2.2 Organ Systems and Their Functions

Human Body Systems

Composition and Function

The human body operates through several organ systems that coordinate to maintain homeostasis, the stable internal environment your cells need to survive. Each system handles specific tasks, but none works in isolation. The major organ systems are:

• Cardiovascular system
• Respiratory system
• Digestive system
• Urinary system
• Nervous system
• Endocrine system
• Musculoskeletal system
• Integumentary system

Organ System Responsibilities

The cardiovascular system (heart and blood vessels) transports oxygen, nutrients, hormones, and waste products throughout the body. Think of it as the delivery and pickup network.

The respiratory system (lungs and airways) handles gas exchange: pulling in oxygen and expelling carbon dioxide.

The digestive system (gastrointestinal tract plus organs like the liver and pancreas) breaks food down into absorbable nutrients and eliminates solid waste.

The urinary system (kidneys, ureters, bladder, urethra) filters blood, removes metabolic waste, and maintains fluid and electrolyte balance.

The nervous system (brain, spinal cord, peripheral nerves) receives, processes, and transmits signals to coordinate body functions and respond to stimuli. It works on a timescale of milliseconds.

The endocrine system (hormone-secreting glands) regulates growth, metabolism, development, and other processes. Compared to the nervous system, its effects are slower but longer-lasting.

The musculoskeletal system (bones, joints, muscles) provides structural support, protects internal organs, and enables movement.

The integumentary system (skin, hair, nails) acts as a protective barrier, regulates temperature, and contributes to sensory perception.

Cardiovascular, Respiratory, Digestive, and Urinary Systems

Cardiovascular System Anatomy and Physiology

The cardiovascular system consists of the heart, a four-chambered muscular pump, and a network of blood vessels (arteries, veins, and capillaries).

• The heart is divided into right and left sides, each with an atrium (receiving chamber) and a ventricle (pumping chamber).
  • The right side pumps deoxygenated blood to the lungs (pulmonary circulation).
  • The left side pumps oxygenated blood to the rest of the body (systemic circulation). The left ventricle has a thicker wall because it must generate enough pressure to push blood through the entire systemic circuit.
• Blood flows through the heart in one direction, enforced by four valves:
  • Tricuspid valve: right atrium → right ventricle
  • Pulmonary valve: right ventricle → pulmonary artery
  • Mitral (bicuspid) valve: left atrium → left ventricle
  • Aortic valve: left ventricle → aorta
• Arteries carry blood away from the heart, veins carry blood back to the heart, and capillaries are the thin-walled exchange sites where gases and nutrients move between blood and tissues.

Respiratory System Anatomy and Physiology

The respiratory system includes the upper respiratory tract (nose, pharynx, larynx) and the lower respiratory tract (trachea, bronchi, lungs).

• The lungs contain millions of alveoli, tiny air sacs where gas exchange occurs. Their enormous combined surface area (roughly 70 $m^2$ in an adult) makes diffusion efficient.
• Breathing relies on the diaphragm and intercostal muscles. During inhalation, these muscles contract and expand the thoracic cavity, creating negative pressure that draws air into the lungs. During exhalation at rest, the muscles relax and the lungs recoil passively.
• Gas exchange happens by diffusion across the alveolar-capillary membrane: oxygen moves from alveoli into the blood (down its partial pressure gradient), and carbon dioxide moves from the blood into the alveoli.

Digestive System Anatomy and Physiology

The digestive system includes the gastrointestinal (GI) tract and accessory organs (liver, gallbladder, pancreas).

• Digestion involves two parallel processes:
  • Mechanical digestion: physical breakdown through chewing, churning in the stomach, and peristalsis (wave-like smooth muscle contractions that move food along the GI tract).
  • Chemical digestion: enzymatic breakdown by substances like salivary amylase (starches), pepsin (proteins in the stomach), and pancreatic lipase (fats).
• The small intestine is the primary site of nutrient absorption. Its inner lining is covered with villi and microvilli, finger-like projections that dramatically increase surface area for absorption.
• The large intestine absorbs water and electrolytes from the remaining material and forms, stores, and eventually eliminates feces.

Urinary System Anatomy and Physiology

The urinary system consists of the kidneys, ureters, bladder, and urethra.

• The kidneys filter blood, remove waste products (like urea), and regulate fluid and electrolyte balance. Each kidney contains roughly one million nephrons, the functional units responsible for filtration.
• Each nephron has two main components:
  • The glomerulus: a capillary tuft where blood is filtered under pressure. Small molecules (water, ions, glucose, urea) pass into the tubule; large molecules (proteins, blood cells) stay in the blood.
  • The tubule system (proximal tubule, loop of Henle, distal tubule, collecting duct): processes the filtrate by reabsorbing useful substances (glucose, amino acids, most water) and secreting additional waste.
• Urine produced by the kidneys travels through the ureters to the bladder for storage, then exits the body through the urethra.

Nervous and Endocrine Systems Regulation

Nervous System Structure and Function

The nervous system is divided into the central nervous system (CNS), which includes the brain and spinal cord, and the peripheral nervous system (PNS), which connects the CNS to the rest of the body.

• The neuron is the basic functional unit. It transmits signals in two forms:
  • Electrical signals (action potentials) travel along the neuron's axon.
  • Chemical signals (neurotransmitters) cross the synapse to communicate with the next neuron or target cell.
• The CNS integrates sensory information, makes decisions, and sends motor commands to effectors (muscles and glands).
• The PNS is further divided into:
  • Somatic nervous system: controls voluntary movements (skeletal muscle).
  • Autonomic nervous system: controls involuntary functions (heart rate, digestion, respiration). It has two branches: the sympathetic ("fight or flight") and parasympathetic ("rest and digest") divisions, which generally oppose each other to fine-tune organ function.

Endocrine System Structure and Function

The endocrine system uses glands that secrete hormones directly into the bloodstream. These hormones travel to target cells bearing the right receptors and trigger specific cellular responses.

• Major endocrine glands: hypothalamus, pituitary, thyroid, parathyroid, adrenal glands, pancreas, and gonads (ovaries and testes).
• Hormones are classified into three chemical categories:
  • Peptide hormones (e.g., insulin): water-soluble, bind to surface receptors.
  • Steroid hormones (e.g., cortisol, estrogen): lipid-soluble, can cross cell membranes and bind to intracellular receptors.
  • Amine hormones (e.g., epinephrine, thyroid hormones): derived from amino acids, with varying mechanisms.
• The endocrine system works closely with the nervous system to maintain homeostasis, but its effects tend to be slower in onset and longer in duration compared to neural signaling.

Neuroendocrine Interaction and Feedback Loops

The hypothalamic-pituitary axis is the primary link between the nervous and endocrine systems.

• The hypothalamus detects changes in the internal environment and secretes releasing or inhibiting hormones that control the anterior pituitary gland. For example, thyrotropin-releasing hormone (TRH) from the hypothalamus stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH).
• The posterior pituitary doesn't synthesize hormones itself. Instead, it stores and releases two hormones produced by the hypothalamus: antidiuretic hormone (ADH), which promotes water reabsorption in the kidneys, and oxytocin, which stimulates uterine contractions and milk ejection.

Feedback loops are the control mechanisms that keep hormone levels in the right range:

• Negative feedback is the most common type. When hormone levels rise above a set point, the system reduces secretion; when levels drop, secretion increases. Example: high blood levels of thyroid hormone inhibit further TRH and TSH release, preventing overproduction.
• Positive feedback amplifies a response rather than dampening it. This is less common and typically drives a process to completion. Example: during childbirth, oxytocin stimulates uterine contractions, which push the baby against the cervix, which triggers more oxytocin release, intensifying contractions until delivery.

Musculoskeletal and Integumentary Systems

Musculoskeletal System Structure and Function

The musculoskeletal system combines the skeletal system (bones, cartilage, ligaments) with the muscular system (skeletal muscles, tendons, fascia).

Skeletal system:

• Functions include support, protection of internal organs, movement, mineral storage (calcium and phosphorus), and hematopoiesis (blood cell production in red bone marrow).
• Bones are classified by shape: long (femur), short (carpals), flat (skull bones), and irregular (vertebrae). They're composed of dense compact bone on the outside and porous spongy bone on the inside.
• Bones connect at joints, which are stabilized by ligaments (bone-to-bone connective tissue) and allow varying degrees of movement depending on joint type.

Muscular system:

• Skeletal muscles attach to bones via tendons and generate movement through contraction.
• Each muscle is made of bundles of muscle fibers. Inside each fiber are myofibrils, which contain repeating units called sarcomeres, the basic contractile units.
• Muscle contraction follows the sliding filament theory:
  1. A motor neuron stimulates the muscle fiber, triggering calcium ion ($Ca^{2+}$) release from the sarcoplasmic reticulum.
  2. $Ca^{2+}$ binds to troponin on the thin filaments, exposing binding sites on actin.
  3. Myosin heads on the thick filaments attach to actin, pull the thin filaments inward (power stroke), then detach and repeat.
  4. The sarcomere shortens as thin and thick filaments slide past each other, producing contraction.

Integumentary System Structure and Function

The integumentary system consists of the skin, hair, nails, and associated glands.

Skin structure:

The skin is the body's largest organ and has two main layers, with a third layer beneath:

• Epidermis: the outermost layer, made of stratified squamous epithelium. It provides a waterproof barrier and contains melanocytes, which produce melanin (the pigment responsible for skin color and UV protection).
• Dermis: the deeper layer, composed of connective tissue. It houses blood vessels, nerve endings, hair follicles, and glands, providing strength, elasticity, and nourishment.
• Hypodermis (subcutaneous layer): sits below the dermis and consists mainly of adipose tissue for insulation and energy storage.

Skin functions: protection against pathogens and physical damage, temperature regulation (via sweating and blood vessel dilation/constriction), vitamin D synthesis (triggered by UV exposure), and sensory perception (touch, pressure, temperature, pain).

Hair and nails:

Both are composed of keratinized cells. Hair grows from follicles and comes in three types: vellus (fine body hair), terminal (thicker scalp/body hair), and lanugo (fine hair covering a fetus). Nails (nail plate, bed, and root) protect the fingertips and toes and assist with fine motor tasks like picking up small objects.

Glands:

• Eccrine sweat glands: found throughout the body, produce a dilute (hypotonic) sweat for temperature regulation via evaporative cooling.
• Apocrine sweat glands: concentrated in axillary (armpit) and genital regions, produce a thicker secretion. Bacteria on the skin metabolize this secretion, which is what causes body odor.
• Sebaceous glands: associated with hair follicles, secrete sebum (an oily substance) that lubricates and waterproofs skin and hair.
• Ceruminous glands: modified apocrine glands in the ear canal that produce cerumen (earwax), which protects and lubricates the ear canal.