Lecture Notes
Circulation and Gas Exchange
CIRCULATION
1) Circulation: movement of material from one part of body to another, or movement of materials
between internal fluids and external environment. For cells inside a body to survive, material received
from the body surface must be moved to the inner cells.
2) Materials to circulate: nutrients and usually oxygen (into and throughout body); wastes (collected from
all parts of body and exported), also hormones for communication. Heat is also circulated by the
circulatory system. Not all circulatory systems transport the same things. Insects have no need to
circulate oxygen, for example.
3) Single-celled organisms rely on purely physical forces, mainly diffusion, to circulate gases, nutrients,
and wastes. Exchange occurs across the plasma membrane.
4) Sponges circulate environmental water through their bodies. Bath sponges (leuconoid body form)
have branched networks of tubes through which water circulates.
5) Cnidarians and flatworms (except tapeworms) have gastrovascular cavities. These distribute
nutrients and oxygen throughout the body, and collect wastes and CO2. Movement of the fluid is
achieved by body movement.
6) Some organisms (e.g. roundworms, rotifers) use the fluid in their body cavity for circulation.
7) In all of the above taxa, gas exchange also occurs through the skin.
8) Molluscs, annelids, arthropods, chordates, echinoderms all have separate circulatory systems,
consisting of tubes and sometimes sinuses filled with fluid and pumped by one or more hearts.
9) In an open circulatory system (arthropods, most mollusks), the fluid is called hemolymph and at
least some of the time circulates through part of all of the body cavity. Compared to closed circulatory
systems, hemolymph volume is high, flow rate is low. Hemolymph often has other functions (e.g.
arthropod molting). Some jumping spiders use hemoplymph for hydraulically extending limbs.
10) In a closed circulatory system (annelid, chordates, cephalopods), blood confined to tubules.
11) Fluids in transport system have plasma (water with dissolved nutrients and minerals) and formed
elements (cells and other natural solid particles).
12) The cells found in blood or hemolymph are collectively called “hemocytes.” These include the red
and white blood cells found in vertebrates. A variety of different forms of hemocytes are found among
the animal phyla.
13) Vertebrate blood formed elements are red blood cells (RBCs), white blood cells (WBCs), and
platelets.
14) RBCs are packed with hemoglobin that binds with oxygen, making the blood a more efficient oxygen
transporter. The evolutionary trend from amphibians to mammals is toward smaller RBCs.
Mammalian RBC are “enucleated.” This means they lack a nucleus. RBCs are bright red when
oxygenated, but become darker when deoxygenated. A vein carrying deoxygenated blood appears
blue in color.
15) WBCs are scavengers that attack and digest foreign microorganisms and chemicals that may enter the
blood. They also digest and remove dead body cells. They come in different types, but most digest
unwanted material by phagocytosis.
16) There are three general kinds of vessels. Arteries carry blood away from the heart and towards the
capillaries. Capillaries are very tiny vessels with walls thin enough for gases and nutrients to diffuse
through. All living body cells must be near a capillary if they are to survive. Veins return blood from
the capillaries to the heart.
17) See Fig 26.7 on p430. Fish hearts have only two chambers (1 atrium and 1 ventricle). Blood is
pumped in a circuit from heart to gill capillaries to systemic capillaries to heart. Blood flow is slowed
considerably during passage through capillaries.
18) Amphibian hearts are three chambered (2 atria and 1 ventricle). Unlike fish, amphibians have two
circuits of blood flow. The pulmonary circuit: from heart to gas exchange surfaces to heart, and the
systemic circuit: from heart to body tissues to heart. The single ventricle pumps blood into both
circuits.
19) In reptiles, birds and mammals, there are two ventricles. There is a more efficient pumping of
deoxygenated blood to the lungs and oxygenated blood to the systemic circuit.
20) In order to function, capillaries must be permeable. When blood passes through capillaries, there is
thus a leakage of water and proteins out of the blood. This fluid that enters the body cavities is
collected by the vessels of the lymphatic system and drained back into the bloodstream. The
lymphatic system also has lymph nodes which are important in destroying foreign particles and cellular
debris.
GAS EXCHANGE
21) Most animals use aerobic respiration to burn food. This means they consume O2 and produce CO2.
Since respiration occurs in each cell, this means that they must transport O2 to each cell and eliminate
CO2 from each cell.
22) Respiratory surfaces are the surfaces of the animal across which O2 diffuses. Apparently it is not
possible for cells to actively transport O2 across membranes, since there is no known case of a cells
actively pumping O2 across a membrane. Thus, O2 must be taken in by diffusion.
23) Another limitation on respiratory surfaces is that they must be made of living cells, which will die if
they dry out. Thus, animals in terrestrial environments must protect their respiratory surfaces from
drying.
24) Respiratory surfaces in animals may be categorized as 1) tracheal systems (e.g. insects); 2) cutaneous
exchenge (i.e., across the skin) (e.g. annelids, flatworms); 3) gills (e.g. fish, crustaceans); 4) lungs (e.g.
tetrapod vertebrates, arachnids).
25) Cutaneous exchange is the simplest form of gas exchange. It usually only provides sufficient gas
exchange in animals with a high surface area to volume ratios (e.g. worms, small and thin).
26) Gills may be simple projections of skin in aquatic animal. The tend to be branced or folded into flat
sheets to maximize surface area. Frequently they have a protective covering (e.g. bony fish
operculum).
27) Tracheal systems (found in insects and some other arthropods) consist of branched chitin lined tubes
which open to the outside by spiracles. Since each cell must have gas exchange, the tracheae branch
into smaller and smaller elements until they end in tiny tracheoles, which lie near all the cells of the
body. How is this similar to the vertebrate criculatory system and how is it different?
28) Lungs are similar to gills, but are adapted to gas exchange in air. The respiratory surface still must be
kept moist. Lungs are usually on the inside of the body where the rate of water evaporation is lower.
29) Most vertebrate lungs ventilate by negative pressure (i.e. air is sucked in). Amphibians ventilate their
lungs by positive pressure. Put simply, they gulp air into their buccal cavity (mouth cavity) and then
squeeze it into their lungs.
30) In mammals, contractions of a group of muscles, mainly the diaphram, enlarge the thoracic cavity,
putting negative pressure on the lungs, causing them to inflate and suck in air through the
nasopharyngeal passage ways. Air passes through the nasopharynx, the trachea, the bronchi,
bronchioles, alveoli (gas exchange occurs). Air leaves in the reverse order, thus there is a “two-way
flow of air.”
31) Birds also use negative pressure, but have a one way flow of air that is more efficient than the two way
flow of air found in most other vertebrates. Flow of air is a loop: through bronchus, then ventilating
abdominal air sacs, then through lungs where gas exchange occurs in tubes called parabronchi,, then
ventilating thoracic air sacs, then back out of bronchus. Requires two breathing cycles to get air
through system, but because there is no “backing up”, ventilation is more efficient allowing a higher
rate of oxygen consumption.