Understanding Human Physiology: Cells, Systems, and Processes
Physiology: The Science of Life
Physiology is the scientific study of the functions of living organisms and the physical and chemical factors that influence these functions. It utilizes the scientific method to explain the complex processes responsible for life.
The Cell: The Basic Unit of Life
The cell is the fundamental unit of all living organisms. It possesses three essential characteristics: reproduction, nutrient transport, and oxygen dependence.
The Body: A Complex System of Cells
The human body is composed of trillions of cells, each specialized to perform specific functions. These cells work together to maintain the overall health and well-being of the organism.
Metabolism: Chemical Reactions in the Body
Metabolism encompasses all the chemical reactions that occur within our bodies. These reactions can be categorized as either aerobic (requiring oxygen) or anaerobic (not requiring oxygen).
Internal Environment and Homeostasis
The internal environment refers to the extracellular fluid surrounding cells. Homeostasis is the process of maintaining stable conditions within this environment to ensure proper cellular function.
Functional Systems of the Body
- Transport of extracellular fluid
- Nutrient acquisition
- Elimination of waste products
- Regulation of bodily functions
- Reproduction
Osmosis and Osmotic Pressure
Osmosis is the net movement of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. Osmotic pressure is the pressure exerted by this water movement and is proportional to the number of solute particles in solution.
Biomolecules: The Building Blocks of Life
Carbohydrates
Carbohydrates are the most abundant biomolecules in nature. They serve as a source of energy, provide structural support and protection, act as lubricants and adhesives, and determine intracellular localization signals.
Proteins
Proteins are the most abundant macromolecules in living cells. They are composed of amino acids linked together by peptide bonds. Proteins have diverse functions, including enzymatic activity, transport, nutrient storage, mechanical support, structural roles, defense (e.g., immunoglobulins), and regulation.
Protein Structure
- Primary structure: The sequence of amino acids and the location of disulfide bonds.
- Secondary structure: The folding of the amino acid chain into alpha helices and beta sheets.
- Tertiary structure: The three-dimensional arrangement of the secondary structures.
- Quaternary structure: The spatial arrangement of multiple polypeptide chains.
Enzymes
Enzymes are proteins that increase the rate of chemical reactions. They are highly specific, acting only on a particular substrate. Enzymes work by providing an alternative reaction pathway with a lower activation energy.
Characteristics of Enzymes
- High catalytic activity
- Recovery of initial state after the catalytic cycle
- Specificity for reaction type and substrate
- No change in the nature of the product
Nucleotides
Nucleotides are organic molecules that play essential roles in cellular energy metabolism. They have various functions:
- Polymers of nucleic acids (DNA and RNA) carrying genetic information
- Carriers of chemical energy (e.g., ATP)
- Involvement in enzyme formation
- Second messengers in cell signaling
Energy Production
ATP is the primary energy currency of cells. When ATP is broken down to ADP, energy is released. Metabolic systems generate ATP from ingested food.
Protein Synthesis
Protein synthesis involves two main steps:
- Transcription: DNA is transcribed into RNA by the enzyme RNA polymerase.
- Translation: RNA is translated into protein by ribosomes, which read the mRNA sequence and assemble amino acids accordingly.
Action Potential
An action potential is a rapid electrical signal that travels along the axon of a neuron. It is generated by the opening and closing of voltage-gated sodium and potassium channels.
Oxygen Transport
Hemoglobin (Hb) in red blood cells binds to oxygen and transports it from the lungs to body tissues. Each Hb molecule can carry four oxygen molecules.
Carbon Dioxide Transport
Most carbon dioxide is transported in the blood as bicarbonate ions. A small amount is carried by Hb.
Neuromuscular Transmission and Muscle Contraction
Neuromuscular transmission is the process by which a nerve impulse is transmitted from a motor neuron to a muscle fiber, leading to muscle contraction.
Steps of Neuromuscular Transmission and Muscle Contraction
- Arrival of motor neuron action potential
- Release of acetylcholine (ACH)
- Binding of ACH to receptors and opening of ion channels
- Generation of muscle action potential
- Breakdown of ACH by acetylcholinesterase
- Transmission of action potential along muscle fiber
- Activation of dihydropyridine receptors (DHPR)
- Release of calcium from sarcoplasmic reticulum
- Calcium binding to troponin and exposure of actin active sites
- Myosin head binding to actin
- ATP hydrolysis and power stroke
- Release of ADP and phosphate
- Sliding of actin filaments
- Release of ADP and return to resting state
- Calcium reuptake and muscle relaxation
Physiology of Organs and Systems
Liver Function
The liver receives blood from the hepatic artery and portal vein. It performs various functions, including bile secretion, carbohydrate metabolism, fat metabolism, protein metabolism, vitamin storage, drug detoxification, and phagocytosis.
The Nephron
The nephron is the functional unit of the kidney. It consists of the glomerulus, proximal tubule, Henle loop, distal tubule, and collecting duct. Nephrons regulate blood volume and composition.
Sex Hormones
Testosterone is the primary male sex hormone, responsible for male sexual development and characteristics. It also influences metabolism and protein synthesis.
Blood Glucose Regulation
Blood glucose levels are tightly regulated by hormones such as insulin and glucagon. Insulin lowers blood glucose, while glucagon raises it.
Thyroid Hormones
Thyroid hormones, primarily T4 and T3, regulate metabolism and energy expenditure.
Growth Hormone
Growth hormone stimulates cell growth and protein synthesis.
Urine Formation
The kidneys produce either diluted or concentrated urine depending on the body’s hydration status. Antidiuretic hormone (ADH) plays a key role in regulating urine concentration.
Understanding human physiology is crucial for comprehending the intricate processes that sustain life and maintain health. By exploring the functions of cells, organs, and systems, we gain insights into the remarkable complexity and resilience of the human body.