNAL PUMP

PERISTALTIC PUMP

• All pumps involve muscle contraction

B

Types of channelsTypes of channelsOPEN CIRCULATORY SYSTEM• circulatory fluid comes in

direct contact with the tissues in spaces called sinuses

CLOSED CIRCULATORY SYSTEM • circulatory fluid remains

within the blood vessels and does not come in direct contact with the tissues

• extracellular fluid separated into plasma and interstitial fluid compartments

C

D

E

Types of fluidTypes of fluid• Hemolymph – fluid that circulates within an open

circulatory system (single ECF compartment)• Blood – fluid that circulates within a closed circulatory

system (plasma + cells)• Interstitial fluid – extracellular fluid that directly bathes

the tissues• Lymph – fluid that circulates in the secondary system of

vertebrates called the lymphatic system

LYMPHINTERSTITIAL FLUID

PLASMAF G H

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(2) Evolution of vertebrate circulatory systems

VERTEBRATES

HAGFISH

FISH

LUNGFISH

REPTILES (MOST) BIRDSMAMMALS

AMPHIBIANS

CROCODILES

Key players in evolution of the vertebrate cardiovascular system

I

Evolution of circulatory systems - overview

• First evolved to transport nutrients

• Very early on began to serve a respiratory function

• O2 is the nutrient most tissues can’t do without for long

• Consequently, O2 requirements typically determine blood circulation requirements of an organism

• There is an evolutionary correlation between metabolic demands and circulatory capacity for oxygen transport

Can you think of any environmental and/or physiological changes that would significantly increase the oxygen delivery

requirements of an organism?

Common features of vertebrate circulatory systems

• Closed circulatory system

• General plan involves two or more contractile chambers of myocardial tissue, with valves to ensure unidirectional blood flow

• Two or more heart chambers

• Progressive increase in separation of blood flow to gas Progressive increase in separation of blood flow to gas exchange organs and blood flow to rest of body exchange organs and blood flow to rest of body

GOAL FOR THE LECTUREBe able to describe the increasing (1) number of heart

chambers and (2) separation of lung/body blood flow through the circulatory system in the evolution of vertebrates [fish,

lungfish, amphibians, reptiles, crocodilians birds and mammals].

(2a) All vertebrates have a closed circulatory system

• The circulatory systems of vertebrates consist of a heart and The circulatory systems of vertebrates consist of a heart and a closed system of vessels containing a closed system of vessels containing blood separate from blood separate from tissue fluid. tissue fluid.

Closed systems evolved

independently in some

invertebrates as a result of high oxygen demand or low oxygen

environment

BLOOD TOWARDS HEART

BLOOD AWAY FROM HEART

Advantages of a closed circulatory system

• Closed systems generally support higher levels of metabolic activity than open systems because:

1. Closed systems can generate higher pressures, blood flows more rapidly through vessels than interstitial space quicker nutrient & waste transport

2. Resistance in blood vessels can be changed blood flow can be more tightly regulated and more easily redirected to specific tissues

3. Cellular elements and transport molecules kept within vessels (e.g haemoglobin)

(2b) All vertebrate hearts generate (2b) All vertebrate hearts generate force through contraction of myocardial force through contraction of myocardial

tissuetissue• Two different types of myocardium:

• Spongy – meshwork of loosely connected cells• Compact – tightly packed cells arranged in a regular pattern

• Relative proportions vary among species

COMPACT MYOCARDIU

MSPONGY

MYOCARDIUM

COMPACT MYOCARDIU

M

ENDOCARDIUM

ENDOCARDIUM

J

K

(2c) The number of heart chambers has increased during vertebrate evolution• Vertebrate hearts are the first to have two chambersVertebrate hearts are the first to have two chambers

• All vertebrates have at least one atrium and one ventricleAll vertebrates have at least one atrium and one ventricle

• Atrium Atrium = receiving chamber, = receiving chamber, ventricle ventricle = muscular pump= muscular pump

• The vertebrate heart evolved from two chambers in fish The vertebrate heart evolved from two chambers in fish to three in amphibians and reptiles and four in to three in amphibians and reptiles and four in crocodilians, mammals, and birds. crocodilians, mammals, and birds.

Fish Heart• First group to develop a multi-chambered heart

• Focal gas exchange (gills) requires more efficient circulatory system working at higher pressure

• Two chambers allow separate collection and pumping of blood

• Atrium: thin-walled sac

• Ventricle: muscular cavity

• Mostly spongy myocardium

• Emergence of polarised contraction (posterior anterior)

ATRIUM

VENTRICLE

L

• Ventricle located ventral to atrium (gravity helps blood flow)

• Most of pressure from ventricular contraction dissipated passing through gills (high resistance)

• Blood flowing to tissues is at relatively low pressure (but, good enough for tuna, marlin…)

Fish Circulation

M

Lungfish Heart• Evolved “lung-like” gut pocket and

vessel carrying blood from lung to heart

• Heart receives both O2-rich blood (from lung) and O2-poor blood (from tissue)

• Three-chambered heart (2 atria, 1 ventricle)

• Partial separation of oxygenated and deoxygenated blood increased respiratory efficiency

• Extent of chamber division varies between lungfishes, related to requirement for air breathing

• Mostly spongy myocardium

Is the heart above from an Australian (fish-like) or an

African lungfish (mostly air-breathing)? How did you

decide?

LEFT ATRIUM

RIGHTATRIUM

N

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• First separation of pulmonary (lung) and systemic (body) circulations

• Not complete separation of oxygen-rich and oxygen-poor blood, but in practice there is very little mixing

Lungfish Circulation

LUNG

TISSUESO

Amphibian Heart• Three-chambered heart (2 atria, 1 ventricle)

• Oxygenated blood from the lungs left atrium, de-oxygenated blood from tissues right atrium

• Two types of blood enter a single ventricle

• Suprisingly little mixing, mechanism unclear, trabeculae may help to maintain separation

• Mostly spongy myocardium Although amphibian heart is similar to lungfish heart,

they evolved separately

P

Amphibian Circulation• Partially separated pulmonary and systemic circuits

• Lung (high-resistance) no longer between heart and body higher systemic pressures

•Gas exchange in amphibians also occurs at skin, buccopharyngeal mucosa

• Only the pulmonary circuit has a separate venous return to heart.

RQ

S

Reptile Hearts (turtles, snakes, lizards)

•Three-chambered heart (2 atria, 1 ventricle)

•But, complex ventricular structure (3 sub-chambers divided by muscular ridges)

•Ridges separate flow of oxygenated and deoxygenated blood

•Mostly compact myocardium

•Two aortas rather than one

Ridges are small in turtles (significant mixing of

blood), large in lizards and snakes (efficient

separation)

T

Reptile Circulations(turtles, snakes, lizards)

• Left aorta takes oxygenated blood from left ventricle to body (as usual), right aorta takes blood from both ventricles to body• Reptile stops breathing contraction of blood vessels in lung increased resistance• Blood diverted from lungs into systemic circuit when not breathing

LUNG

TISSUESU

Crocodilian Heart• 4 chambered heart, two ventricles completely separated by septum (right ventricle is the new chamber)

• 2 aortas

• Blood bypasses lungs when animal submerged (pulmonary pressure increases, opens valve).

Valve closed when

breathing

Valve open underwater

TO BODYTO BODYTO LUNGS

TO LUNGS

V

Crocodilian Circulation

LUNG

TISSUES

Bypass circuit to “miss” the lungs if the animal is submerged

W

• Four chambered hearts with valves to prevent the backflow of blood. – Atria are thin-walled, ventricles are thick-

walled– Ventricles separated by intraventricular

septum

• Complete separation of oxygenated and de-oxygenated blood

• Very different pressures in pulmonary and systemic circuits

• Compact myocardium

Avian and Mammalian HeartsAvian and Mammalian Hearts

X

The full septa of avian and mammalian hearts evolved separately, increased metabolic

demands of endothermy likely driving force

RIGHT ATRIUM

LEFT ATRIUM

RIGHT VENTRICLELEFT VENTRICLE

2. To the lungs

1. From the body3. From the lungs

4. To the body

Blood flow through bird and Blood flow through bird and mammalian heartsmammalian hearts

Avian and Mammalian CirculationsAvian and Mammalian Circulations

Y Z

Advantages of separate pulmonary and systemic circulations

1) Oxygenated and deoxygenated blood cannot mix - meaning systemic circulation always receiving blood with highest O2 content

2) Maximises respiratory gas exchange

3) Pulmonary and systemic circuits can operate under different pressures.

But, incomplete separation is not necessarily a disadvantage…

no benefit sending blood to lungs if you’re not breathing!

IMPROVED RESPIRATORY

/ CARDIOVASCU

LAR EFFICIENCY

VERTEBRATES

HAGFISH

FISH

LUNGFISH

REPTILES (MOST) BIRDSMAMMALS

AMPHIBIANS

CROCODILES

Summary of heart chamber evolution

AA

Summary of vertebrate circulatory system evolution

BB

(2d) Other evolutionary changes in cardiovascular system

(1) SPECIALISED ELECTRICAL CONDUCTION(1) SPECIALISED ELECTRICAL CONDUCTION• Pacemaker cells present very early in evolution Pacemaker cells present very early in evolution rhythmic activity rhythmic activity

(e.g. peristalsis in chordates)(e.g. peristalsis in chordates)

• Fish and amphibia first to demonstrate ordered contraction (AV node Fish and amphibia first to demonstrate ordered contraction (AV node apex apex base), no specialised conduction tracts but trabeculae base), no specialised conduction tracts but trabeculae contribute to ordercontribute to order

• Mammals have specialised conduction pathways (Bundle of His, Mammals have specialised conduction pathways (Bundle of His, Purkinje fibres)Purkinje fibres) better coordinated contraction better coordinated contraction

(2) MYOCARDIAL CELL REPLICATION(2) MYOCARDIAL CELL REPLICATION• The ability to efficiently replace lost myocardial cells in adulthood

disappeared around the appearance of endothermy (warm-bloodednes)

• Mammals cannot generate significant numbers of new ventricular myocytes after birth

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Evolution of cardiovascular systems in vertebrates - summary

TYPE OF MYOCARDIU

M

NUMBER OF HEART

CHAMBERS

NUMBER OF AORTAS

SEPARATION OF

PULMONARY & SYSTEMIC

CIRCULATIONS

FISH Spongy 2 1 None

LUNGFISH Spongy 3 1 Partial

AMPHIBIA Spongy 3 1 Partial

REPTILES Compact 3 (complex ventricle)

2 Partial (bypass)

CROCODILIANS

Compact 4 2 Partial (bypass)

BIRDS AND MAMMALS

Compact 4 1 Complete

• Circulatory systems require a pump, tubes and a fluid. In the vertebrate cardiovascular system, the heart is the pump, blood vessels are the tubes and blood is the fluid.

• The main function of the cardiovascular system is transport of nutrients (including oxygen) and waste.

• Oxygen availability is crucial for vertebrate tissues, thus evolution of the vertebrate cardiovascular system has been strongly influenced by changes in oxygen availability or oxygen requirements (e.g. move from aquatic to terrestrial environment, shift from dispersed to focal gas exchange, shift from cold- to warm-bloodedness)

• Evolution of the vertebrate cardiovascular system has involved (1) a shift from spongy to compact myocardium, (2) an increase in the number of heart chambers and (3) increasing separation of the pulmonary and systemic circulations 37

Lecture Summary

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Test your understanding•Why is the two-chambered fish heart more efficient than the one-chambered hearts of their chordate ancestors?

•List three advantages to the organism of having separate pulmonary and systemic circulations.

• Briefly describe three differences between the cardiovascular systems (including hearts) of amphibia and mammals.

• Using a specific example, briefly explain the role of oxygen requirements in the evolution of vertebrate cardiovascular systems.

• How has the structure of myocardial tissue changed throughout vertebrate evolution?

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