2.2 Organ Systems and Their Functions

The human body is a complex machine with interconnected systems working together. From the heart pumping blood to the lungs exchanging gases, each organ system plays a crucial role in keeping us alive and functioning.

Understanding how these systems work is key to grasping human physiology. We'll look at the main organ systems, their structures, and how they interact to maintain balance in the body. This knowledge forms the foundation for studying health and disease.

Human Body Systems

Composition and Function

• The human body is composed of several organ systems that work together to maintain homeostasis and support life
• The major organ systems include:
  • Cardiovascular system
  • Respiratory system
  • Digestive system
  • Urinary system
  • Nervous system
  • Endocrine system
  • Musculoskeletal system
  • Integumentary system

Organ System Responsibilities

• The cardiovascular system, consisting of the heart and blood vessels, transports oxygen, nutrients, hormones, and waste products throughout the body
• The respiratory system, which includes the lungs and airways, facilitates gas exchange, allowing the intake of oxygen and the removal of carbon dioxide
• The digestive system, comprising the gastrointestinal tract and associated organs, breaks down and absorbs nutrients from food and eliminates waste products
• The urinary system, which includes the kidneys, ureters, bladder, and urethra, filters blood, removes waste products, and maintains fluid and electrolyte balance
• The nervous system, consisting of the brain, spinal cord, and peripheral nerves, receives, processes, and transmits signals to coordinate body functions and respond to internal and external stimuli
• The endocrine system, composed of various glands that secrete hormones, regulates growth, development, metabolism, and other physiological processes
• The musculoskeletal system, which includes the bones, joints, and muscles, provides support, protection, and movement for the body
• The integumentary system, consisting of the skin, hair, and nails, serves as a protective barrier, regulates body temperature, and aids in 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 and a ventricle
    • The right side pumps deoxygenated blood to the lungs
    • The left side pumps oxygenated blood to the body
  • Blood flows through the heart in a unidirectional manner, controlled by four valves (tricuspid, pulmonary, mitral, and aortic valves)
  • Arteries carry blood away from the heart, veins carry blood back to the heart, and capillaries facilitate gas and nutrient exchange between the blood and tissues

Respiratory System Anatomy and Physiology

• The respiratory system consists of the upper and lower respiratory tracts (nose, pharynx, larynx, trachea, bronchi, and lungs)
  • The lungs are composed of lobes and contain millions of alveoli, the site of gas exchange between the air and the blood
  • Breathing is controlled by the diaphragm and intercostal muscles, which create negative pressure in the thoracic cavity, allowing air to flow into the lungs during inhalation
  • Gas exchange occurs by diffusion, with oxygen moving from the alveoli into the blood and carbon dioxide moving from the blood into the alveoli

Digestive System Anatomy and Physiology

• The digestive system consists of the gastrointestinal tract and associated organs (mouth, esophagus, stomach, small intestine, large intestine, rectum, anus, liver, gallbladder, and pancreas)
  • Mechanical digestion (chewing and peristalsis) and chemical digestion (enzymes and secretions) break down food into smaller molecules that can be absorbed by the body
  • The small intestine, lined with villi and microvilli to increase surface area, is the primary site of nutrient absorption
  • The large intestine absorbs water and electrolytes and forms and stores feces

Urinary System Anatomy and Physiology

• The urinary system consists of the kidneys, ureters, bladder, and urethra
  • The kidneys filter blood, remove waste products, and maintain fluid and electrolyte balance
    • They contain millions of nephrons, the functional units of the kidney
    • Each nephron consists of a glomerulus (blood filtration) and a tubule system (filtrate processing and essential substance reabsorption)
  • Urine is produced by the kidneys, transported through the ureters to the bladder for storage, and eliminated from 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: brain and spinal cord) and the peripheral nervous system (PNS: sensory and motor divisions)
  • The neuron is the basic functional unit of the nervous system and transmits electrical (action potentials) and chemical (neurotransmitters) signals
  • The CNS processes information from sensory receptors, makes decisions, and sends motor commands to effectors (muscles and glands)
  • The PNS is further divided into the somatic nervous system (voluntary movements) and the autonomic nervous system (involuntary functions like heart rate, digestion, and respiration)

Endocrine System Structure and Function

• The endocrine system consists of glands that secrete hormones directly into the bloodstream, which travel to target cells and tissues to regulate physiological processes
  • Major endocrine glands include the hypothalamus, pituitary, thyroid, parathyroid, adrenal, pancreas, and gonads (ovaries and testes)
  • Hormones can be classified as peptides, steroids, or amines and bind to specific receptors on target cells to initiate cellular responses
  • The endocrine system works closely with the nervous system to maintain homeostasis and regulate processes like growth, development, metabolism, reproduction, and stress response

Neuroendocrine Interaction and Feedback Loops

• The hypothalamus and pituitary gland form the hypothalamic-pituitary axis, linking the nervous and endocrine systems
  • The hypothalamus secretes releasing and inhibiting hormones that control hormone secretion from the anterior pituitary gland
  • The posterior pituitary gland stores and releases hormones produced by the hypothalamus (antidiuretic hormone and oxytocin)
• Feedback loops (negative and positive) are essential mechanisms for regulating hormone levels and maintaining homeostasis
  • Negative feedback loops maintain stable hormone levels by decreasing secretion when levels are high and increasing secretion when levels are low
  • Positive feedback loops amplify hormone secretion and play crucial roles in processes like childbirth and lactation

Musculoskeletal and Integumentary Systems

Musculoskeletal System Structure and Function

• The musculoskeletal system consists of the skeletal system (bones, cartilage, and ligaments) and the muscular system (skeletal muscles, tendons, and fascia)
  • The skeletal system provides support, protection, movement, mineral storage, and hematopoiesis (blood cell production)
    • Bones are classified by shape (long, short, flat, and irregular) and are composed of compact and spongy bone tissue
    • Bones are connected at joints, which are stabilized by ligaments and allow for various degrees of movement
  • Skeletal muscles, attached to bones via tendons, generate movement through contraction
    • They are composed of bundles of muscle fibers containing myofibrils made up of sarcomeres (basic contractile units of muscle)
    • Muscle contraction is triggered by calcium ion release and the sliding of thin (actin) and thick (myosin) filaments past each other (sliding filament theory)

Integumentary System Structure and Function

• The integumentary system consists of the skin, hair, nails, and various glands (sweat, sebaceous, and ceruminous)
  • The skin is the largest organ and is composed of two main layers: epidermis and dermis (with the hypodermis or subcutaneous layer beneath)
    • The epidermis is the outermost layer, composed of stratified squamous epithelium, providing a waterproof barrier and containing melanocytes (pigment-producing cells)
    • The dermis is the deeper layer, containing connective tissue, blood vessels, nerve endings, hair follicles, and glands, providing strength, elasticity, and nourishment to the skin
  • The skin serves several functions: protection, temperature regulation, vitamin D synthesis, and sensory perception
• Hair and nails are skin appendages composed of keratinized cells
  • Hair (vellus, terminal, and lanugo) grows from hair follicles and helps with insulation, protection, and sensory perception
  • Nails (nail plate, bed, and root) protect the distal ends of fingers and toes and aid in fine motor tasks
• Glands in the integumentary system include:
  • Eccrine sweat glands (throughout the body) produce a hypotonic solution for temperature regulation
  • Apocrine sweat glands (axillary and genital regions) produce a thicker, oily secretion that bacteria can metabolize, leading to body odor
  • Sebaceous glands (associated with hair follicles) secrete sebum to lubricate and waterproof the skin and hair
  • Ceruminous glands (modified apocrine glands in the ear canal) produce cerumen (earwax) to protect and lubricate the ear canal