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BIO204 Physio Lecture note 6.docx

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University of Toronto Mississauga
Ingo Ensminger

BIO204 Nov.24/2011 Chapter 44: Gas exchange and circulation (pg.1038-1052) 44.4 How are oxygen and carbon dioxide transported in blood?  Blood is a connective tissue that consists of cells in a watery extracellular matrix called plasma.  The remainder of the blood is made up of formed elements: red blood cells, platelets, and several types of white blood cells.  Blood carries CO and oxygen between cells and lungs, transports nutrients from digestive tarct to other tissues in body, moves waste to kidney and liver processing, conveys hormones from glands to target tissues, delivers IS cells to sites of infection and distributes heat from deeper organs to surface.  Platelets are cell fragments that minimize blood loss from ruptured blood vessels by releasing material that assists in the formation of clots.  White blood cells are part of the immune system and fight infections.  Red blood cells transport oxygen from the lungs to tissues throughout the body, and play a critical role in transporting carbon dioxide from tissues to lungs. In humans, red blood cells make up 99.9% of the formed elements.  Everything made in bone marrow.  Red blood cells contain an oxygen–carrying molecule called hemoglobin. Structure and function of haemoglobin  Oxygen found In high [] of blood. Blood had high oxygen carrying capacity because O re2dily binds to haemoglobin found in RBC.  By increasing oxtgen carrying capacity of blood, hemoglobim made it possible for cellular respiration rates to increase. High rates of ATP production support high rates of growth, movement, digestion and other activities.  Hemoglobin consists of four polypeptide chains, each of which binds to a nonprotein group called heme.  Each heme contains an iron ion (Fe ) that can bind to an oxygen molecule. Each hemoglobin molecule can thus bind up to four oxygen molecules.  In blood, 98.5% of the oxygen is bound to hemoglobin (Figure 44.13). Cooperative binding  The oxygen–hemoglobin dissociation curve plots the percentage saturation of hemoglobin versus the partial pressure of oxygen in blood (Figure 44.14).  This curve is sigmoidal, indicating that the oxygen displays cooperative binding to hemoglobin.  This means that the binding of oxygen to a hemoglobin subunit makes the remaining subunits more likely to bind oxygen. Conversly, the loss of a bound oxygen make to loss of additional oxygens more likely.  Cooperative binding of oxygen by hemoglobin results in greater oxygen delivery that noncooperative binding (Figure 44.15). How do temperature and pH affect haemoglobin?  Hemoglobin is also sensitive to changes in pH and temperature.  CO 2roduced by exercisinf muscle reacts with water in blood to form carbonic acid, which dissociates and releases a hyrdrogen ion. As a result pH drops.  Decreases in pH and increases in temperature alter hemoglobin’s conformation such that it is more likely to release 2 at all values of P2 .  This phenomenon is known as the Bohr shift.  Bohr shift is important because it makes haemoglobin more likely to release oxygen during exercise or other conditions in which Po 2s high, pH is low and tissues under oxygen stress. Comparing hemoglobins  Fetuses have a fetal hemoglobin that has a higher affinity for oxygen than adult hemoglobin.  This ensures an adequate supply of oxygen as the fetus develops. CO t2ansport and the buffering of blood pH  CO 2hat is produced by cellular respiration enters the blood, where it is quickly converted to bicarbonate ions and protons in a reaction catalyzed by carbonic anhydrase.  The protons produced by the carbonic anhydrase reaction induce the Bohr shift, which makes hemoglobin more likely to release oxygen.  The partial pressure of CO 2n blood drops when CO is c2nverted to bicarbonate, maintaining a strong partial pressure gradient favoring the entry of CO 2nto red blood cells (Figure 44.18).  Carbonic anhydrase activity makes CO uptake more tissues more efficient. Once bicarbonate ion form 2 in RBC, they diffuse into blood plasma along [] gradient. Bicarbonate ions via an antiporter that exchanges bicarbonate with chloride ions.  Protons produced by reaction stay inside RBC. Small amounts build up in plasma.  When hemoglobin is not carrying O , i2 has a high affinity for protons and acts as a buffer—a compound that minimizes changes in pH.  In the lungs, CO 2iffuses from the blood into the alveoli, and the carbon dioxide partial pressure (Pco ) 2 in the blood declines.  Bicarbonate is converted back to CO , 2hich then diffuses into the alveoli and is exhaled from the lungs.  Hemoglobin picks up O du2ing inhalation, and the cycle begins again. 44.5 circulatory systems  The function of a circulatory system is to carry blood or hemolymph into close contact with every cell in the body.  The most sophisticated circulatory systems consist of: –one or more pumps called hearts –tough, thick–walled arteries that take blood away from the heart under high pressure –capillaries whose walls are just one cell thick, allowing the exchange of gases and other molecules with tissues in networks called capillary beds –veins that return blood to the heart under low pressure.  Some animals lack some or all of these or have no circulatory system. What is an open circulatory system?  In open circulatory systems, hemolymph is actively pumped throughout the body but is not confined exclusively to blood vessels.  The overall pressure in the system is low.  Key observation about hemolymph is that it comes into direct contact with tissues-meaning the molecules being exchanged between hemolymph and tissue sdo not have to diffuse across walls of a blood.  Hemolymph is a bloodlike tissue that transports wastes and nutrients and may also contain oxygen– carrying pigments, some cells, and clotting agents.  Hemolymph returns to heart when heart relaxes and its internal pressure drops below that in cavity.  One salient characteristic of an open circulatory systm is that hemolymph is under low pressure. Hemolymph flow rates may also be kow. Makes open system suitable for organisms with not that high oxygen demands.  The low pressure of open circulatory systems favors relatively sedentary organisms that do not have high oxygen demands.  Open circulatory system moves hemolymph throughout an animal’s body is much the same way a ceiling fan moves air throughout room in house. What is closed circulatory system?  In closed circulatory systems, blood is completely contained within blood vessels and flows in a continuous circuit through the body under pressure generated by the heart.  Because blood is confined to vessels, the system can generate enough pressure to maintain a high flow rate.  Blood flow can also be directed in a precise way. Regulatory systems can direct blood to specific vessels and thus to specific locations.  Found in vertebraes. Types of Blood vessels  In a closed circulatory system, the heart ejects blood into a large artery, usually called the aorta.  Although all arteries have both muscle fibers and elastic fibers in their walls, the walls of the aorta are dominated by elastic fibers, which allow the aorta to expand when blood enters it under high pressure from the heart.  Arteries carry blood away from the heart.  Arterioles, the smallest arteries, have muscle fibers called sphincters wrapped around their circumference.  The sphincters allow the diameter of the vessel to be carefully regulated in response to signals from the nervous system.  Capillaries are the smallest vessels and are the site where gases, nutrients, and wastes are exchanged between the blood and other tissues.  Veins carry blood back to the heart after it passes through the capillaries.  Because blood is under relatively low pressure by the time it exits the tissues, veins have much thinner walls and much larger
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