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David Sadava, David M. Hillis, H. Craig Heller, May R. Berenbaum La nuova biologia. blu Anatomia e fisiologia dei viventi S
Circulatory System and Gas Exchange
How Is the Circulatory System Made? A circulatory system (also known as the cardiovascular system) consists of: • Muscular pump—the heart • Fluid—blood • Series of conduits—blood vessels It’s a closed systems (completely contain the fluid in a continuous system of vessels) with four-chambered hearts and completely separate pulmonary and systemic circuits.
The Human Heart and Circulation Valves prevent back-flow of blood: • Atrioventricular (AV) valves between atria and ventricles prevent back-flow when ventricles contract. • Pulmonary valve and aortic valve between ventricles and major arteries prevent back-flow when ventricles relax.
How Does the Mammalian Heart Function? The right atrium receives deoxygenated blood from the body through two large veins: • Superior vena cava—blood from upper body • Inferior vena cava—blood from lower body Blood passes from the right atrium through an AV valve into the right ventricle. The atrium contracts, then the ventricle contracts. The AV valve closes and blood is pumped through the pulmonary artery to the lungs.
How Does the Mammalian Heart Function? Pulmonary veins return oxygenated blood to the left atrium. Oxygenated blood moves into the left ventricle through an AV valve. The left atrium contracts, then the left ventricle. The aortic valve opens and blood flows into the aorta.
How Does the Mammalian Heart Function? In the cardiac cycle both sides of the heart contract at the same time—first the atria, then the ventricles. Two phases: • Systole—when ventricles contract • Diastole—when ventricles relax
How Does the Mammalian Heart Function? Cardiac cells are in electrical contact with each other through gap junctions—action potentials can spread rapidly. Large groups of cardiac muscle contract in unison— essential for pumping blood efficiently. Pacemaker cells can initiate action potentials without input from the nervous system. The primary pacemaker is the sinoatrial node.
The Heartbeat (Part 1) The action potential in the atria stimulates the atrioventricular node. The node consists of non-contracting cells that send action potentials to the ventricles via the bundle of His.
The Heartbeat (Part 2) Fibers spread throughout the ventricles and are called Purkinje fibers.
What Are the Properties of Blood Vessels? Arteries carry blood away from the heart and branch into arterioles that feed into capillary beds. Capillaries: tiny, thin-walled vessels; site of exchange between blood and tissue fluid. Venules drain the capillary beds and form veins, which deliver blood back to the heart.
What Are the Properties of Blood Vessels? The anatomy of different types of vessels reflects their functions. Walls of large arteries have extracellular collagen and elastin fibers, which enable them to withstand high blood pressures. Smooth muscle cells in artery and arteriole walls constrict or dilate those vessels.
What Are the Properties of Blood Vessels? Arterioles are called resistance vessels because their resistance can vary to control the blood flow to specific tissues. Beds of capillaries lie between arterioles and venules. Capillary walls are thin and permeable to water and many solutes. Blood flows slowly through capillaries, allowing time for exchange.
What Are the Properties of Blood Vessels? Capillary walls are a single layer of endothelial cells and are permeable to water, ions, and small molecules, but not to large proteins. Permeability varies widely in different capillary beds, water and small solutes pass through spaces in the capillary wall, in some cases through holes called fenestrations.
What Are the Properties of Blood Vessels? Precapillary sphincters on arterioles can shut off blood supply. Auto-regulation depends on smooth muscle being sensitive to its chemical environment. Hyperemia: Low O 2 and high CO 2 levels cause smooth muscle to relax, increasing blood supply to capillary bed.
What Are the Properties of Blood Vessels? Blood flows through large arteries rapidly at high pressures, but when it reaches the capillaries, pressure and rate of flow decrease. The sum total cross-sectional area of the capillaries is much greater than that of any other class of vessels.
What Are the Properties of Blood Vessels? Blood returning from the upper body is assisted by gravity. Blood returning from below the heart is assisted by skeletal muscle contractions that squeeze the veins. One-way valves in the veins prevent back-flow.
How Is the Circulatory System Controlled and Regulated? The cardiovascular control center in the medulla controls heart rate and vessel constriction. Information on blood composition and pressure comes from baroreceptors (stretch receptors) and chemoreceptors in the aorta and carotid arteries.
What Are the Properties of Blood? Blood is a connective tissue made of cells in a liquid extracellular matrix, called plasma. Packed-cell volume, or hematocrit, is the percent of blood that is red blood cells. Normal hematocrit is 42– 46%, but can vary: higher in people who live at high altitudes because low O 2 stimulates production of more red blood cells.
What Are the Properties of Blood? Mature erythrocytes, or red blood cells, are biconcave, flexible discs packed with hemoglobin. They transport respiratory gases; the shape gives them a large surface area for gas exchange, and flexibility enables them to squeeze through narrow capillaries. Red blood cells are generated in the bone marrow. Erythropoietin, a hormone released in the kidney in response to hypoxia, controls red blood cell production.
What Are the Properties of Blood? Bone marrow stem cells also produce megakaryocytes that break off cell fragments called platelets. Platelets initiate blood clotting when activated by collagen exposed in damaged blood vessels. They release chemical clotting factors that activate other platelets and also form a sticky plug at the damaged site.
What Are The Major Diseases? Atherosclerosis is “hardening of the arteries. ” The endothelial lining of arteries is damaged by high blood pressure, smoking, high-fat diet, or microorganisms. Plaque forms at damage sites.
What Are the Major Diseases? The coronary arteries supply blood to the heart muscle. Atherosclerosis in coronary arteries reduces blood flow; marked by chest pain and shortness of breath. Coronary thrombosis—a thrombus that forms in a coronary artery can lead to a heart attack, or myocardial infarction. An embolus is a piece of a thrombus that breaks loose. • It may cause an embolism if it lodges in a smaller blood vessel. • Arteries narrowed by plaque are likely places for an embolism. • If the embolism is in the brain, the cells fed by that artery will die, causing a stroke.
Why Do Animals Need a Circulatory System? Some animals do not need circulatory systems: • Single-celled organisms exchange materials directly with the environment. • Some aquatic multicellular organisms are small or thin, so that all cells are close to the environment to allow exchange. • Other aquatic organisms have branched gastrovascular systems that bring the external environment inside the animal (e. g. , sponges). • Large animals without a circulatory system tend to be inactive or sessile.
Why Do Animals Need a Circulatory System? Open circulatory systems (fluid leaves the circulatory system and moves between the cells) are found in arthropods and mollusks. Closed circulatory systems: • Blood is kept separate from the interstitial fluid. • Blood is pumped through the vascular system by one or more hearts. Earthworms have a closed system.
How Have Vertebrate Circulatory Systems Evolved? In fishes, the heart pumps blood to the gills and then the rest of the body in a single circuit. Turtles, snakes, and lizards have three-chambered hearts. Birds and crocodiles have four completely separated chambers. Amphibians circulatory system. Birds and mammals have four -chambered hearts and completely separate pulmonary and systemic circuits.
What Physical Factors Govern Respiratory Gas Exchange? Animals must exchange the respiratory gases O 2 and CO 2. Gas exchange systems are made up of: • Specialized surfaces where gas can move between the body and the environment • Mechanisms that ventilate the environmental side and perfuse the internal side. Passive diffusion is the only means of gas exchange across these surfaces.
How Do Human Lungs Work? Air enters the human lung through the mouth or nasal passage, which join in the pharynx. Below the pharynx, the trachea leads to the lungs— at the beginning is the larynx, or voice box.
How Do Human Lungs Work? The trachea branches into two bronchi, then into bronchioles, and then into alveoli—the sites of gas exchange. The combined surface area of the alveoli is about 70 m 2. Alveoli have thin walls and are surrounded by capillaries. The diffusion path between blood and air is less than two micrometers.
The Human Respiratory System
How Do Human Lungs Work? Human lungs are suspended inside thoracic cavity. The diaphragm is a sheet of muscle at the bottom of the cavity. The pleural membrane covers each lung and lines the thoracic cavity. The pleural space contains fluid to help the membranes slide past each other during breathing. A surfactant reduces the surface tension of a liquid.
How Do Human Lungs Work? Inhalation begins when the diaphragm contracts while exhalation begins when the diaphragm stops contracting and relaxes. The intercostal muscles between the ribs can also change the volume of the thoracic cavity. Abdominal muscles also participate.
What Adaptations Maximize Respiratory Gas Exchange? A spirometer measures the amount of air breathed in and out. Tidal volume (TV) is the amount of air that moves in and out per breath when at rest. Inspiratory (IRV) and expiratory (ERV) reserve volumes are the additional amounts of air that we can forcefully inhale or exhale. Vital capacity (VC) = TV + IRV + ERV. Athletes generally have high vital capacity. VC decreases with age because lung tissue stiffens.
What Adaptations Maximize Respiratory Gas Exchange? Residual volume (RV) = air that can’t be expelled from the lungs. Allows calculation of the functional residual volume (FRV) which equals ERV + RV.
How Is Breathing Regulated? Breathing is controlled by neural circuits in the brain stem. Axons of respiratory motor neurons in the medulla form the phrenic nerve, which innervates the diaphragm and initiates inhalation. Neurons in the pons help regularize the basic respiratory rhythm.
How Does Blood Transport Respiratory Gases? Blood plasma carries some O 2 in solution, but most vertebrate blood contains molecules that bind reversibly to O 2 is picked up where the partial pressure is high and is released where the partial pressure is lower. Red blood cells contain the protein hemoglobin, which has 4 polypeptide subunits. Each polypeptide surrounds an iron-containing heme group that can bind a molecule of O 2.
How Does Blood Transport Respiratory Gases? The change in affinity is reflected in the increased steepness of the O 2 binding curve. The influence of O 2 binding by one subunit on the O 2 affinity of the other subunits is called positive cooperativity.
How Does Blood Transport Respiratory Gases? Blood must also carry CO 2 away from tissues. CO 2 is highly soluble and readily diffuses into the blood plasma, where it is converted into bicarbonate ions (HCO 3–). In endothelial cells and red blood cells carbonic anhydrase speeds up the conversion.
How Does Blood Transport Respiratory Gases? Myoglobin in muscle cells is a single polypeptide molecule that can bind one molecule of O 2. It has a higher affinity for O 2, and binds it at low PO 2 values when hemoglobin molecules would release their O 2. It provides a reserve for high metabolic demand for O 2.
What Physical Factors Govern Respiratory Gas Exchange? Diffusion rate of O 2 in water is very slow. This limits the size and shape of invertebrate species without internal systems for gas exchange. They are mostly small; some have flat bodies to increase surface area; or thin bodies built around a central cavity.
What Adaptations Maximize Respiratory Gas Exchange? Adaptations to maximize respiratory gas exchange: • Increase surface area • Maximize partial pressure gradients • Minimize diffusion path length • Minimize diffusion that takes place in an aqueous medium
What Adaptations Maximize Respiratory Gas Exchange? Insects have a tracheal respiratory system. Fish gills use countercurrent flow to maximize gas exchange.
What Adaptations Maximize Respiratory Gas Exchange? Bird lungs have unidirectional air flow, instead of bidirectional as in mammals. Interconnected air sacs receive inhaled air but are not sites of gas exchange. Air enters through the trachea, which divides into primary bronchi, which extend to the posterior air sacs and branch into secondary bronchi.
Adapted from Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014 Inc. All rights reserved