Water Balance and Circulation in Animals
February 13, 2009
p. 994 -1000
(Animal Circulation Tables and Figures)
1. Introduction
Animal cells (like plant cells) need to exchange minerals, gases, wastes with
the external environment.
If organism is small (like protozoans) with high S:V in an aquatic environment, this exchange is not a problem.
Even relatively large invertebrates (e. g., sponges, hydra, nematodes) are small enough, and have a body plan (S:V) that allows most all cells ready access to the external environment.
Problem only arises (as with plants) with multicellularity and transition to land. The solution, as with plants, is a vascular system that allows materials dissolved in fluid to be distributed to and carried from all cells (think xylem/pholoem).
2. The Heart
The simplest circulatory systems are open circulatory systems found in most mollusks, and arthropods.
The “heart” is a muscular tube that contracts in peristaltic waves and sends fluid through a network of channels. This fluid accumulates and drains into a central cavity where the heart can redistribute it. Think of basting a turkey. Because there is no separation between the circulating fluid and the extracellular fluids of the body tissues, the fluid is called hemolymph. Low pressure due to low resistance.
As body size and physiological complexity increases, we need more efficient way to move minerals, gases, wastes. Closed circulatory systems are found in annelids, cephalopod mollusks, vertebrates. Blood is contained and separate from extracellular fluids. We can now refer to blood and lymph as separate fluids. Problem with closed system is pressure requirement. Narrower vessel increases resistance to flow. A vessel of ½ diameter gives 16X resistance. (Resistance proportional to the 4th power of radius)
Flow α pressure/resistance
Coonsequently, there is an increase in the structural complexity of the heart. These adaptations first appear in fishes.
Necessitated by the closed system and the evolution of gills that facilitate gas exchange.
The fish heart is a tube with four specialized structures: sinus venosus, atrium, ventricle, conus arteriosus. They contract in sequence, and valves insure the unidirectional flow of blood (now separate from lymph)).
Blood from heart to gills, picks up oxygen, sheds CO2, then on to remainder of body. Problem is pressure drop going through the gill capillaries. This slows down circulation and can limit O2 distribution.
With the development of lungs in amphibians, a double circulation developed. Here, blood is pumped through pulmonary arteries to lungs, then returns to the heart to be pumped through the systemic artery through the rest of the body.
More efficient maintenance of pressure and speed for better gas delivery.
Amphibians have a 3 chambered heart: two atria and a single ventricle where mixing is reduced by internal channels and baffles.
Ventricle > skin/lungs > left atrium > ventricle > body > right atrium > ventricle
More efficient still is the 4 chambered heart of some reptiles (crocodilians) birds and mammals. Two completely separated systems: pulmonary and systemic.
Right ventricle > lungs > left atrium > left ventricle > body > right atrium
Increased efficiency of the double circulatory system is thought to have been linked with endothermy. More efficient circulation is needed to maintain internal body heat.
Considerable increase in pressure from open to closed 4 chambered heart system; look at plumbing.
3. Plumbing
Plumbing to move blood around body. Arteries, arterioles, veins and venules have same basic structure: innermost endothelium, elastic fibers, smooth muscle, connective tissue. Too thick to exchange materials
Arteries and arterioles carry blood away from the heart. See the greatest pressure. Thick muscular elastic walls. Can constrict to shunt blood supply around. Smooth muscle.
Blood pressure cuff systolic, diastolic to force viscous blood through capillaries, must be considerable pressure. Typical (normal) is 120/75 where 120 mm Hg is systolic pressure (during ventricular contraction) and 75 is diastolic pressure when ventricles are relaxed. Hypertension is defined as systolic pressure > 150 mm Hg OR diastolic pressure > 90 mm Hg.
Capillaries – Diameter of red blood cell. Walls 1 cell thick, only endothelium, so molecules and ions can pass from plasma to tissues by diffusion, by filtration, and by transport through cells. 50,0000 miles, every cell in body within 100 μ (10-6 m)of a capillary.
Venules and Veins;- large diameter, distensible, thinner walls although same tissue layers. Valves prevent backward flow. Pressure in veins only ~1/10 that in arteries. Pressure not sufficient to return blood to heart without skeletal muscle assist. Varicose veins.
4.
Water Balance
Appreciable fluids lost even from closed circulatory system. Most from arteriole end of capillaries due to high pressure. Some lost fluid returned near venous end of capillary by osmosis. The rest enters lymphatic system to be returned to circulatory system near subclavian (collarbone) veins.
Contraction of smooth muscles layer in arteries/arterioles/venules/veins reduces XC area, increases resistance, decreases flow. Vasoconstriction. Relaxation (vasodilation).
Precapillary sphincters have same effect. Blood flow can be directed as needed.
Blood is largely water (~92%). Functions:
More details in Energy theme, Defense theme.
1. Why do animals need a
circulatory system and why talk about during this water theme?
2. Describe the forces behind blood flow.
3. What animals have open circulatory systems? What animals have closed
circulatory systems? What is the advantage of closed circulatory systems?
4. What are the forces behind capillary exchange of fluids? What happens if
there is too much fluid moving into the interstial spaces?
5. Trace the blood flow through the mammalian heart.
6. What is blood pressure and how is it measured in humans? What do the blood
pressure numbers indicate?
Key Words: blood, lymph, closed circulatory system, open
circulatory system, hemolymph, sinus venosus, atrium, ventricle, conus
arteriosus, arteries, arterioles, capillaries, venules, veins, precapillary
sphincters.
