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Small differences in heat along the length of the exchanger sum up to create a large overall temperature gradient from beginning to end. The longer the system, the greater the overall differential will be
- secretin primary function is to induce a flow of bicarbonate ions from the pancreas to the small intestine. Bicarbonate neutralizes the acid arriving from the stomach.
Habitats with large numbers of photosynthetic organisms tend to be relatively oxygen rich while habitats where most organisms live off existing organic material tend to be oxygen poor
• The most important issue in oxygen availability is often surface area, which has a large impact on oxygen’s ability to diffuse into water
• Many small animals exchange gases by direct diffusion across the body surface. They mostly must live in wet environments.
• Large animals, or those that live in dry environments, need a specialized organ for gas exchange.
Fick’s law identifies traits that allow animals to maximize the rate at which oxygen and carbon dioxide diffuse across surfaces.
Specifically, Fick’s law states that all gases, including O2 and CO2, diffuse in the largest amounts when three conditions are met:
1. The surface area for gas exchange is large.
2. The respiratory surface is extremely thin.
3. The partial pressure gradient of the gas across the surface is large.
-Countercurrent flow makes fish gills extremely efficient in extracting oxygen from water, because it ensures that the difference in the partial pressure of O2 and CO2 in water versus blood is large over the entire gas-exchange surface.
• In aquatic habitats, ventilation tends to disrupt water and electrolyte balance, and homeostasis must be maintained by an active osmoregulatory system.
• In contrast, breathing leads to a loss of water by evaporation in terrestrial environments.
CO2 + H2O = H2CO3 = H+ + HCO3- (extra CO2 in blood reacts with water in the blood and cerebrospinal fluid)
The release of hydrogen ions lowers the blood and CSF pH, which is sensed by specialized neurons, leading to the medullary respiratory center increasing the breathing rate, which returns the partial pressures of these two gases to resting levels.
• Blood leaving human lungs has a Po2 greater than that of muscles and other tissues. This difference creates a diffusion gradient that unloads O2 from hemoglobin to the tissues.
• The oxygen-hemoglobin equilibrium curve, or oxygen dissociation curve, plots the percentage saturation of hemoglobin in RBCs versus the Po2 in blood within tissues.
The most remarkable feature of the oxygen-hemoglobin equilibrium curve is that it is sigmoidal, or S-shaped. The pattern occurs because the binding of each successive oxygen molecule to a subunit of the hemoglobin molecule causes a conformational change in the protein that makes the remaining subunits much more likely to bind oxygen.
• Cooperative binding makes hemoglobin exquisitely sensitive to changes in the Po2 of tissues.
• In other words, in response to a relatively small change in tissue Po2, there is a relatively large change in the percentage saturation of hemoglobin.
• Hemoglobin is also sensitive to changes in pH and temperature. • Decreases in pH and increases in temperature alter hemoglobin’s
conformation such that it is more likely to release O2 at all values of Po2.
– This phenomenon is known as the Bohr shift.
• The Bohr shift makes hemoglobin more likely to release oxygen during exercise or other conditions in which Pco2 is high, pH is low, and tissues are under oxygen stress.
• Research shows that in hard-working tissues, the combination of increased temperature, lower pH, and lower Po2 causes hemoglobin to become almost completely deoxygenated.
• Oxygen-delivery systems based on hemoglobin are thus extremely efficient.
• Hemoglobin molecules from different individuals or species may vary in ways that affect fitness.
• Fetuses have a fetal hemoglobin that has a higher affinity for oxygen than adult hemoglobin.
– As a result, oxygen is transferred from the mother’s blood to the fetus’s blood, thus ensuring an adequate supply of oxygen as the fetus develops.
-CO2 that is produced by cellular respiration enters the blood and RBCs, where it is quickly converted to bicarbonate ions and protons in a reaction catalyzed by this enzyme
-Carbonic anhydrase catalyzes the formation of carbonic acid from carbon dioxide in water. CO2 that diffuses into red blood cells is quickly converted to bicarbonate ions and protons, so the most CO2 is transported in blood in the form of the bicarbonate ion, HCO3-.
1. The protons produced by the carbonic anhydrase reaction induce the Bohr shift, which makes hemoglobin more likely to release oxygen.
2. The Pco2 in blood drops when CO2 is converted to bicarbonate, maintaining a strong partial-pressure gradient favoring the entry of CO2 into red blood cells.
• In the alveoli, a partial-pressure gradient favors the diffusion of CO2 from plasma and RBCs to the atmosphere.
• Hemoglobin releases protons, which combine with bicarbonate to form CO2, which then diffuses into the alveoli and is exhaled from the lungs.
• Hemoglobin picks up O2 during inhalation, and the cycle begins again.
– Tiny animals have a small enough volume that diffusion over their body surface is adequate to keep them alive.
– Jellyfish and corals have a large, highly folded gastrovascular cavity that offers a large surface area for molecular exchange.
– The flattened bodies of flatworms and tapeworms give them a high surface area/volume ratio; molecular exchange thus occurs over the body surface.
• Hemolymph is pumped into blood vessels that empty into an open, fluid-filled space, and is returned to the heart when the heart relaxes, lowering its internal pressure.
• Hemolymph transports wastes and nutrients and may also contain oxygen-carrying pigments, some cells, and clotting agents.
• Hemolymph is under relatively low pressure in an open circulatory system. As a result, hemolymph flow rates may also be low.
• The low pressure of open circulatory systems makes them most suitable for relatively sedentary organisms that do not have high oxygen demands.
• Insects are the exception to this rule—they overcome the limitations imposed by low hemolymph pressure via their tracheal respiratory system, which delivers oxygen directly to the tissues.
• A limitation of the open circulatory system is that, without discrete, continuous vessels, the flow of hemolymph cannot be directed toward tissues that have a high oxygen demand and CO2 buildup.
• Crustaceans are an important exception to this rule; they have a network of small vessels that can preferentially send hemolymph to tissues with the highest oxygen demands.
-blood flows in a continuous circuit through the body under pressure generated by a heart. Because blood is confined to vessels, a closed system can generate enough pressure to maintain a high flow rate.
• Blood flow can also be directed in a precise way in a closed circulatory system.
• Closed circulatory systems are found in vertebrates and a few other lineages where individuals tend to be active.
• For example, annelids, most of which live as active burrowers and hunters, are able to obtain and circulate enough oxygen to support intense muscular activity.
• A similar situation occurs in squid, octopuses, and other cephalopods that hunt down prey.
• Closed circulatory systems contain an array of blood vessels, each of which has a distinct structure and function.
1. Permeate all tissues.
2. Eventually join with one another to form larger vessels.
3. Return excess interstitial fluid, in the form of lymph, to the major veins entering the heart.
• For the lymphatic system to work, a solute concentration gradient is needed to bring fluid into capillaries via osmosis.
– The fluid that leaks out of capillaries must have low osmolarity; thus, capillaries have to act as filters that retain large proteins as fluid leaks out into the surrounding tissues.
Research isolated the large, negatively charged protein albumin, which is effectively excluded from interstitial fluid.
– Albumin keeps blood solute concentrations high, maintaining a strong osmotic gradient that brings fluid back into capillaries.
• Some animals have a bypass vessel running from the right ventricle directly into the systemic circulation. It shunts blood from the pulmonary to the systemic circulation when an animal is underwater, greatly reducing blood flow to the lungs when the animal is not breathing.
1. Blood returns from the body to the right atrium.
2. Blood enters the right ventricle through the right AV valve.
3. Blood is pumped through the pulmonary valve, into the pulmonary artery, and to the lungs.
4. Blood returns from the lungs, via the pulmonary veins, to the left atrium.
5. Blood enters the left ventricle through the left AV valve.
6. Blood is pumped through the aortic valve, into the aorta, and to the body.
• The contraction of the left ventricle sends oxygenated blood at high pressure out the aorta and into the arteries, capillaries, and veins that form the systemic circulation.
• One-way valves ensure that blood follows only this path. – If heart valves are damaged or defective, the resulting backflow
can be heard through a stethoscope as a heart murmur.
Blood pressure measured in the systemic arterial circulation at the peak of ventricular ejection into the aorta
• People with blood pressures consistently higher than 140/90 mm Hg have high blood pressure, or hypertension, a serious health
concern because it can lead to a variety of circulatory system defects.
• Abnormally high blood pressure puts mechanical stress on arteries; if the walls of an artery fail, the individual may experience heart attack, stroke, kidney failure, and burst or dilated blood vessels.
1. The SA node originates a signal.
2. The signal from the SA node is propagated over the atria, which contract simultaneously and fill the ventricles.
3. The signal is conducted to the AV node, which relays the signal to the ventricles after they fill completely with blood.
4. The electrical impulse is rapidly transmitted through both ventricles, causing them to contract as the atria relax.
5. The final electrical event occurs as the ventricles relax and their cells recover.
force that blood exerts on the walls of arteries, capillaries, and veins.
• Blood pressure drops dramatically as blood moves through the capillaries, because the total cross-sectional area of blood vessels in the circulatory system increases greatly.
• The drop in blood pressure decreases the rate of blood flow to allow sufficient time for gases, nutrients, and wastes to diffuse between tissues and blood in the capillaries.
The nervous system, along with certain chemical messengers in the circulation, can accurately control blood flow to various tissues by contracting or relaxing the arteriolar sphincters.
• Decreases in blood pressure elicit a powerful homeostatic response. • Falling blood pressure is detected by baroreceptors in the walls of
the heart and the major arteries.
1. Cardiac output is increased by an increase in both heart rate and the amount of blood pushed out by the ventricles.
2. Arterioles serving the capillaries of noncritical tissues are constricted to divert blood to more critical organs.
3. The veins are constricted, shifting blood volume toward the heart and arteries to maintain blood pressure and flow to vital organs.
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