Introduction to Physiology

• Physiology is the study of how living organisms function, encompassing mechanical, physical, and biochemical functions of living organisms.
• Understanding physiology is crucial for comprehending life processes from the molecular level to the whole organism.
• It explores the mechanisms that allow organisms to survive and thrive in their environments.

Levels of Organization

• Physiological study spans various levels of biological organization.
• Molecular physiology examines the interactions of molecules like proteins and nucleic acids.
• Cellular physiology focuses on cells, the basic units of life, including their structure and function.
• Tissue physiology concerns the function of tissues, which are groups of similar cells performing specific tasks.
• Organ physiology studies the functions of organs, such as the heart, lungs, and kidneys.
• Systemic physiology looks at the coordinated functions of organ systems, like the nervous, endocrine, and cardiovascular systems.
• Organismal physiology considers the integrated functions of the entire organism.

Homeostasis

• Homeostasis is the maintenance of a stable internal environment despite external changes.
• It is essential for the survival of cells, tissues, and organs.
• The concept was coined by Walter Cannon.
• Physiological variables like body temperature, blood pressure, and blood glucose are tightly regulated.
• Feedback mechanisms, both negative and positive, are critical for maintaining homeostasis.

Negative Feedback

• Negative feedback is the primary mechanism for maintaining homeostasis.
• It works by counteracting changes to return a variable to its set point.
• A sensor detects a deviation from the set point.
• A control center receives information from the sensor and activates an effector.
• The effector produces a response that opposes the initial change, restoring the set point.
• Example: Regulation of body temperature. If body temperature rises, sweating occurs to cool the body down.

Positive Feedback

• Positive feedback amplifies the initial change, moving the variable further from its set point.
• It is less common than negative feedback and usually involved in processes with a clear endpoint.
• Example: Blood clotting. The initial clotting factors activate more clotting factors, leading to rapid clot formation.
• Another example: Childbirth. Uterine contractions stimulate the release of oxytocin, which further enhances contractions until the baby is born.

Control Systems

• Physiological functions are controlled by various systems.
• The nervous system provides rapid, short-term control through electrical and chemical signals.
• The endocrine system provides slower, long-term control through hormones.
• Local control systems involve paracrine and autocrine signaling, where cells communicate with nearby cells through chemical messengers.

Cellular Physiology

• Cells are the fundamental units of life.
• Cellular physiology focuses on processes within cells, such as membrane transport, cell signaling, and energy production.
• Membrane transport: Movement of substances across the cell membrane, including passive and active transport.
• Cell signaling: Communication between cells through chemical signals like hormones and neurotransmitters.
• Energy production: Cellular respiration in mitochondria to produce ATP, the energy currency of the cell.

Membrane Transport

• Passive transport does not require energy.
• Simple diffusion: Movement of molecules from an area of high concentration to an area of low concentration.
• Facilitated diffusion: Movement of molecules across the membrane with the help of transport proteins.
• Osmosis: Movement of water across a semipermeable membrane from an area of high water concentration to an area of low water concentration.
• Active transport requires energy (ATP).
• Primary active transport: Uses ATP directly to move molecules against their concentration gradient, e.g., sodium-potassium pump.
• Secondary active transport: Uses the energy stored in ion gradients to move other molecules, e.g., co-transport of glucose and sodium.

Cell Signaling

• Cells communicate through chemical signals.
• Ligands bind to receptors on the cell surface or inside the cell.
• Signal transduction pathways convert the signal into a cellular response.
• Types of cell signaling include:
• Endocrine signaling: Hormones travel through the bloodstream to target cells.
• Paracrine signaling: Signals affect nearby cells.
• Autocrine signaling: Signals affect the cell that produced them.
• Synaptic signaling: Neurotransmitters transmit signals across synapses.

Energy Production

• Cells produce energy in the form of ATP through cellular respiration.
• Glycolysis: Glucose is broken down into pyruvate in the cytoplasm.
• Krebs cycle (Citric acid cycle): Pyruvate is converted into acetyl-CoA, which enters the Krebs cycle in the mitochondria.
• Electron transport chain: Electrons are passed along a series of protein complexes in the mitochondrial membrane, generating ATP.
• Anaerobic respiration: In the absence of oxygen, cells can produce ATP through fermentation.

Muscle Physiology

• Muscle tissue is responsible for movement.
• Three types of muscle tissue: skeletal, smooth, and cardiac.
• Skeletal muscle: Voluntary, striated muscle attached to bones.
• Smooth muscle: Involuntary muscle found in the walls of internal organs.
• Cardiac muscle: Involuntary, striated muscle found in the heart.

Skeletal Muscle Contraction

• Skeletal muscle contraction is controlled by the nervous system.
• Motor neurons release acetylcholine at the neuromuscular junction.
• Acetylcholine binds to receptors on the muscle fiber, causing depolarization.
• Depolarization leads to the release of calcium from the sarcoplasmic reticulum.
• Calcium binds to troponin, exposing the myosin-binding sites on actin.
• Myosin heads bind to actin, forming cross-bridges.
• Myosin heads pull actin filaments towards the center of the sarcomere, shortening the muscle fiber.
• ATP is required for myosin heads to detach from actin and reset for another cycle.

Cardiovascular Physiology

• The cardiovascular system transports blood, oxygen, nutrients, and hormones throughout the body.
• Key components: heart, blood vessels (arteries, veins, capillaries), and blood.
• Heart: Pumps blood through the circulatory system.
• Blood vessels: Provide pathways for blood flow.
• Blood: Carries oxygen, nutrients, and waste products.

Cardiac Cycle

• The cardiac cycle consists of systole (contraction) and diastole (relaxation).
• Atrial systole: Atria contract, pushing blood into the ventricles.
• Ventricular systole: Ventricles contract, pushing blood into the pulmonary artery and aorta.
• Diastole: Atria and ventricles relax, filling with blood.
• Heart rate: Number of cardiac cycles per minute (beats per minute).
• Stroke volume: Volume of blood ejected from the ventricle per beat.
• Cardiac output: Volume of blood pumped by the heart per minute (heart rate x stroke volume).

Respiratory Physiology

• The respiratory system is responsible for gas exchange (oxygen and carbon dioxide).
• Key components: lungs, airways (trachea, bronchi, bronchioles), and diaphragm.
• Ventilation: Movement of air into and out of the lungs.
• External respiration: Exchange of gases between the lungs and the blood.
• Internal respiration: Exchange of gases between the blood and the tissues.

Pulmonary Ventilation

• Inspiration: Air flows into the lungs when alveolar pressure decreases.
• Diaphragm contracts and flattens, increasing the volume of the thoracic cavity.
• External intercostal muscles contract, lifting the rib cage.
• Expiration: Air flows out of the lungs when alveolar pressure increases.
• Diaphragm relaxes and returns to its dome shape, decreasing the volume of the thoracic cavity.
• Internal intercostal muscles contract, depressing the rib cage.

Renal Physiology

• The renal system (kidneys) filters blood, removes waste products, and regulates fluid and electrolyte balance.
• Key functions include:
• Filtration: Movement of fluid and solutes from the blood into the nephron.
• Reabsorption: Movement of water and solutes from the nephron back into the blood.
• Secretion: Movement of solutes from the blood into the nephron.
• Excretion: Elimination of waste products in the urine.

Nephron Function

• The nephron is the functional unit of the kidney.
• Glomerulus: Filters blood, producing filtrate.
• Proximal convoluted tubule: Reabsorbs most of the water, glucose, amino acids, and sodium.
• Loop of Henle: Establishes a concentration gradient in the medulla of the kidney.
• Distal convoluted tubule: Regulates sodium, potassium, and pH balance.
• Collecting duct: Collects urine and transports it to the renal pelvis.

Endocrine Physiology

• The endocrine system regulates body functions through hormones.
• Hormones are chemical messengers that travel through the bloodstream to target cells.
• Key endocrine glands: pituitary gland, thyroid gland, adrenal glands, pancreas, ovaries, and testes.

Hormone Action

• Hormones bind to receptors on target cells, initiating a cellular response.
• Steroid hormones: Lipid-soluble, bind to intracellular receptors, affecting gene transcription.
• Peptide hormones: Water-soluble, bind to cell surface receptors, activating signal transduction pathways.
• Examples:
  • Insulin: Regulates blood glucose levels.
  • Thyroid hormones: Regulate metabolism.
  • Cortisol: Regulates stress response.

Neurophysiology

• The nervous system controls and coordinates body functions through electrical and chemical signals.
• Key components: brain, spinal cord, and nerves.
• Neurons: Nerve cells that transmit electrical signals.
• Neuroglia: Support cells that protect and nourish neurons.

Neuron Function

• Neurons transmit signals through action potentials.
• Resting membrane potential: The electrical potential difference across the neuron membrane when it is not stimulated.
• Depolarization: The membrane potential becomes more positive, making the neuron more likely to fire an action potential.
• Repolarization: The membrane potential returns to its resting value.
• Action potential: A rapid, short-lasting change in membrane potential that travels along the axon.
• Synaptic transmission: Release of neurotransmitters from the presynaptic neuron, which bind to receptors on the postsynaptic neuron, transmitting the signal.

Physiology studies the functions of living organisms, covering mechanical, physical, and biochemical aspects. It is crucial for understanding life processes at all levels, from molecular interactions to organismal functions. It explores how organisms survive and thrive.