Animal physiology
Animal physiology is a branch of biology that investigates the various functions and processes that sustain life in animals, focusing on their organs, organ systems, and cells. It encompasses the study of how different systems, such as the neuromuscular, cardiovascular, and respiratory systems, operate and maintain homeostasis. Early thinkers like Aristotle laid the groundwork for physiological understanding, while William Harvey is recognized as a pivotal figure for applying experimental methods to explore bodily functions. Research in this field often employs comparative physiology, examining how diverse species adapt their physiological processes to different environments, such as aquatic versus terrestrial habitats.
Animal physiology is characterized as a synthetic science that integrates concepts from biology, chemistry, mathematics, and physics to explain bodily functions. The field has significant implications for human and animal health, as well as evolutionary biology, but it also raises ethical considerations regarding animal testing. Experimental approaches often utilize model organisms, such as fruit flies, to glean insights about genetic and physiological phenomena. Recent studies have highlighted the impact of climate change on animal physiology, revealing how species are adapting to environmental shifts. Understanding these processes is crucial for comprehending broader ecological dynamics and the evolutionary history of life on Earth.
Animal physiology
Animal physiology is a subdiscipline of biology that focuses on the properties, functions, and homeostatic processes that support life in animals, including their organs, organ systems, and cells. Early Greek philosophers like Aristotle used deductive reasoning—the use of observations, logic, and intuition—to explain function. William Harvey (1578-1657) is generally considered the father of modern physiology, as he was among the first and most successful in employing inductive reasoning (experimentation) to explain body function. Inductive reasoning uses the scientific method to study unsolved problems, which begins with a hypothesis. Experiments are designed to test the hypothesis, and data are collected using quantified measurements. The result is always expressed in quantified terms. For example, results may indicate that the heart rate increased from thirty to forty-five beats per minute. These results are analyzed and interpreted in terms of the original hypothesis. The interpretation usually leads to new hypotheses, which are then tested with new experiments.
The Study of Systems
Classically, physiology has been divided into several organ systems—neuromuscular (nerves and muscles), cardiovascular (circulatory), pulmonary (lungs), renal (kidneys), digestive (nutrition), and endocrine (hormones). This nomenclature is based on mammalian systems and must be modified to include such alternative systems as gills (branchial system) in aquatic animals.
Physiology has been characterized as a synthetic science because it synthesizes biology, chemistry, mathematics, and physics to describe body functions. The objective is to explain a given physiological function in terms that obey the laws of chemistry and physics. This often involves model building. A physiological model is a construct of hypotheses, which can be qualitative or quantitative. Harvey proposed a model in which the blood circulates from the heart to arteries and then to capillaries which connect to veins to return the blood to the heart. He then did experiments to prove this. Quantitative or mathematical models are used to describe many functions using mathematical constructs and equations.
Physiology is studied at the molecular, cellular, tissue, organ, system, and organismic levels. There are several branches of physiology. Mammalian, fish, and insect physiologists restrict their studies to a certain group. General physiology seeks to describe functions that are common to all life forms, such as cell membrane function. Comparative physiology examines how different groups of animals accomplish similar goals while living in completely different circumstances, for example, in aquatic environments, exchanging oxygen and carbon dioxide with water versus living on land and exchanging these respiratory gases with air.
Comparative studies are important for several reasons. First, the acquisition of each new piece of specific knowledge raises questions about its broader applicability. Once it is known that land animals need oxygen in the air to breathe, the question arises: If a fish spends its entire life underwater, why does it not drown? The answer is that it uses gills to extract oxygen efficiently from water (with a notoriously scarce oxygen supply) and to excrete carbon dioxide. Comparative physiology also helps to better understand evolution. The evolution of the vertebrate cardiovascular system from fish with two-chambered hearts to amphibians and most reptiles with three-chambered hearts to birds and mammals with four-chambered hearts has attracted much interest in how the circulation of oxygen-rich blood is kept separate from oxygen-poor blood. Finally, comparative physiology can be used to study simple physiological systems in primitive animals, such as invertebrates, to help explain more complex systems in more advanced animals, such as mammals.
Gaining Knowledge Through Experimentation
Research and experimentation in animal physiology have important applications for human and animal health, evolutionary research, and gene engineering. However, the ethical considerations of testing on animals are often debated, and many professional organizations that investigate animal physiology have guidelines for their scientists. Some scientists have developed models or robots that replicate particular animal behaviors or processes that aid research, but these are not perfect. Fruit flies (Drosophila melanogaster) are often used in studies because their genetic makeup is similar to that of humans and other mammals, allowing a sort of proxy experiment model. Research using fruit flies has provided evidence concerning genetics, ecology, alcohol and drug addiction, neurodegenerative disorders, and various diseases.
Our knowledge of how information is carried by neurons (the individual fibers of nerves) began with studies of squid neurons. Squids have giant neurons; they are so large that physiologists could insert electrodes inside them to discover the electrical events that produce nervous impulses and, thus, information transfer. Beyond this, the neurons are so large that it is possible to remove their contents and substitute artificial solutions to see how this alters neural impulse production. This is how it was discovered that the ions sodium and potassium are responsible for neural impulses. This research started in the 1920s, and developing technology has shown that mammalian neural function operates essentially the same way as squid neural function.
Another example of such use of comparative physiology has led to the understanding of kidney function. In the early twentieth century, it was observed that kidney tubules in amphibians, such as frogs and salamanders, are large enough to allow samples of nephric tubular fluid to be removed and analyzed. Such studies led to the discovery that the initial event in urine formation is the filtration of blood plasma in glomerular capillaries and that the final urine composition results from selective reabsorption and filtration of water and solutes in nephrons.
In a longitudinal study of birds that died after flying into Chicago’s skyscrapers from the late 1990s to the early 2020s, researchers found that birds across species are shrinking due to climate change while their wingspan is slightly increasing. This research provides crucial insight into the physiological implications of the changing environment on birds and other animals, highlighting the urgent need for further studies. Further research found that competition and light pollution have also played a role in the shrinking of polar bears, fish, and some animals’ eyes, emphasizing the multifaceted impact of climate change on animal physiology.
Studies focusing on animals’ adaptive evolution provide scientists with information on the evolutionary processes that resulted in modern-day animals, including humans, and help predict changes and possible issues. For example, using gene analysis, specialized imaging, and the fossil record, scientists now understand that the velvet worm (phylum Onychophora) is an important evolutionary link between arthropods and annelids. Some fish, like the Lobe-finned fish (clade Sarcopterygii) and sea robins (phylum Chordata), use their fins in a similar way to walking. Scientists speculate these animals are likely ancestors of the tetrapods and are an evolutionary link to the transition of vertebrates from the water to land. Some animals experience rapid adaptive physiological changes when introduced to a new environment. Tawny owls (Strix aluco) and peppered moths (order Lepidoptera) have changed color in the twenty-first century to blend into their changing environment. Understanding this physiological process and other changes in animal physiology allows a better understanding of the Earth, all living things, climate change, and much more.
Principal Terms
Artery: A blood vessel that carries blood from the heart to the body tissues
Capillary: A small vessel that connects arteries to veins; this is where respiratory gases, nutrients, and wastes are exchanged between blood and tissues
Glomerulus: A specialized capillary in the kidneys that filters blood
Nephron: A tubular structure in the kidneys that extracts filtrate, reabsorbs nutrients and other valuable substances and secretes wastes
Neuron: An individual nerve cell; nerves are made up of many neurons bundled together
Vein: A blood vessel that returns blood to the heart
Bibliography
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