Very small or very thin organisms need no special system for internal transport.
Many invertebrates have an open system, with blood vessels opening into
a general circulatory cavity or hemocoel. Vertebrates have a closed
circulatory system: their hearts pump blood from atrium to ventricle
and then through the major arteries; veins return blood to the heart.
Simple forms of transport:
- Cytoplasmic streaming (cyclosis): Cytoplasm in all eucaryotic cells
continually flows and changes direction.
- Diffusion: Passive transport in all organisms, effective only at distances
of a few cells. This may suffice for organisms in which each part is only a few
cells away from a body surface, but larger animals need circulatory systems.
Internal transport in plants uses xylem and phloem.
Open circulatory systems: Systems in which a body cavity or
hemocoel contains most of the circulating fluid, as in insects.
- The pumping action of a heart drives fluid forward through
an aorta, then through a series of arteries.
No veins exist; used blood seeps into sinuses that drain
into the hemocoel.
Closed circulatory systems: Systems in which blood is everywhere
contained in vessels, as in all vertebrates.
- The heart may have 2 to 4 chanbers. The heartbeat originates
from a pacemaker at the sinoatrial node. Highest pressure,
at maximum contraction, is called systole; lowest pressure is
called diastole.
- In mammals, the right atrium pumps oxygen-poor blood from the
body's tissues into the right ventricle, which pumps it through
the pulmonary arteries into the lungs. The left atrium
meanwhile pumps oxygen-rich blood from the lungs into the left ventricle,
which pumps it through the aorta for distribution throughout the body.
- Arteries carry blood from the heart to the body's tissues.
- Veins return the blood from the body's tissues back to the heart.
Valves in veins prevent the blood from flowing backward.
- Vertebrate blood is always red because of the oxygen-carrying pigment
hemoglobin, carried in red blood cells (erythrocytes).
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