Introduction to Physiology

• Physiology is the study of how living organisms function, encompassing mechanical, physical, and biochemical functions
• It explores how organs, cells, and biomolecules work individually and collectively to maintain life
• It involves understanding the interactions between different systems in the body and their responses to internal and external stimuli

Levels of Organization

• Physiology spans multiple levels of organization, from molecular to systemic
• Molecular physiology examines the interactions of molecules, including proteins, nucleic acids, and lipids, and how these interactions influence cellular function
• Cellular physiology studies the functions of individual cells, including membrane transport, cell signaling, and energy production
• Tissue physiology focuses on the integrated function of cells within tissues, such as muscle contraction, nerve impulse transmission, and epithelial transport
• Organ physiology investigates the specific functions of organs like the heart, lungs, kidneys, and brain
• Systemic physiology studies the coordinated function of multiple organ systems working together to maintain homeostasis
• Organismal physiology examines the overall function of the entire organism, including its adaptations to the environment

Homeostasis

• Homeostasis is the maintenance of a stable internal environment despite external changes
• It is essential for the survival and function of cells, tissues, and organs
• Physiological systems use feedback mechanisms to maintain homeostasis
• Negative feedback reverses deviations from a set point, stabilizing the system
• Positive feedback amplifies a change, leading to a rapid response, but it is less common due to its potential for instability
• Examples of homeostatic variables include body temperature, pH, blood glucose levels, and blood pressure

Cell Physiology

• Cells are the basic structural and functional units of living organisms
• Cell membranes regulate the movement of substances in and out of cells
• Membrane transport mechanisms include passive transport (diffusion, osmosis, facilitated diffusion) and active transport (primary and secondary)
• Cell signaling involves communication between cells through chemical messengers like hormones, neurotransmitters, and growth factors
• Signal transduction pathways convert extracellular signals into intracellular responses
• Cellular metabolism includes catabolic processes (breakdown of molecules) and anabolic processes (synthesis of molecules)
• Energy production in cells occurs through cellular respiration (glycolysis, Krebs cycle, oxidative phosphorylation) and fermentation

Muscle Physiology

• Muscle tissue is specialized for contraction, enabling movement and force generation
• There are three types of muscle tissue: skeletal, smooth, and cardiac
• Skeletal muscle is voluntary and striated, responsible for movement of bones and joints
• Smooth muscle is involuntary and non-striated, found in the walls of internal organs and blood vessels
• Cardiac muscle is involuntary and striated, forming the walls of the heart
• Muscle contraction involves the interaction of actin and myosin filaments
• In skeletal muscle, the sliding filament mechanism is regulated by calcium ions and the proteins troponin and tropomyosin
• Smooth muscle contraction is regulated by calcium ions and calmodulin
• Cardiac muscle contraction is similar to skeletal muscle but with unique features like automaticity and long refractory period

Nerve Physiology

• Nerve tissue is specialized for communication, transmitting electrical and chemical signals throughout the body
• Neurons are the basic functional units of the nervous system, consisting of a cell body, dendrites, and an axon
• Nerve impulses, or action potentials, are rapid changes in membrane potential that propagate along the axon
• Action potentials are generated by the opening and closing of voltage-gated ion channels
• Synapses are junctions between neurons where neurotransmitters are released to transmit signals
• Neurotransmitters bind to receptors on the postsynaptic cell, causing either excitation or inhibition
• The nervous system is divided into the central nervous system (brain and spinal cord) and the peripheral nervous system (nerves outside the CNS)
• The peripheral nervous system includes the somatic nervous system (voluntary control of skeletal muscles) and the autonomic nervous system (involuntary control of internal organs)

Cardiovascular Physiology

• The cardiovascular system transports blood, oxygen, nutrients, hormones, and waste products throughout the body
• The heart is a muscular pump that circulates blood through the pulmonary and systemic circuits
• Blood vessels include arteries, veins, and capillaries
• Arteries carry blood away from the heart, veins carry blood to the heart, and capillaries are the site of exchange between blood and tissues
• Blood pressure is the force exerted by blood against the walls of blood vessels
• Blood pressure is regulated by the autonomic nervous system, hormones, and local factors
• Cardiac output is the amount of blood pumped by the heart per minute, determined by heart rate and stroke volume
• Blood consists of plasma (liquid portion) and formed elements (red blood cells, white blood cells, and platelets)
• Red blood cells transport oxygen, white blood cells are involved in immune defense, and platelets are involved in blood clotting

Respiratory Physiology

• The respiratory system is responsible for gas exchange, taking in oxygen and eliminating carbon dioxide
• The respiratory system includes the lungs, airways, and respiratory muscles
• Ventilation is the movement of air into and out of the lungs, driven by pressure gradients
• Gas exchange occurs in the alveoli, where oxygen diffuses into the blood and carbon dioxide diffuses out
• Oxygen is transported in the blood bound to hemoglobin in red blood cells
• Carbon dioxide is transported in the blood as dissolved gas, bicarbonate ions, or bound to hemoglobin
• Respiration is regulated by the brainstem, which controls the rate and depth of breathing
• Chemoreceptors in the brain and blood vessels monitor oxygen and carbon dioxide levels, influencing respiratory rate

Renal Physiology

• The renal system (kidneys) regulates fluid and electrolyte balance, blood pressure, and waste excretion
• The nephron is the functional unit of the kidney, consisting of the glomerulus, proximal tubule, loop of Henle, distal tubule, and collecting duct
• Glomerular filtration filters blood plasma into the nephron
• Tubular reabsorption returns essential substances from the filtrate back to the blood
• Tubular secretion moves waste products from the blood into the filtrate
• Urine concentration is regulated by antidiuretic hormone (ADH), which increases water reabsorption
• The kidneys also produce hormones like erythropoietin (stimulates red blood cell production) and renin (regulates blood pressure)

Endocrine Physiology

• The endocrine system regulates body functions through the release of hormones
• Hormones are chemical messengers that travel through the bloodstream to target cells
• Endocrine glands include the pituitary gland, thyroid gland, adrenal glands, pancreas, ovaries, and testes
• Hormones can be classified as steroid hormones, peptide hormones, or amino acid derivatives
• Steroid hormones bind to intracellular receptors, while peptide hormones bind to cell surface receptors
• Hormone secretion is regulated by feedback mechanisms, neural control, and other hormones
• The hypothalamus and pituitary gland form a neuroendocrine axis that controls many endocrine functions

Gastrointestinal Physiology

• The gastrointestinal (GI) system is responsible for digestion, absorption, and elimination of food
• The GI tract includes the mouth, esophagus, stomach, small intestine, large intestine, rectum, and anus
• Digestion involves the breakdown of food into smaller molecules by mechanical and chemical processes
• Absorption is the movement of digested nutrients from the GI tract into the bloodstream
• Motility is the movement of food through the GI tract by peristalsis and segmentation
• Secretion involves the release of enzymes, hormones, and other substances into the GI tract to aid digestion
• The stomach secretes hydrochloric acid and pepsin to begin protein digestion
• The small intestine is the primary site of nutrient absorption, with the aid of enzymes from the pancreas and bile from the liver
• The large intestine absorbs water and electrolytes and forms feces for elimination