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Chapter six Internal Fluids and Respiration Dr. Mohamed Ibrahim Abdi “Soojeede” Master of HSM at KU INTRODUCTION • Single-celled organisms live in direct contact with their environment. They obtain nutrients and oxygen and release wastes directly across the cell surface. These organisms as so small that no special internal system of transport. • Most other multicellular organisms, because of their size, activity, and complexity, need a specialized circulatory system to transport nutrients and respiratory gases to and from all tissues of the body. Internal Fluid Environment • The body fluid of a single-celled organism is cellular cytoplasm, a liquid-gel substance in which the various membrane systems and organelles are suspended. • In multicellular animals body fluids are divided into two main phases, intracellular and extracellular. – The intracellular phase (also called intracellular fluid) is the collective fluid inside all the body’s cells. – The extracellular phase (or fluid) is the fluid outside and surrounding the cells. • In animals having closed circulatory systems extracellular fluid is further subdivided into blood plasma and interstitial (intercellular) fluid. – Blood vessels contain plasma, – whereas interstitial fluid, or tissue fluid as it is sometimes called, occupies spaces surrounding the cells in the body. Interstitial fluid is constantly formed from plasma by filtration through capillary walls. Composition of the Body Fluids • All these fluid spaces—plasma, interstitial, and intracellular—differ from each other in solute composition, but all have one feature in common: they are mostly water. • Body fluids contain many inorganic and organic substances in solution. Principal among these are inorganic electrolytes and proteins. Sodium, chloride, and bicarbonate ions are the chief extracellular electrolytes, whereas potassium, magnesium, and phosphate ions and proteins are the major intracellular electrolytes. • The two subdivisions of extracellular fluid—plasma and interstitial fluid—have similar compositions except that plasma has more proteins. Composition of Blood • Among invertebrates that lack a circulatory system (such as flatworms and cnidarians) it is not possible to distinguish a true “blood.” These forms possess a clear, watery tissue fluid containing some phagocytic cells, a little protein, and a mixture of salts similar to seawater. The “blood” of invertebrates with open circulatory systems is more complex and is often called hemolymph (Gr. haimo, blood, L. lympha, water). In vertebrates, blood is a complex liquid tissue composed of plasma and formed elements, mostly red cells (also called corpuscles), suspended in plasma. Types of Circulatory Systems in Animals • Simple Circulatory Systems • The circulatory system varies from simple systems in invertebrates to more complex systems in vertebrates. The simplest animals, such as the sponges (Porifera), do not need a circulatory system because diffusion allows adequate exchange of water, nutrients, and waste, as well as dissolved gases (figure a). • Fish Circulatory Systems • Fish have a single circuit for blood flow and a two-chambered heart that has only a single atrium and a single ventricle (figure a). The atrium collects blood that has returned from the body, while the ventricle pumps the blood to the gills where gas exchange occurs and the blood is re-oxygenated; this is called gill circulation. The blood then continues through the rest of the body before arriving back at the atrium; this is called systemic circulation. Amphibian Circulatory Systems • In amphibians, reptiles, birds, and mammals, blood flow is directed in two circuits: one through the lungs and back to the heart (pulmonary circulation) and the other throughout the rest of the body and its organs, including the brain (systemic circulation). • Amphibians have a three-chambered heart that has two atria and one ventricle. The two atria receive blood from the two different circuits (the lungs and the systems). There is some mixing of the blood in the heart's ventricle, which reduces the efficiency of oxygenation. • There are another Amphibians which are unique in that they have a third circuit that brings deoxygenated blood to the skin in order for gas exchange to occur; this is called pulmocutaneous circulation. Reptile Circulatory Systems • Most reptiles also have a three-chambered heart similar to the amphibian heart that directs blood to the pulmonary and systemic circuits. The ventricle is divided more effectively by a partial septum, which results in less mixing of oxygenated and deoxygenated blood. Some reptiles (alligators and crocodiles) are the most primitive animals to exhibit a four-chambered heart. • Mammal and Bird Circulatory Systems • In mammals and birds, the heart is also divided into four chambers: two atria and two ventricles. The oxygenated blood is separated from the deoxygenated blood, which improves the efficiency of double circulation and is probably required for the warm-blooded lifestyle of mammals and birds. Mammalian Heart • The mammalian heart is a muscular organ located in the thorax and covered by a tough, fibrous sac, the pericardium. Blood returning from the lungs collects in the left atrium, passes into the left ventricle, and is pumped into the body (systemic) circulation. Blood returning from the body flows into the right atrium, and passes into the right ventricle, which pumps it into the lungs. Respiration • Energy bound up in food is released by oxidative processes, usually with molecular oxygen as the terminal electron acceptor. Physiologists find it is suitable to distinguish two separate but interrelated respiratory processes: – cellular respiration, the oxidative processes that occur within cells, and – external respiration, the exchange of oxygen and carbon dioxide between the organism and its environment. Problems of Aquatic and Aerial Breathing • The two great arenas of animal evolution water and land— are vastly different in their physical characteristics. The most obvious difference is that air contains far more oxygen—at least 20 times more—than does water. • The most advanced fishes with highly efficient gills and pumping mechanisms may use as much as 20% of their energy just extracting oxygen from water. By comparison, the cost for mammals to breathe is only 1% to 2% of their resting metabolism. • In general evaginations of the body surface, such as gills, are most suitable for aquatic respiration; invaginations, such as lungs and tracheae, are best for air breathing. Respiratory Organs • Gas Exchange by Direct Diffusion Protozoa, sponges, cnidarians, and many worms respire by direct diffusion of gases between organism and environment. We have noted that this kind of cutaneous respiration is not adequate when the cellular mass exceeds approximately 1 mm in diameter. • Gas Exchange Through Tubes: Tracheal Systems Insects and certain other terrestrial arthropods (centipedes, millipedes, and some spiders) have a highly specialized type of respiratory system, It consists of a branching system of tubes (tracheae) that extends to all parts of the body. The smallest end channels are fluidfilled tracheoles, less than 1 um in diameter. Air enters and diffuses out the tracheal system through valvelike openings (spiracles). Efficient Exchange in Water: Gills • Gills of various types are effective respiratory devices for life in water. Gills may be simple external extensions of the body surface, such as dermal papulae of sea stars. Most efficient are internal gills of fishes and arthropods. • Fish gills are thin filamentous structures, richly supplied with blood vessels arranged so that blood flow is opposite to the flow of water across the gills. This arrangement, called countercurrent flow, provides the greatest possible extraction of oxygen from water. Gas Exchange through: Lungs • The most airbreathing vertebrates possess lungs, highly vascularized internal cavities. – Amphibian lungs vary from simple, smooth-walled, baglike lungs. – The total surface available for gas exchange is much increased in lungs of reptiles which are subdivided into numerous interconnecting air sacs. – Most elaborate of all are mammalian lungs complexes of millions of small sacs, called alveoli, each veiled by a rich vascular network. – In birds, lung efficiency is improved vastly by adding an extensive system of air sacs. that serve as air reservoirs during ventilation.