Chapter 42: Circulation and Gas Exchange

 

CIRCULATION

 

Every organism must exchange nutrients and wastes with its environment.  Cells are the ultimate exchange sites of nutrients and wastes.  In organisms with a closed circulatory system, the blood plasma is forced out of the capillaries, bathing cells with dissolved O2 and other nutrients.  That portion of the blood plasma that is forced out of the capillaries is called “interstitial fluid”.  This interstitial fluid returns to capillaries via osmotic pressure, carrying with it dissolved CO2 and other wastes.  While diffusion is sufficient for the exchange of dissolved gases and some nutrients, diffusion is much too slow a process for most transport needs:

Proportional to distance2

eg. Glucose:      1 sec to travel 100 um

                              100 sec to travel 1 mm

                              3 years to travel 1 m

Systems for internal transport needed for any animal more than a few millimeters thick

In lungs, O2 diffuses across thin epithelium into blood (lots of surface area)

CO2 able to diffuse in the opposite direction due to diffusion gradient

 

Chemicals transported between blood and interstitial fluid, bathing cells--greatly decreases distance for diffusion

 

Internal Transport in Invertebrates

Hydra, Aurelia only 2 cells thickness between exterior and gastrovascular cavity (serves for both digestion and circulation, flagellated cells lining cavity), then able to diffuse to outer layer--Fig. 42.1

 

Planarians have flat shape, much-ramified gastrovascular cavity

adding more layers to body makes diffusion inadequate

 

A. Open Circulatory Systems – Cool!

insects, other arthropods, mollusks (Fig. 42.2)

hemolymph = "blood", bathes internal organs; serves function of both blood, interstitial fluid (no distinction between the two, hence the unique term)

circulated by simple body movements, squeezes fluid through sinuses (spaces between organs)

coupled with simple heart (one or coupled dorsal vessels)

hemolymph enters heart through ostia (pores)

relaxation of heart causes hemolymph to be drawn in; expelled as heart contracts (creating vacuum)

 

B. Closed Circulatory Systems - Fig. 42.2

earthworm simplest; 2 main vessels: dorsal to anterior, ventral to posterior, with side connections

dorsal vessel = main heart, pumping via peristaltic waves

5 pairs of anterior vessels = auxillary hearts, connect dorsal and ventral vessels laterally

 

C. Vertebrates (cardiovascular systems)

heart + blood vessels + blood

atrium (or 2 atria) = chambers that receive blood

ventricle ( or 2 ventricles) = chambers that pump out

 

blood vessels up to 100,000 km in human body (eg.comparable to 1000 m inside a 2 cm capsule)

1. arteries--blood carried away from heart to organs

2. branch to arterioles within organs

3. branch to capillaries, network that penetrates to cells in tissues

diffusion across capillary walls with interstitial fluid surrounding cells

4. capillaries rejoin as venules and

5. veins returning to atrium of heart

 

Vertebrate circulation (Fig. 42.3)

A. Fish--2-chambered heart, single circuit blood flow

ventricle to gills (diffuse out into capillary beds, O2 diffuses in, CO2 diffuses out) to systemic tissues (diffuse into capillary beds again) to veins to atrium

 

flow to capillary beds causes drop in pressure, so blood flows to tissues rather slowly; aided by body movement during swimming

 

B. Frogs, other amphibians

three-chambered heart, double circulation

one ventricle supplies both: systemic circuit to all other organs, returning to right atrium

and pulmonary circuit to lungs and skin, returning to the left atrium

 

double circulation insures good blood flow to all organs; pressure reestablished after oxygenation

 

frog single ventricle does allow some mixing of O2+ and O2 - blood; ridge in center diverts most O2+ from left atrium to systemic circuit, O2- from right atrium to pulmonary circuit

 

reptile single ventricle partially closed by a septum; in crocodiles septum completely divides into two ventricles

so, selective pressure caused evolution of more efficient circulation with greater time spent on land (less chance to oxygenate blood through skin)

 

Bird and Mammalian heart--4-chambered, double circulation

2 atria, 2 complete ventricles;  O2+ and O2- blood never mix

left side-oxygenated only

this + double circulation enhances O2 delivery to all parts of the body

 

endotherms require more O2 for higher levels of cellular respiration to maintain constant body temperature (again, selective pressure leading to evolutionary change)

 

Mammalian heart (Fig 42.4) human as example

a. atria thin-walled collection chambers for blood returning to heart; pump blood only to ventricle (short distance)

b. ventricles thicker-walled, much more muscular, esp. left ventricle (leads to systemic circuit)

c. aorta leads from left ventricle to rest of body

d. all systemic blood returns through venae cavae:

           superior (cranial, anterior) vena cava from anterior limbs and head

           inferior (caudal, posterior) vena cava from posterior limbs and trunk

 

e. pulmonary circuit sends blood to lungs to be oxygenated; returns blood to heart

           passes from right ventricle through pulmonary arteries to each lung

           capillary beds in alveoli exchange O2 for CO2

           pulmonary veins return to left atrium

 

Skip: Maintaining the Heart's Rhythmic Beat pp. 877

 

Microcirculation and Blood Distribution

flow  of blood from arterioles to venules via capillary nets

a. some are thoroughfare channels--direct links, always open

b. true capillaries branch from these; flow into these is controlled by precapillary sphincters (ring of smooth muscle tissue)

 

only 5-10% of capillaries open at any one time

flow regulated via arteriole contraction and dilation, sphincters

eg. after large meal, blood flow to wall of digestive tract increases

strenuous exercise diverts blood flow to skeletal muscles

always well-supplied: brain, kidneys, liver, heart

 

Capillary Exchange (Fig. 42.12)

transfer between blood and interstitial fluid bathing cells

capillary wall (endothelium)- single layer of flattened cell, overlapping at edges (leaky)

 

materials exchanged from capillary to tissue:

a.  vesicles form by endocytosis (inside cell), release on opposite side (exocytosis)

b. diffusion between blood and interstitial fluid

           small molecules move via concentration gradient across endothelial cells

           also in clefts between adjacent cells

c.  transport through clefts via bulk flow: movement of fluid under pressure

           small molecules, H2O eg. sugars, salts, O2, urea

           blood cells, proteins too large

direction of movement depends on difference between hydrostatic pressure (HP = fluid) and osmotic pressure (OP = concentration)  Fig. 42.13

           HP tends to force fluid out of capillary

           OP favors entry of water into capillary due to high solute concentration inside

           at arterial end HP> OP, net exodus of blood (fluid) and small solutes (proteins remain due to selectively permeable membrane)

           HP decreases with loss of fluid

           at venous end, HP<OP, tendency for fluids to reenter; >99% of fluids regained

 

Lymphatic System -- Fig 43.4

 

Returns the 1% of  blood fluids not reabsorbed from interstitial fluid; also some blood proteins lost through "leaky" capillary walls

 

3 functions:

return of blood components described above

diffuse back into lymph capillaries intertwined with blood capillaries

drains back into circulatory system at two locations near shoulders

 

pooling of interstitial fluid in tissues can cause edema (bloating)

eg. elephantiasis caused by parasitic worms blocking lymph vessels

eg. severe protein deficiency --bloated belly syndrome

body begins to cannibalize its own blood proteins, causing decrease in osmotic pressure, slower return of blood fluids to capillaries

 

1)      lymph nodes filter lymph, attack viruses and bacteria

specialized white blood cells divide rapidly in response to infection, causing swelling, tenderness in lymph nodes

 

2)     lymphatic system absorbs most fats digested in small intestines (chylomicrons), transports them to the circulatory system

 

RESPIRATION 

 

Gas Exchange in Animals

 

major function of circulatory system is transport of O2 and CO2 between respiratory organs and other parts of body

gas exchange--supports cellular respiration by supplying O2, removing CO2--Fig. 42.18

 

respiratory medium = source of O2 (air, water)

respiratory surface = portion of an animal's body where gas exchange occurs (Fig. 42.19)

must be moist for gases to dissolve and diffuse across membranes

must be large enough to supply gas exchange needs of animal

general structure--of gill, lung, etc. is thin, moist epithelium rich in blood supply

may be entire skin eg. earthworms, amphibians if small enough, large surface/volume ratio

OR, localized region, with extensive folding or branching to enlarge functional surface area

           aquatic animals--gills, bathed in water, external

           terrestrial animals--cannot use gills (would lose too much water to evaporation; fine filaments would stick together in air; use either: lungs (vertebrates) or tracheae (insects)--internal, narrow openings to outside

 

I. Gills--evaginations or outfoldings of body surface

a. distributed over whole body surface eg. echinoderms, annelids

b. specialized regions with large surface area

clam--cilia to move water across surface

crustaceans--gills enclosed in exoskeleton in thorax, specialized appendages to ventilate

 

[O2] much lower in water than in air, esp. in high temp., salinity

means to increase efficiency of gills:

1) ventilation--means to increase air flow

eg. arthropods--crayfish, lobsters

fish--water flows through mouth and pharyngeal slits, coordinated movement of mouth and bony operculum pumps water across gills--major energy drain (Fig. 42.20)

2) countercurrent exchange (Fig 42.21)

allows 80% O2 take-up

arrangement of capillaries in gills to enhance gas exchange

flow of blood in capillaries opposite of water flow

ie. most deoxygenated blood entering at rear of lamella

encounters least oxygenated water--still a concentration gradient favoring O2 transfer to blood

as blood becomes more saturated with O2 moving towards front of lamella, encounters higher O2 conc. in water

maintains favorable diffusion gradient along whole length of capillary

 

II. Tracheae--insects, some spiders

air has much higher [O2] than water

speed of diffusion of O2 and CO2 also greater in air

ventilation less necessary, due to constant availability of O2 ie. less energy usage

 

problem: loss of H2O across moist respiratory surface

solution: invagination of surfaces within body

 

tracheae (Fig. 42.22)--tiny air ducts reinforced with chitin rings, branch throughout body, and reach to nearly every cell surface

entry to body via spiracles over surface of body, lead to tracheoles at cell membranes, fluid content varies with O2 demand to allow more diffusion

some large insects ventilate to aid air diffusion--bellows-like body movements

air sacs collect O2 near organs of high demand

 

III. Lungs of Terrestrial Vertebrates

 

Respiratory surface restricted to one area of the body, depends on circulatory system to distribute O2, remove CO2

·        Must be highly vascularized to provide enough surface for absorption

·        Eg. vascular mantle of land snails, book lungs of spiders

·        Surfaces must be moist for gas diffusion to occur

·        Evaporation minimized by narrow passages to exterior of body

 

Frog lungs balloon-like, smooth, resp. occurs on surface only

 

Mammalian lungs honeycombed with epithelium = area of a tennis court! (Fig 42.23)

Lungs in the thoracic (pleural) cavity, inside a double-walled fluid-filled sac

Outer wall of sac (parietal pleura) attaches to chest cavity

Inner wall of sac (visceral pleura) attaches to lungs

Direction of air flow : nostrils to nasal cavity to pharynx past glottis (opening) to larynx to trachea, branches into two (primary) bronchi leading to each lung, subdivides into secondary and tertiary bronchi and finally bronchioles (diameter of <1mm) and ends in alveoli (air sacs) made of thin epithelium where gases exchanged across cell membranes with the capillaries of the pulmonary circuit

 

Ventilation of the lungs is via inhaling and exhaling (breathing)

 

Frogs employ positive pressure ventilation by lowering floor of mouth, drawing air in through nostrils. Nostrils then closed, floor of mouth raised to force air down to lungs via positive pressure (pushing). Elastic recoil of lungs forces air back out (Lab Video)

 

Mammals use negative pressure ventilation (suction or pulling) (Fig. 42.24)

Changing volume of the thoracic cavity increases the lung volume, decreases air pressure at the alveoli < atmospheric pressure, causing air to rush in via nostrils, mouth

 

a)      at rest, via diaphragm

·        muscle contracts (shortens), moves downward and flattens, increasing thoracic volume, dropping air pressure within, causing suction

·        relaxation of diaphragm pushes air out

b)      during exercise, rib cage muscles also contract to further expand thoracic cavity

 

Birds use a combination of positive and negative pressure ventilation (Fig. 42.25)

Have 8-9 pairs of air sacs in addition to lungs lined with parabronchi

These provide a bellows action to ventilate, also help the bird be more flight-worthy

·        First inhalation pulls most air to posterior air sacs

·        First exhalation and second inhalation pushes air into lungs and anterior air sacs

·        Second exhalation finally eliminates most of the air from the cycle

 

Control of breathing located at the medulla oblongata in the  brain stem (Fig 42.26)

Send impulses to inhale to rib muscles, diaphragm (10-14 X/min.)

Monitors pH of blood, which changes with CO2 concentration: 

CO2 + H2O forms H2CO3, lowering the pH

Other sensors in carotid arteries and aortal walls also monitor blood pH, send impulses to medulla

Hyperventilation occurs when too many breaths purge CO2 from blood, therefore no signals to inhale from breathing control center

 

Respiratory Pigments and Oxygen Transport

O2 - very little soluble in H2O of blood plasma

Mostly carried bound to respiratory pigments (hemoglobin for most vertebrates)

 

CO2 Transport (Fig. 42.29)

Only 7% carried as CO2 in blood plasma

23% binds to multiple amine groups of hemoglobin

70% in blood in form of bicarbonate ions (HCO3-)

 

CO2 at tissue diffuses into red blood cells, converted into bicarbonate

H+ ions attach to hemoglobin, buffers the blood from pH drop (7.34 venous vs. 7.4 arter.)

HCO3- diffuses out into plasma

In lungs, CO2 reconstituted, exhaled

Reversible equation:      CO2 +  H2O  n H2CO3  n  HCO3- + H+

 

Diving Mammals have special adaptations for higher oxygen demand

Eg. Weddell seal, a large predator, spends up to 1 hour under water, 200-500M down

a)      store extra O2

                                                            Humans                            Seals

·        volume of blood/kg weight                1X                                 2X

·        % in lungs                                          36%                               5%

·        % in blood                                          51%                              70%

·        % in myoglobin (muscular protein)   13%                              25%

·        size of spleen                                        ?                                  24L

 

Spleen contracts during dive, releasing extra blood and O2 supply

b)      O2 conservation during dive

·        pulse slows

·        O2 consumption slows overall

·        most blood routed to essential organs

·        Muscles switch from aerobic respiration to fermentation