Human Physiology Overview

Fundamental Concepts of Mammalian Life

• Mammalian life is defined by six key characteristics: metabolism, excitability, communication, reproduction & fertility, differentiation, and growth.

The 6 Characteristics of Life

• Metabolism: Chemical processes forming or degrading substances (anabolic and catabolic). Catabolism uses nutrients and oxygen to produce energy (ATP). Anabolism uses ATP to create substances. Glucose + Oxygen → ATP + Water + Carbon Dioxide Amino acids + ATP → Proteins

• Nutrients: Macro-nutrients include carbohydrates (glucose, starch), proteins (essential and non-essential amino acids), and fat (saturated and unsaturated fatty acids, cholesterol). Micro-nutrients include vitamins (water or fat soluble), trace elements (zinc, copper, iron), and minerals (sodium, potassium, magnesium).

• Energy Supply: Proteins - 1g = 4.2kcal Fat - 1g = 9.2kcal Carbohydrates - 1g = 4.1 kcal Alcohol - 1g = ~7 kcal 1kcal = 4.19 kJ

• Energy Demand: Basic energy demand is 1500-1700 kcal (6.3-7.1 MJ/day) at rest. White-collar workers use 8.4-9.6 MJ/day, blue-collar (physical) workers use 15-20 MJ/day, and athletic activities require up to 4.3 MJ per hour.

• Excitability: An organism's ability to react to environmental changes. "Triggers" include light, sound, temperature, and motion. Sensory organs receive information processed by the brain. Biological information is coded as electric voltage (potential) at cell membranes. A resting cell maintains a stable potassium/potassium gradient (-70 to -100 mV), while stimulated cells experience potassium outflux and simultaneous sodium influx, producing an electric current.

• Communication: Communication uses various systems. Nerves use transmitter substances; hormones are produced by endocrine glands (thyroid, pancreas, ovaries, testicles); and immune system messengers include interleukins and interferons. The nervous system uses electrochemical coupling, where an electrical impulse is converted into a chemical signal at synapses and then back into an electric signal to continue transmission.

• Reproduction & Fertility: Reproduction is the cell's ability to divide into two daughter cells (mitotic) and is fundamental for regeneration and tissue repair. Daughter cells contain 23 chromosome pairs (46 chromosomes), as do identical “mother cells". Meiotic reproduction produces cells with half the chromosomes (23 chromosomes). Reproduction is necessary for fertilization and is unique to germ cells

• Differentiation: A single cell in an organism initially performs all life functions (omnipotency), but differentiated cells perform specialized functions, losing their original broader capabilities. A specialized cell has a singular function.

• Growth: Growth has three mechanisms: an increase in the number of cells, an increase in the size of cells, and/or an increase in extracellular volume .

Regulation and Adaptation

• Regulation: Maintaining a stable internal environment despite external changes.

• Adaptation: Adjusting supply to meet demand.

Principle of Homeostasis

• Vital parameters (blood pressure, core temperature, pH, blood glucose level) are continuously monitored by sensors and automatically maintained within a normal range.

The Regulatory Circuit 1

• Vital parameters must be maintained within physiological limits at all times. Sensors/receptors continuously monitor these parameters and automatically correct any deviations.

The Regulatory Circuit 2

• A sensor measures the actual value. A regulator compares this value with the nominal value (physiological range) and adjusts actions. The results of these actions are observed and reported to the regulator (negative feedback mechanism).

Examples of Regulation

• Temperature regulation: Body temperature is maintained near 37.5°C through sweat, blood vessel dilation, and shivering.

• pH regulation: A 7.36-7.42 pH is maintained by buffers (like blood proteins) and the kidneys and lungs (controlling CO₂ removal).

Feedback control

• The hypothalamus is the main control center for endocrine function Hormone regulation is via short- and long-loop feedback mechanisms: Thyroid hormone (T3, T4) regulation, using TRH and TSH. and Gonadotropins (LH, FSH) to regulate estrogen and progesterone levels for pregnancy and the menstrual cycle .

Adaptation to Physical Work

• Forced respiration: Increased oxygen uptake.

• Accelerated glycolysis: Increased muscle and liver glycogen breakdown.

• Muscle vessel dilation: Increased blood supply.

• Accelerated heart rate: Increased blood supply.

Adaptation to Altitude

• Decreased air pressure: Reduced oxygen pressure at high altitudes.
• Accelerated respiration + heart rate: Maintaining O₂ supply to the body in spite of a lower concentration in the air.