Description

All cells need to be capable of exchange with their environment:

•      All reactions in cells depend on resources moving in, waste products moving out

–     all cells must have access to this exchange

•      Single-celled & simple multi-celled: all cells have contact with medium

–     diffusion distances are short

•      Larger multi-celled: some/most cells isolated from external environment

–     diffusion distances are too long

–     circulatory systems connect those cells to the outside

–     bring resources to the cell, carry waste away

Circulatory systems vary in complexity:

•      Gastrovascular cavities: lack specialized circulatory system

–     have highly branched gut/body cavity

–     high SA:V, short diffusion differences

–     e.g., cnidarians, flatworms

•      Specialized circulatory system: 3 basic components

–     circulatory fluid: carries resources/wastes

–     interconnecting tubes: through which fluid travels

–     heart: muscular pump

•      Open circulatory systems: e.g., arthropods, most mollusks

–     circulatory fluid (hemolymph) in direct contact with organs; same as interstitial fluid

–     advantages: lower pressures, can use fluid as hydrostatic skeleton

•      Closed circulatory systems: e.g., annelids, vertebrates

–     circulatory fluid (blood) in vessels, separate from interstitial fluid

–     advantages: faster delivery of O2, easier to regulate

Even among vertebrates, there is variation in circulatory systems:

•      Parts of the vertebrate circulatory system = cardiovascular system:

–     pumping heart with 2+ chambers

•      atrium: receives blood

•      ventricle: pumps blood away

–     arteries: carry blood away from heart; branch into arterioles

–     branch further into capillaries: where exchange takes place; capillary bed: network of capillaries

–     capillaries converge into venules; converge into veins: carry blood back to heart

–     remember: arteries & veins distinguished by direction they carry blood

–     more energy organism/organ needs, the more complex circulation

•      Single circulation: in fishes with 2-chambered hearts

–     blood passes through 2 capillary beds during circuit

–     Runs at lower pressure, so lower velocity

•      Aided by swimming muscles

•      Double circulation: in tetrapods with 3- or 4-chambered hearts (4 in mammals)

–     Blood pumped through two separate circuits

•      right side pulmonary circuit: to lungs

•      left side systemic circuit: to body

–     maintains higher pressure/velocity of blood

•      Variation across major groups: amphibians and (non-bird) reptiles have 3-chambered heart

11 Steps in the flow of blood through both circuits:

1. right ventricle pumps blood to lungs
2. via the pulmonary arteries
3. blood flows through capillary beds of the left & right lungs (gas exchange)
4. blood returns to left atrium via pulmonary veins (blood is oxygenated)
5. left ventricle pumps blood out to body
6. via the aorta (including coronary arteries to the heart)
7. one branch leads to capillary beds in the head & arms
8. another branch leads to capillary beds in the abdomen & legs
9. deoxygenated blood drains from the head & arms via superior vena cava
10. deoxygenated blood drains from the abdomen & legs via the inferior vena cava
11. both empty to the right atrium
    
    and the process continues...

The cardiac cycle alternates pumping and filling:

•      Cardiac cycle: complete sequence of contraction/pumping (systole) and relaxation/filling (diastole)

–     heart rate: 72 beats per minute (average resting rate)

–     stroke volume: 70 mL per ventricle

–     cardiac output: ca. 5 L/minute (per ventricle)

•      4 valves keep blood from flowing wrong direction

–     one-way flaps, bigger than the opening they cover

–     atrioventricular valve (AV): between chambers

–     semilunar valves: between ventricles and arteries

–     heart murmur: defective valve leads to back-flow

The heart provides its own “pacemaker”:

•      Pacemaker: autorhythmic cells of heart; contraction based upon own electrical impulses

–     begins at sinoatrial node: cause atria to contract

–     relayed by atrioventricular node: after 0.1 s delay, ventricles contract

–     nervous system can speed-up or slow-down rate with activity level