Human Physiology

Human Physiology is the branch of biological knowledge that explains how the human body functions as an integrated living system. It describes the physical and chemical processes that allow cells, tissues, organs, and organ systems to sustain life. These processes include movement, respiration, circulation, digestion, communication between cells, temperature regulation, energy production, growth, and adaptation to internal and external changes.

The human body is a dynamic structure organized into levels of increasing complexity:

• Cells are the smallest functional units.
• Tissues are groups of similar cells performing common tasks.
• Organs are combinations of tissues working together.
• Organ systems coordinate multiple organs for large-scale physiological functions.
• The organism is the complete integration of all systems.

The sequence illustrates how physiological organization progresses from microscopic structures to the complete functioning organism.

Homeostasis

The central principle of physiology is homeostasis, the maintenance of stable internal conditions despite external variation.

Examples include:

• Body temperature regulation
• Blood glucose control
• Blood pressure stabilization
• Fluid balance
• Blood pH regulation

This stability depends on feedback mechanisms.

The relationship between change detection and correction can be represented as:

A negative feedback system reduces deviation and restores equilibrium.

This loop continuously restores physiological stability.

Cellular Physiology

Cells perform all essential life processes:

• Energy production
• Protein synthesis
• Ion transport
• Waste elimination
• Signal transmission

Energy generation depends on ATP production:

ATP supports:

• Muscle contraction
• Nerve impulses
• Active transport
• Biosynthesis

The cell membrane controls exchange through selective permeability.

Diffusion follows concentration gradients:

where:

• J is diffusion flux
• D is diffusion coefficient
• dC/dx is concentration gradient

Nervous System Physiology

The nervous system coordinates rapid communication using electrical signals.

Its major divisions are:

• Central nervous system
• Peripheral nervous system

Neurons transmit action potentials through membrane voltage changes.

Resting membrane potential is approximately:

When threshold is reached, depolarization occurs.

Signals travel along axons to target cells, enabling sensation, movement, and cognition.

Cardiovascular Physiology

The cardiovascular system transports oxygen, nutrients, hormones, and wastes.

It consists of:

• Heart
• Blood vessels
• Blood

Cardiac output is:

If:

• Heart Rate = 70 beats/min
• Stroke Volume = 70 mL/beat

Then:

This equals 4.9 liters per minute.

Blood pressure depends on resistance and flow:

Respiratory Physiology

The respiratory system exchanges gases.

Functions include:

• Oxygen intake
• Carbon dioxide elimination
• Acid-base regulation

Ventilation is:

If tidal volume is 500 mL and respiratory rate is 12 breaths/min:

Gas exchange occurs in alveoli through diffusion.

Digestive Physiology

Digestion transforms food into absorbable nutrients.

Major steps:

1. Ingestion
2. Mechanical digestion
3. Chemical digestion
4. Absorption
5. Elimination

Energy balance is:

Positive balance stores energy as fat.

Negative balance mobilizes reserves.

Endocrine Physiology

The endocrine system regulates slower long-term processes through hormones.

Major glands:

• Pituitary
• Thyroid
• Adrenal glands
• Pancreas
• Gonads

Hormonal concentration often follows exponential decay:

This explains gradual hormonal clearance from blood.

Muscular Physiology

Muscles produce force and movement through actin-myosin interaction.

Force depends on:

• Fiber recruitment
• Fiber length
• Neural stimulation frequency

Mechanical work is:

If a muscle lifts 100 N through 2 m:

Renal Physiology

The kidneys regulate:

• Water balance
• Electrolytes
• pH
• Waste removal

Filtration rate is central to renal performance:

Normal filtration preserves internal chemical stability.

Integration of Human Physiology

Human physiology is unified through continuous communication between systems.

Examples:

• Exercise increases heart rate, ventilation, and metabolism
• Stress activates hormones and neural responses
• Digestion alters circulation and endocrine release

The body functions as an interconnected network maintaining life through coordination, adaptation, and regulation.

This integration defines the complete physiological operation of the human organism.