Overview of Human Body Systems and Functions

Homeostasis

The body's way of keeping internal conditions stable, even when the outside environment changes.

Negative Feedback

Reverses a change to bring the body back to normal.

Positive Feedback

Strengthens or reinforces a change.

Dynamic Equilibrium

Conditions are kept within a range, not a fixed number; the body is always making small adjustments.

Receptor

Senses a change (e.g., temperature change).

Integration Center

Processes info (often the brain or spinal cord).

Effector

Carries out the response (e.g., muscles, glands).

Intracellular Fluid (ICF)

Fluid inside cells, makes up about 60% of total body water (≈ 40 liters in an average adult).

Extracellular Fluid (ECF)

Fluid outside cells, makes up about 40% of total body water (≈ 15 liters total).

Interstitial Fluid (IF)

Fluid between cells (≈ 12 L).

Plasma

Fluid portion of blood (≈ 3 L).

Diffusion

The movement of molecules from an area of high concentration to low concentration; no energy (ATP) is needed.

Osmosis

A special type of diffusion — the movement of water across a membrane toward a higher solute concentration.

Passive Transport

Molecules move down their concentration gradient (high → low) without ATP.

Simple diffusion

Small molecules pass through the membrane.

Facilitated diffusion

Uses channels or carriers (e.g., glucose transport).

Active Transport

Molecules move against their concentration gradient (low → high) and requires ATP.

Primary active transport

Uses ATP directly (e.g., Na⁺/K⁺ pump).

Secondary active transport

Uses energy from another substance's gradient.

Endocytosis

Large molecules are brought in via vesicles.

Exocytosis

Large molecules are expelled via vesicles.

Integumentary System

Acts as a barrier to protect against physical damage, pathogens, and water loss.

Integumentary System

Regulates body temperature through sweating and blood flow to the skin.

Integumentary System

Produces vitamin D when exposed to UV light (needed for calcium absorption).

Integumentary System

Contains sensory receptors for touch, pressure, pain, and temperature.

Integumentary System

Excretes wastes through sweat (like salts and small amounts of urea).

Integumentary System

Assists in immune defense through skin barrier and inflammatory response.

Epidermis

Outermost layer of the skin; avascular and provides a protective barrier.

Dermis

Middle layer of the skin containing blood vessels, nerve endings, sweat glands, and connective tissue.

Hypodermis

Deepest layer of the skin made of adipose tissue for insulation and energy storage.

Osteoblasts

Build new bone matrix (bone formation) and secrete collagen while promoting calcium deposition.

Osteoclasts

Break down bone tissue (bone resorption) and help regulate calcium and phosphate levels in blood.

Osteocytes

Mature bone cells that maintain bone tissue and communicate with other bone cells to coordinate remodeling.

Osteoporosis

A condition where bone mass decreases, making bones weak and fragile, affecting 1 in 3 women and 1 in 5 men over age 50.

Hip fractures

Up to 24% mortality in the first year; 40% lose ability to walk independently; 60% need assistance long-term.

Skeletal Muscle

Striated and voluntary muscle attached to bones, responsible for body movement.

Cardiac Muscle

Striated and involuntary muscle found only in the heart, connected by intercalated discs for synchronized contraction.

Smooth Muscle

Non-striated and involuntary muscle found in the walls of hollow organs, moving substances through internal pathways.

Sarcomere

The basic unit of a muscle's striated appearance and contraction, composed of Z lines, A band, I band, and H zone.

Motor Unit

A motor neuron and all the muscle fibers it controls; small motor units allow precise control, while large motor units enable powerful contractions.

Action Potential Activation

Nerve impulse arrives at the neuromuscular junction, triggering acetylcholine release and subsequent muscle contraction.

CNS

Central Nervous System; includes the brain and spinal cord and processes and integrates information.

PNS

Peripheral Nervous System; includes all nerves outside the CNS that carry information to and from the CNS.

Afferent

Sensory pathways that carry signals to the CNS.

Efferent

Motor pathways that carry signals from the CNS to muscles or glands.

Somatic Nervous System

Controls voluntary movements of skeletal muscles.

Autonomic Nervous System

Controls involuntary functions such as heart rate, glands, and smooth muscle activity.

Sympathetic Nervous System

Increases heart rate, dilates pupils, and decreases digestion in response to stress or danger.

Parasympathetic Nervous System

Slows heart rate, constricts pupils, and promotes digestion and energy storage.

Glial Cells

Supporting cells in the nervous system.

Glial Cells

They help neurons by protecting, nourishing, insulating, and cleaning up around them.

Role of Glial Cells

They do not send electrical signals, but they are essential for healthy brain and nerve function.

Astrocytes

Maintain the environment around neurons; help form the blood-brain barrier.

Oligodendrocytes

Make myelin in the central nervous system (CNS).

Schwann cells

Make myelin in the peripheral nervous system (PNS).

Microglia

Act like immune cells; clean up waste and protect against invaders.

Ependymal cells

Help move and produce cerebrospinal fluid (CSF).

Cell body (soma)

Contains the nucleus and organelles.

Dendrites

Receive signals from other neurons.

Axon

Sends signals away from the cell body.

Axon terminals

Release neurotransmitters to communicate with the next cell.

Action Potentials

An electrical signal that travels along the axon.

Threshold

Triggered when the cell reaches a certain voltage.

Depolarization

Sodium (Na⁺) channels open → Na⁺ rushes in → inside becomes more positive.

Repolarization

Potassium (K⁺) channels open → K⁺ exits → inside becomes more negative again.

Neurotransmitters

Chemicals like acetylcholine (ACh) or norepinephrine (NE) released from one neuron to send a message to the next.

Synaptic gap

The space into which neurotransmitters are released.

Steroid hormones

Made from cholesterol (e.g., cortisol, testosterone).

Amino acid derivatives

Modified amino acids (e.g., thyroid hormones, epinephrine).

Peptide hormones

Chains of amino acids (e.g., insulin, glucagon, ADH).

Hydrophobic hormones

Can cross the cell membrane and bind to receptors inside the cell.

Hydrophilic hormones

Cannot cross the cell membrane and bind to membrane receptors.

Short-term stress response

The body releases epinephrine (adrenaline), increasing heart rate, blood pressure, and energy availability.

Long-term stress response

The body releases cortisol from the adrenal cortex, increasing glucose levels and suppressing nonessential functions.

Albumin

Most abundant plasma protein; maintains osmotic (oncotic) pressure.

Globulins

Include antibodies (immunoglobulins) used for immune defense.

Fibrinogen

Helps in blood clotting by forming fibrin threads.

Red Blood Cells (Erythrocytes)

Carry oxygen using hemoglobin; lack a nucleus and live about 120 days.

White Blood Cells (Leukocytes)

Involved in immune response; types include neutrophils, lymphocytes, monocytes, eosinophils, and basophils.

Platelets (Thrombocytes)

Small fragments that help form clots to stop bleeding.

Blood Types

Determined by antigens on the surface of red blood cells.

Cardiac Output (CO)

Amount of blood the heart pumps in one minute; Formula: CO = Heart Rate (HR) × Stroke Volume (SV).

Systole

Contraction phase (pumps blood out).

Diastole

Relaxation phase (chambers fill with blood).

Pulmonary Ventilation

Breathing - air in/out of lungs

Boyle's Law

Pressure and volume are inversely related.

Inhalation

Diaphragm and intercostal muscles contract, increasing thoracic cavity volume which drops intrapulmonary pressure and allows air to flow in.

Exhalation

Muscles relax, decreasing thoracic cavity volume which rises intrapulmonary pressure and allows air to flow out.

Intrapulmonary pressure

Pressure in alveoli.

Intrapleural pressure

Pressure in pleural cavity, always slightly negative to keep lungs inflated.

External Respiration

Gas exchange between alveoli and blood where O₂ diffuses into blood and CO₂ diffuses into alveoli.

Gas Transport

Oxygen is mostly carried by hemoglobin; carbon dioxide is carried mostly as bicarbonate (HCO₃⁻), some bound to hemoglobin, and a small amount dissolved in plasma.

Internal Respiration

Gas exchange between blood and body tissues

Alveolus

Surfactant: Secreted by cells in alveoli

Surfactant

Reduces surface tension, preventing alveoli from collapsing

Surface tension

Created by water lining the alveoli

Respiratory membrane

Thin barrier where gas exchange happens

Gas Exchange

Occurs by diffusion across the respiratory membrane

Gas Transport

Oxygen: 98.5% bound to hemoglobin, 1.5% dissolved in plasma

Carbon Dioxide Transport

~70% as bicarbonate (HCO₃⁻), ~20% bound to hemoglobin, ~10% dissolved in plasma