Organismal Biology

Key Concepts

Chapter  13 - Animals -
Distribution of Materials

Fluids

Animal fluids exist at many levels of internal organization.

Intracellular fluids, things like cytoplasm and nucleoplasm, are a mixture of water (at close to a marine concentration, more on that later) and all of the ions and molecules needed to produce a living cell system.  The concentration of water is very important to keeping chemistry running at proper rates.

Extracellular fluids must be in osmotic balance (equal dilutions) with the intracellular fluids, to prevent a dangerous gain or loss of water.  Cells in a multicellular system, just like single-celled organisms, require a watery environment to be active.  Around those cells is interstitial fluid, generally originating from the fluid component of blood in animals that have some sort of blood.  Most nutrients are distributed through the watery component of circulated fluid.

Most animals do have some form of circulation, which follows two basic approaches.  In an open circulation system, the main pump pushes a blood called hemolymph into sinuses, open body spaces, where it moves around inside a nearly hollow body, and is eventually sucked back into the pump.  This is an adequate mode of circulation in animals with relatively low metabolisms, and therefore low oxygen needs;  the circulation is not great at moving things quickly, so in high-metabolism animals, a separate distribution system for oxygen and / or a set-up to better move oxygen from absorption area to areas of need may be in place.  In a closed circulation system, the blood circulates in vessels:  arteries moving blood out from the pump, thin capillaries for material distribution and pickup, and veins collecting blood and moving it back to the pump.  Capillaries are generally leaky so as to better move materials (otherwise the materials have to go through the membrane linings), and closed systems often have lymph systems to gather leaked fluids and return them to the blood.  A number of animals groups have systems that significantly combine open and closed structures.

Blood, other than its plasma's nutrient, respiratory, and waste components, has other dissolved components.  Since materials like lipid components and oxygen dissolve poorly in water, there are often carrier proteins to help them stay dissolved.  Many materials in the blood are regulatory components.  These include osmoregulation molecules, small proteins added or removed to adjust the dilution levels of the blood.  There are hemostasis molecules, activated when the system is injured and losing blood (involved in coagulation / clotting).  There are hormones, carrying messages from an area that monitors aspects of biology to organs that can affect those aspects.  In many invertebrates, hormones are made by neurosecretory cells;  in vertebrates, by endocrine glands.  There is likely to be some sort of immunity molecules in the blood.  These might be general antibiotics or specific antibodies.  In terrestrial animals, there is a significant amount of nitrogen dissolved in the blood - this is purely because there is so much nitrogen in the atmosphere and it dissolves fairly well.  It's different from the nitrogenous waste compounds produced by protein metabolism.

Blood generally has cellular components for a few functions.  Thrombocytes (or platelets) are involved in hemostasis.  Many types of leukocytes are involved in recognizing invaders or damaged cells and reacting.  They must be able to move out of closed systems and into interstitial fluid;  they use the lymph system to get back into the blood, and use lymph nodes as storage / staging areas.  In some animals, erythrocytes may package oxygen carriers, which may also just be dissolved in the plasma.

Respiration

Virtually all animals are aerobic, using a method of respiration that requires oxygen to efficiently shift energy from large molecules like glucose to the highly usable energy molecule ATP.  That means that animal tissues require not just a continuous supply of fuel molecules, but oxygen as well.  Respiration of most fuel compounds produces carbon dioxide, which is lost to the environment in respiratory systems.  Respiration of proteins produces toxic nitrogenous wastes, which do not easily leave through respiratory surfaces and require specialized removal systems, dealt with below.

Oxygen is somewhat limited by how much is available from the environment.  Aquatic environments contain oxygen, but oxygen has limited ability to dissolve in water, an ability that decreases with temperature (but generally, animal metabolic rates increase with temperature).  Oxygen tends to have a much higher presence in air.  When aquatic organisms were dealing with a movement onto the land, the higher oxygen levels would have been a negative, based upon oxygen's high chemical activity, but it was also an evolutionary opportunity;  terrestrial organisms' metabolic rates rose with the new oxygen levels, to the point where those species that later became aquatic again rarely were able to be active breathing just water.

Of course, oxygen must reached the cells in order to be used.  This can use the animal's outer surface for exchange - cutaneous respiration - but any such exchange requires a thin, wet surface.  In aquatic animals, this is not an issue; in terrestrial animals, cutaneous respirers must live in wet and/ or extremely humid conditions that reduce water loss across the exchange surfaces.  Entering oxygen must then reach internal cells.  In small or flat animals, this can be accomplished with simple diffusion.  In larger animals, no matter how the oxygen enters, some sort of distribution system is necessary.  That has been discussed above.

Entry surfaces for larger animals are almost always increased in various ways.  Gills, rather than being just flat, well-vascularized membranes, are generally invaginated, with the surface repeatedly folded inward;  lungs,  even though available oxygen levels are much higher, are often evaginated, folded outward to present a lot of surface-to-blood availability.  Tricks to increase oxygen uptake from water often include countercurrent systems, with the blood flow going opposite to water flow through the gills - as water going through loses oxygen by diffusion to the blood, it continually passes oxygen-depleted blood, sustaining the concentration gradient.

With terrestrial animals, the tissue fluids will hold a very limited amount of oxygen compared to the air.  There is a need to charge the blood, but not to fully deplete the air, which can't really be done - the blood will never hold an equal concentration to normal atmosphere of oxygen.  Systems that use a lot of oxygen, or use it quickly, may increase surface with internal spaces filled with air, and may adjust how incoming and outgoing air does or doesn't mix.

There is a non-lung system used in insects.  With an open circulation system that is not efficient enough to deliver oxygen quickly, they have  tracheal system of dedicated tubes that connect all cells directly to the outside air.  This gets lots of oxygen in when needed, but also is a lot of potential water loss, so the system under all but high-need conditions is to some extent flooded with fluid.

Internal Stability, Homeostasis

The chemistry of animals has a long list of needs:  input of oxygen and fuel, nutrients for building appropriate molecules and sustaining that chemistry.  Cells must interact with their surroundings but sustain a very precise balance of water-dissolved materials inside to perform their functions.  Any individual must be set up to help every cell maintain internal stability, homeostasis.

In order for cell chemistry to work properly, there must be a stable concentration of materials in a uniform amount of water.  Not surprisingly, as organisms evolved in the early oceans, the dilution of materials inside cells worked best when it matched the dilution outside, and the descendants of those organisms, the marine organisms, are generally osmotic conformers.  The levels of dissolved materials do not individually match the surrounding water, just their overall concentration, which matches so that the water concentration is the same.

In nonstable marine environments, where water concentration can fluctuate, such as tidal pools, estuaries, even outflow areas for major rivers, organisms must be able to deal with those fluctuations.  Adaptations that allow organisms to survive a rise or fall of dilutions around them also set up adaptations to inland water systems and eventually terrestrial systems.

Tidal areas flushed with rain and freshwater systems present a much higher water concentration than a living cell could match and survive.  For organisms to evolve into those systems, there are three primary adaptations.  For plants, fungi, and a number of protozoan animals, having cell casings prevents the inflow of water from swelling and bursting the cells.  These cells have to prevent loss of materials to the dilute surroundings, but inflow of water has to match outflow because the cells can't get bigger.  A second adaptation is to pump excess water back out, as is done in protozoan contractile vacuoles as well as animal kidneys.  What makes this tricky is that there are no actual molecular water pumps;  moving water involves moving particles, many of which are nitrogenous waste particles that need to leave anyway, but also particles that are needed.  Needed particle are taken back quickly and the remaining dilute fluid is sent back into the surrounding water.  The third adaptation is to waterproof every surface possible.  As vertebrate bony fish evolved in freshwater, this meant waterproofing every surface but the gills and digestive system.  Over time, bony fish tissues became a bit more dilute than their marine ancestors, and their marine descendants are still a bit more dilute than their surroundings.

Many of the adaptations that were useful in these ecosystems were usable out of the water, which makes tidal areas and shallow freshwater systems the likeliest staging areas for the transition to terrestrial species.

Excretion

Respiratory processes take the energy of bonds in carbon compounds and shift that energy to bonds in much-more-easily-used ATP.  Large molecules, like starches and lipids, are reduced to small bits for the process.  Protein molecules can be reduced to carbon-based bits, but that releases a nitrogen compound, ammonia, as what is called a nitrogenous waste.

Ammonia is very reactive, and would be very toxic if allowed to build up in tissues.  It is also very soluble in water, so it will quickly diffuse out into aquatic environments, where several prokaryote species make use of its chemical potential and convert it to nitrites and nitrates, used by plants to build their own proteins.  This means that, for many aquatic organisms, ammonia is the primary nitrogenous waste.

A major problem for terrestrial animals involves the potential accumulation of toxic ammonia in the sealed eggs they produce.  In these animals, ammonia is processed to uric acid, a nontoxic white crystal.  Uric acid is close to nonsoluble in water, so mobilizing and removing it in terrestrial animals is a little tricky, but it is the primary nitrogenous waste of terrestrial arthropods, reptiles, and birds.

In mammals which produce a placenta, an exchange surface between the blood of the embryo and mother, the primary nitrogenous waste is urea.  Urea dissolves in water, even if not as well as ammonia, but it is much less toxic than ammonia, if not as nontoxic as uric acid.  Urea is also produced in cartilage fish and stored in the liver, where it provides a certain amount of buoyancy.

Even the simpler groups have dedicated waste-removal structures to get toxic by-products quickly out of the tissues.  Flatworms have protonephridia, sealed structures with beating flagellar "tufts" (also called flame cells) that help more wastes out ducts.  Mollusks, annelids, and a few other phyla use metanephridia, which draw in hemolymph from sinuses, and which concentrate wastes for removal.

More advanced systems use a 3-step process for dealing with removing wastes:  1)  blood is filtered, so all particles below a certain size leave;  2)  any particles which have left but which are still needed are reabsorbed back into the blood;  3)  materials that would not go through the filter but need to be removed are added / secreted to the collecting fluid.

Crustacean arthropods typically use antennal glands to remove ammonia.  Insects and many terrestrial arthropods use malpighian tubules to move uric acid into the digestive system.  Vertebrate kidneys use a cluster of leaky capillaries, a nephron, as a filter, and a set of tubes to reabsorb and add materials.  

 Temperature Controls

Cellular chemistry rates are affected strongly by temperature, which is basically a measure of how quickly particles are moving.  Many processes work best at particular temperatures, so keeping tissues at those temperatures can be very important.

The vast majority of marine environments are very temperature-stable.  Ocean water temperatures tend to vary only near the surface, land, and major currents, but any variation occurs slowly.   Marine organisms, therefore, do not generally require chemistry with a broad temperature profile.

The major "staging areas" from which aquatic organisms moved toward being terrestrial organisms, tidal pools and flowing fresh water, required chemistry with more temperature flexibility, sometimes enzymes that do the same thing but at different optimum temperatures.  Most animals are poikilotherms, whose internal temperatures are mostly linked to the temperature of their surroundings.  Some animals (just two groups, the birds and mammals, and a few species, and some systems in animals) are homeothermic, where fuel and oxygen are devoted to sustaining a particular internal temperature.  This increases chemical efficiently but has energetic requirements.

There are adjustments that get made to stabilize temperatures.  In poikilotherms, behavioral adjustments include selecting locations, timing of activity, and using solar heating.  There may be some metabolic adjustments, such as "shivering" wing muscles to get them warm enough to be usable.

In homeotherms, behavioral adjustments can be used to help minimize heat loss / energy usage for heat generation.  Chemistry may be different between core and extremity areas.  Evaporative cooling may be used  in warm environments or when metabolic rates generate too much heat.

An extreme challenge comes in environments where the water in cells might freeze, requiring a number of different adaptations.  In many poikilotherms, a period of dormancy occurs during cold winter months, usually sheltered away from the full effect of freezing temperatures (although a few can survive hard freezing and thawing with special anti-crystallization chemicals).  Some produce a freezing-resistant egg, so each generation spans at most a year.  A very few might migrate to warmer environments and return.  Migration is more common in response to changes in available food or water, but some of those are generated by freezing temperatures.

Homeotherm adaptations to freezing environments include insulation, heat barriers that minimize loss to the environment.  These are commonly materials that trap air (a poor conductor of heat), like fur or downy feathers, or blubber-type fat, used in deep water where air would be squeezed out of the other kind of insulators.  Burrows below the frost line can be protective, with animals being active when temperatures are not extreme (or snow cover serves as an insulator).  Energy can be conserved with a reduced-temperature dormancy period such as hibernation (some animals like bats do this during sleep periods, called daily torpor).

Heat gets lost across surfaces, moreso from small animals or extremities.  Some homeotherms like waterfowl use a heat countercurrent system of blood vessels, with outgoing passing close to incoming.  The distant blood has cooled, and as it goes in it picks up heat from the outgoing blood, cooling it;  a lot of the body heat is being used to reheat the incoming blood rather than just being lost to surrounding water (a great heat absorber).

KEY CONCEPTS -
Click on term to go to it in the text.
Terms are in the order they appear.

 Intracellular Fluids
Cytoplasm
Nucleoplasm
Extracellular Fluids
Osmotic Balance
Interstitial Fluid
Open Circulation
Hemolymph
Sinuses
Closed Circulation
Blood
Vessels
Arteries
Capillaries
Veins
Lymph Systems
Plasma Components
Carrier Proteins
Osmoregulation Molecules
Hemostasis
Hormones
Neurosecretory Cells
Endocrine Glands
Immunity Molecules
Antibiotics
Antibodies
Thrombocytes
Platelets
Leukocytes
Lymph Nodes
Erythrocytes
Respiration Process
Nitrogenous Wastes - Intro
Oxygen in Water
Oxygen in Air
Respiratory Surfaces
Cutaneous Respiration
Gill Surfaces
Lung Surfaces
Oxygen Countercurrents
Mixing in Air Respiration
Tracheal Systems
Homeostasis
Osmotic Conformers
Nonstable Marine Environments
Freshwater Adaptations
Contractile Vacuoles
Kidneys & Water
Bony Fish & Osmosis
Waterproofing
Terrestrial Transition
Ammonia
Ammonia to Water
Uric Acid
Eggs, Terrestrial
Placenta
Urea
Protonephridia
Flame Cells
Metanephridia
3-Step Waste Processing
Antennal Glands
Malpighian Tubules
Kidneys & Waste
Nephron
Temperature & Chemistry
Temperature in Oceans
Fluctuating Temperatures
Optimum Temperatures
Poikilotherms
Homeotherms
Temperature Adjustments
Evaporative Cooling
Freezing
Insulation
Hibernation
Daily Torpor
Heat Countercurrent

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