gas exchange and circulation 2

what does the circulatory system do in large animals

carries transport tissues (blood or hemolymph) into close contact with

every cell in the body

“Close contact”

0.1 mm = short diffusion distance + large surface area, e.g. capillary beds

what does bulk transport require

But bulk transport requires a circulatory system + other things are

“circulated” e.g. nutrients, hormones, antibodies, (cancer cells, viruses)

where is open circulatory system found

arthropods, most molluscs

open circulatory system

• 1 or more (tubular) hearts + incomplete set of blood vessels

• Circulatory fluid = hemolymph; actively pumped around the body

• Hemolymph not confined to blood vessels; it flows into open sinuses

• Directly bathes tissues and organs directly (= interstitial fluid)

• Low pressure system = slow O2 delivery but low cost

• Body movements (e.g. wings, legs) contribute to pumping and return of

blood to the heart

where are closed circulatory systems found

Annelids, cephalopods, all vertebrates

closed circulatory system

- Circulatory fluid = blood (connective tissue!)

• Confined to blood vessels so separate from interstitial fluid which bathes tissues

• 1 (or more) hearts pump blood in closed series of blood vessels infiltrating tissues (as

capillaries)

• High blood pressure increases effectiveness, and regulation, of O2 delivery in larger, more active, animals

• But high cost

Capillaries

small and extremely thin-walled (1 cell thick); can be “fenestrated” (holes) – this is where exchange occurs

Arteries

carry blood from heart. tough, thick, elastic walls; recoil maintains blood pressure in capillaries when heart relaxes

Veins

carry blood back to heart, thinner with valves to prevent backflow during low pressure return of blood to the heart; compliant for storage of blood

Blood flow through capillary beds

allows for exchange of

gases, nutrients, wastes

some tissues have only…

5-10% of

their capillaries with blood flowing

through them (but closer to 100% in

brain, heart, kidneys)

blood supply is…

“redistributed” with

time (e.g. to skin for

thermoregulation, away from gut

during exercise)

blood flow is regulated by…

• Heart: amount of blood coming from arteries and arterioles

• Smooth muscle fibers in walls of arterioles + pre-capillary sphincters = peripheral circulation

what does high blood pressure in capillaries create

-interstital fluids and lymph

How do simple solutes (O₂, CO₂, sugars, and salts) move between blood and tissues?

-Simple solutes O2, CO2, sugars, salts diffuse across cells or through spaces between cells (“fenestre”) Movement is aided by bulk flow

How does fluid move in and out of capillaries?

• At arterial end of capillary blood pressure >>>> osmotic pressure – fluid

leaks out (20 L/day)

• At venous end osmotic pressure >>>> blood pressure (proteins

accumulate?) most (?) fluid moves back into capillary (17 L/day)

• Lymphatic system recovers any net fluid loss (~3 L/day)

lymphatic system

Separate set of vessels

running throughout the

body

• Returns lymph to

circulation (at major veins

entering the heart)

• Low pressure system

(valves)

• Flow of lymph aided by

limb movements

Evolution of the vertebrate circulatory system

Single circulation → double circulation

2-chambered heart → 4-chambered heart (completely divided ventricle)

single circulation

Closed circulatory system or cardiovascular system

in fish

amphibians

• 3-chambered heart + double circulation

• Pulmocutaneous circuit – lungs and skin; systemic circuit = body

• Incomplete separation (mixing) of O2ated/deO2ated blood in ventricle

• “Shunting” (redistribution) away from lungs when underwater

amphibians

• 3-chambered heart + double circulation

• Pulmocutaneous circuit – lungs and skin; systemic circuit = body

• Incomplete separation (mixing) of O2ated/deO2ated blood in ventricle

• “Shunting” (redistribution) away from lungs when underwater

reptiles (except birds)

• 3-chambered heart but increasingly partly divided by a septum

• Incomplete separation of blood in ventricle (non-crocodiles)

• Vessel b/t pulmonary/systemic circuit + 4-chambered heart (crocodiles)

• “Shunting” (redistribution) of blood during diving underwater

Mammals and birds

• 4-chambered heart with fully divided ventricle

• No possibility of shunting in adults (but fetus?)

• Very extensive capillary system; coronary circulation

• Convergent evolution in birds and mammals

Pulmonary circulation

relatively) low pressure

circuit to and from lungs

- pulmonary artery

- lung capillaries

- pulmonary vein

Systemic circulation

higher-pressure circuit

to and from the rest of

the body (providing O2,

nutrients to tissues)

- aorta

- capillaries

- superior/inferior vena

cava

mammal circulation overview

Blood flow through the mammalian 4-chambered heart:

a series of valves makes sure blood goes to the right place

Atrioventricular (AV) valves

separate the atrium and ventricle chambers

Semi-lunar valves

in main vessels: pulmonary and aortic valve

Cardiac cycle:

contraction phase (systole) of atria and

ventricles is closely coordinated with relaxation

(diastole) phase

Cardiac muscle cells are

specialised

electrically

coupled via gap junctions

what does Sequential electrical activation lead to

coordinated

contraction of the heart

electrical activation

1. Signal (yellow) initiated by pacemaker cells in sinoatrial (SA) node

2. Signal spreads over atria which contract

3. Signal delayed at AV node

4. Conducting fibers (bundles) rapidly pass signal to bottom of heart and

upwards through ventricles via Purkinje fibers; ventricle contracts

Patterns in blood pressure and blood flow

-Pulsing nature of blood pressure in arteries is due to heart beating

and elastic recoil in main arteries

-Number of capillaries is ENORMOUS and the TOTAL cross-sectional area of capillaries

>>> arteries and veins

-So mechanical resistance (friction) of large SA decreases blood pressure rapidly in

capillaries – but smooths blood flow

where is velocity of blood flow SLOWEST

in capillaries (same vol. through larger SA) – allows time for transfer between blood and tissues

Baroreceptors

specialised pressure-sensitive nerves

Cardiac output

total vol. blood ejected from ventricle/minute

Circulation needs to be regulated to …

match tissues’ metabolic

needs, e.g. inactivity vs. activity

perfusion

Contraction (vasoconstriction) and relaxation (vasodilation) of smooth

muscle (sphincters) in arteriole walls regulates blood flow in to

capillaries/tissues

Cardiac muscle and smooth muscle function controlled by:

• Signals from nervous system (sympathetic and parasympathetic NS)

• Hormones in blood (e.g. atrial naturitic peptide, ANP)

• Local control by endothelium (NO, endothelin)