Circulatory systems – general questions

• What is a circulatory system?• Do all animals have circulatory systems?• Why do animals have circulatory systems?• What kinds of animals lack circulatory systems?

Rate of diffusion• Diffusion is slow over long distances• Einstein’s diffusion equation

t α x2

Distance Time required

100m 5 seconds

200m 20 seconds

500m 2 minutes

1 mm 8 minutes

1 m 16 years

Limits of Diffusion

• Unicellular organisms and some small metazoans lack circulatory systems and rely on diffusion to transport molecules

• Diffusion can be rapid over small distances, but is very slow over large distances

• Rather than rely on diffusion, large animals move fluid through their bodies by bulk flow, or convective transport

• Convection mechanisms are needed to move moleculesrapidly over large distances

• Movement of fluid molecules as a group• Requires input of energy• Physiologists use the symbol Q to represent flow in circulatory

systems• You may also see the symbol Q, the dot indicates that this is a rate

process

Convection systems: speed has a cost

• Diffusion is ‘free’, convection is not• Vertebrate hearts cost ~ 1-4% of resting metabolism• lung and gill ventilation cost ~ 1-10% of resting metabolism

What are the roles for convection systems?1.To carry gases (O2, CO2 and others) to and from tissues2.To carry nutrients, wastes & hormones between organs3.To provide structural support & locomotion4.To provide filtration

Fundamental Components• Fluid: water; cytoplasm; lymph; hemolymph; blood• Energy: ATP kinetic energy via potential energy• Conduits: cellular spaces; sinuses; vessels; circuits

Components of Circulatory Systems

• Circulatory systems move fluids by increasing the pressure of the fluid in one part of the body

• The fluid flows through the body down the resulting pressure gradient

• Three main components are needed• Pump or propulsive structures• A system of tubes, channels, or spaces• A fluid that circulates through the system

Different needs for bulk flow

Different types of pumps

• Skeletal muscle• Pulsating blood vessels

peristalsis• Chambered hearts

One way valves help to ensure unidirectional flow

Figure 9.2

Different types of circulations

• Open - circulatory fluid comes in direct contact with the tissues in spaces called sinuses

• Closed – circulatory fluid remains within the blood vessels and does not come in direct contact with the tissues

Types of Fluid

• Interstitial fluid – extracellular fluid that directly bathes the tissues

• Blood – fluid that circulates within a closed circulatory system

• Lymph – fluid that circulates in the secondary system of vertebrates called the lymphatic system

• Hemolymph – fluid that circulates within an open circulatory system

Evolution of Circulatory Systems

• First evolved to transport nutrients• Very early on they began to serve a respiratory function• Closed systems evolved independently in jawed

vertebrates, cephalopods, and annelids• Closed systems evolved in combination with specialized

oxygen carrier molecules

Evolution of Circulatory Systems, Cont.

Intracellular convection systems

• All cells & unicellular animals• Microfilaments & microtubules move organelles & DNA

Animals That Lack Circulatory Systems

• Sponges, cnidarians, and flatworms• Lack true circulatory systems, but have mechanisms for

propelling fluids around their bodies• Cilia: sponges and flatworms• Muscular contractions: cnidarians

Bulk flow using cilia

A choanocyte cell

water +suspendednutrients

Law of Bulk Flow

Q= P/R

Fundamental physical law exploited by circulatory systems, respiratory systems, excretory systems, digestive systems etc.

Bulk Flow

Bulk Flow

• Hemolymph = plasma (water, ions, proteins, nutrients, hormones, etc.)+ respiratory pigment+ white blood cells

• Blood = plasma + red/pink blood cells + white blood cells

Invertebrate respiratory pigments: intracellular & extracellular1.Red blood cell hemoglobins (Fe):

Some molluscs, annelids, echinoderms2.Pink blood cell hemerythrins (amino acids):

Some marine worms: annelids, sipunculids3.Extracellular hemoglobins & chlorocruorin (Fe):

Some annelids; bivalve molluscs, arthropods4.Molluscan extracellular hemocyanin (Cu):

Cephalopods, gastropods & some bivalves5.Arthropod extracellular hemocyanin (Cu):

Decapod crustaceans

Invertebrate circulatory fluids

All arthropods have an open circulatory system

• Insects: Highly successful terrestrial animals & capable of high • metabolic rates

Role of circulatory system: • To deliver nutrients, immune cells & hormones• Tracheal system moves respiratory gases directly to tissues)• Multiple, segmental hearts• Large dorsal vessel• Sluggish flow rates• Accessory pumps: wings, limbs

All arthropods have an open circulatory system

• Crustaceans: Highly successful aquatic animals & decapods are capable of high metabolic rates

Decapod circulatory system: • Respiratory function, respiratory pigments, gills• Large ostial heart• Many branching outflow vessels• Small tissue sinuses

The decapod crustacean heart

• The ostial heart sits in a pericardial sac that contains hemolymphCardiac cycle• Cardiac muscles contract in unison,

ejecting hemolymph into arteries &stretching suspensory ligaments

• Cardiac muscles relax, recoil of suspensory ligaments expands theheart chamber, drawing hemolymphfrom the sinus via the ostia into theheart

Cardiac control• Neurogenic: CNS controls heart rate &

force of contraction (stroke volume)• Neural control of arterial sphincters

Crayfish cardiac cycle - systole

• Heartbeat is initiated in neurons of cardiac ganglion

• (neurogenic heart)• Neural signal closes ostia• Causes cardiomycocytes to

contract• Volume of heart chamber

decreases, pressure increases

• Blood exits via arteries• Suspensory ligaments are

stretched

Crayfish cardiac cycle - diastole

• When neural signal is absent, cardiomyocytes relax

• Releases tension on suspensory ligaments

• Ligaments spring back• Pulls on the walls of the

heart, increasing volume of heart chamber

• Reduces pressure inside heart

• Ostia open, blood is sucked into the heart

Annelids have open & closed circulatory systems

Three Main Classes• Polychaeta – tube worms• Oligochaeta – earth worms• Hirudinea – leeches

•Most also have vessels that circulate fluid with oxygen carriers•Can be an open (polychaetes) or closed (oligochaetes) systems•Aquatic and terrestrial animals that have low metabolic rates & can live in hypoxic environments

Pumping systems• Chambered heart• Body wall movements ie muscular contractions• Cilia

Earthworms• Five, segmental contractile tubes• Segmental connecting vessels

Molluscs have open & closed circulatory systems

• Bivalves: Sedentary & low metabolic rates• Cephalopods: High locomotory abilities (except Nautilus)

Cephalopod circulatory system• Myogenic, chambered heart• Systemic heart pumps oxygenated

blood to tissues• Paired branchial hearts pump

deoxygenated blood to gills• Coronary arteries • Distributional arteries

Common features• Muscular ‘hearts’• Unidirectional flow, one-way ‘valves’ • Large distribution vessels away from (artery) & to (veins) the heart• Hemolymph in which respiratory pigments may be dissolved

Benefits • No building and maintenance costs• Low resistance to flow b/c there are no small distribution vessels• Lower pump pressures needed for flow saves energy• Hemolymph readily bathes most cells• Large hemolymph volume

Disadvantages • Limited control of blood flow distribution except at the arterial level• Slow circulation/response times• Body movements greatly influence blood pressure & flow

Open vs closed circulatory systems in invertebrates