Organismal Biology #31
GAS EXCHANGE and BODY FLUIDS

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GAS EXCHANGE:

Flatworms and other flattened animals need no special organs for gas exchange because no cell is very far from a body surface. More complex animals use lungs, gills, or tracheal tubes. Anatomy and physiology of gas exchange in land vertebrates: Nostrils take air into nasal cavity, then into pharynx. Floor of pharynx opens behind mouth into larynx (voice-box); entrance to larynx is guarded by epiglottis. The trachea, bronchi, and bronchioles form tree-like branchings within each lung. The lungs have air sacs lined with box-like alveoli. Air exchange: In inhalation (inspiration), diaphragm contracts and moves downward while intercostal muscles raise rib cage.     In exhalation (expiration), muscles relax, rib cage falls, diaphragm springs upward. Gas exchange in alveoli: Oxygen enters capillaries of lung through thin walls; CO2 leaves capillaries and diffuses into air sac. Gas exchange within capillary blood: In lung alveoli, oxygen enters red blood cells, combines with hemoglobin, and is transported as HbO2 (oxyhemoglobin); bicarbonate ions enter blood cells and are split into water and CO2. The reverse occurs in body tissues: oxyhemoglobin breaks down to release oxygen; CO2 and water combine to form bicarbonate ions (HCO3–). Gill systems:   In fishes and many other aquatic animals, thin-walled arteries run through gills with direction of blood flow usually opposite to flow of water (counter-current exchange). Oxygen diffuses into these arteries; CO2 diffuses into surrounding water. Insect tracheal systems: Air diffuses through many branched tubes (tracheae). Air movement is passive most of the time, but, when flying, rhythmic muscular contractions force air in and out.

Illustrations: Body fluids

INTERNAL TRANSPORT:

Very small or very thin organisms need no special system for internal transport. Many invertebrates have an open system, with blood vessels opening into a general circulatory cavity or hemocoel. Vertebrates and annelids have a closed circulatory system: their hearts pump blood from atrium to ventricle and then through the major arteries; veins return blood to the heart. Simple forms of transport: Cytoplasmic streaming (cyclosis): Cytoplasm in all eucaryotic cells continually flows and changes direction. Diffusion: Passive transport in all organisms, effective only at distances of a few cells. This may suffice for organisms in which each part is only a few cells away from a body surface, but larger animals need circulatory systems. Open circulatory systems: Systems in which a body cavity or hemocoel contains most of the circulating fluid, as in insects. The pumping action of a heart drives fluid forward through an aorta, then through a series of arteries. No veins exist; used blood seeps into sinuses that drain into the hemocoel. Closed circulatory systems: Systems in which blood is everywhere contained in vessels, as in all vertebrates. The heart may have 2 to 4 chambers. The heartbeat originates from a pacemaker at the sinoatrial node. Highest pressure, at maximum contraction, is called systole; lowest pressure is called diastole. In mammals, the right atrium pumps oxygen-poor blood from the body's tissues into the right ventricle, which pumps it through the pulmonary arteries into the lungs. The left atrium meanwhile pumps oxygen-rich blood from the lungs into the left ventricle, which pumps it through the aorta for distribution throughout the body. Arteries carry blood from the heart to the body's tissues. Veins return the blood from the body's tissues back to the heart. Valves in veins prevent the blood from flowing backward. Vertebrate blood is always red because of the oxygen-carrying pigment hemoglobin, carried in red blood cells (erythrocytes).

OSMOREGULATION and EXCRETION: Freshwater organisms tend to gain water across membrane surfaces and must actively get rid of it. Land and marine organisms tend to lose water; they must retain water and excrete salt. Vertebrate kidneys filter the blood first, then retrieve (resorb) useful molecules. Osmotic pressure measures the level of dissolved ions in solution. Hypotonic solutions (low osmotic pressure, few dissolved ions):   Cells swell (or may burst) because water diffuses in. Freshwater organisms always gain water from hypotonic surroundings; they void lots of dilute urine and may actively take up some ions. Hypertonic solutions (high osmotic pressure, many ions):   water diffuses out; cells shrink. Marine and land animals lose water across membranes; they excrete concentrated urine or salt-rich fluids. Isotonic solutions: Cells have the same concentration of dissolved ions. Water enters and leaves at the same rate, so cells stay the same size. Simple excretory systems: Some freshwater protists pump water out by contractile vacuoles. Many small aquatic animals allow wastes to diffuse out. Flatworms have single-celled excretory tubules called flame cells. Nephridial systems: Tubules (nephridia) drain coelomic fluid from the body cavity and exchange ions with small blood vessels nearby. Vertebrate kidneys:   Cortex (outer layer) contains mostly glomeruli and convoluted tubules; medulla (inner layer) is made of several medullary pyramids, which contain Henle's loops. Kidney tubules:   Blood plasma is filtered from a series of thin-walled blood vessels (the glomerulus) into Bowman's capsule. In the proximal convoluted tubule, the blood resorbs glucose and some ions. In mammals, Henle's loop resorbs water. In the distal convoluted tubule, more ions return to the blood. Collecting tubules finally concentrate the urine and drain into the renal pelvis, which drains into the ureter. Nitrogen wastes: In mammals, the principal nitrogen waste is urea. Reptiles and insects excrete uric acid instead. Fishes and many other aquatic animals usually excrete ammonium salts. Other organs of excretion: Lungs and gills get rid of CO2. Animals excrete salt and nitrogen wastes through the skin. REVIEW:         Study guide and vocabulary Index             Syllabus Prev rev. March 2020 Next