Respiratory System: Nose & Pharynx

Respiratory System Overview

• The respiratory system facilitates gas exchange, primarily taking in oxygen and expelling carbon dioxide
• It includes the nose, pharynx, larynx, trachea, bronchi, and lungs
• Respiration involves pulmonary ventilation (breathing), external respiration (gas exchange between lungs and blood), transport of respiratory gases, and internal respiration (gas exchange between blood and tissues)

Major Organs: Nose

• The nose provides an airway for respiration, moistens and warms entering air, filters and cleans inspired air, serves as a resonating chamber for speech, and houses olfactory receptors
• The external nose is formed by bone and cartilage, covered with skin
• The nasal cavity is divided by the nasal septum and lined with mucosa
• Nasal conchae increase mucosal surface area and enhance air turbulence in the nasal cavity

Major Organs: Pharynx

• The pharynx (throat) connects the nasal cavity and mouth to the larynx and esophagus
• It is divided into three regions: the nasopharynx, oropharynx, and laryngopharynx
• The nasopharynx serves as an air passageway
• The oropharynx and laryngopharynx serve as passageways for both food and air

Major Organs: Larynx

• The larynx (voice box) provides an open airway, acts as a switching mechanism to route air and food, and functions in voice production
• It is composed of cartilage connected by membranes and ligaments
• The epiglottis covers the laryngeal inlet during swallowing
• Vocal cords vibrate to produce sound as air rushes up from the lungs

Major Organs: Trachea

• The trachea (windpipe) descends from the larynx through the neck into the mediastinum
• Its walls are reinforced with C-shaped cartilage rings
• The trachea is lined with ciliated pseudostratified epithelium

Major Organs: Bronchi

• The trachea divides into two main (primary) bronchi, each entering one lung
• The right main bronchus is wider, shorter, and more vertical than the left
• Each main bronchus branches into lobar (secondary) bronchi, which supply each lobe of the lungs
• Lobar bronchi branch into segmental (tertiary) bronchi
• Bronchioles are smaller branches, and terminal bronchioles are the smallest

Major Organs: Lungs

• The lungs occupy most of the thoracic cavity
• The right lung has three lobes; the left lung has two lobes
• The lungs consist largely of air spaces; the balance of lung tissue consists of stroma, or elastic connective tissue

Alveoli

• Alveoli are tiny air sacs, the main sites of gas exchange
• The alveolar walls are composed primarily of a single layer of squamous epithelium (Type I alveolar cells)
• Type II alveolar cells secrete surfactant, which reduces surface tension in the alveoli
• Alveolar macrophages patrol and clear debris

Respiratory Membrane

• The respiratory membrane is the air-blood barrier composed of the alveolar and capillary walls and their fused basement membranes
• Gas exchange occurs across this membrane by diffusion

Pleurae

• The lungs are surrounded by pleurae; the parietal pleura lines the thoracic cavity, and the visceral pleura covers the external lung surface
• The pleural cavity between the pleurae contains lubricating fluid that reduces friction

Pulmonary Ventilation

• Pulmonary ventilation (breathing) consists of inspiration (inhalation) and expiration (exhalation)
• Inspiration occurs when alveolar pressure is less than atmospheric pressure
• Expiration occurs when alveolar pressure is greater than atmospheric pressure
• Air flows into the lungs down its pressure gradient

Inspiratory Muscles

• The diaphragm and external intercostal muscles are primary inspiratory muscles
• Contraction of the diaphragm increases the volume of the thoracic cavity vertically
• Contraction of the external intercostals increases the anterior-posterior and lateral diameters of the thorax

Expiration

• Expiration is normally a passive process relying on lung elasticity
• Forced expiration involves contraction of abdominal and internal intercostal muscles

Gas Exchange

• External respiration involves gas exchange between the alveoli and the blood
• Internal respiration involves gas exchange between systemic blood and tissue cells
• Gas exchange occurs due to pressure gradients; gases diffuse from areas of higher partial pressure to areas of lower partial pressure

Oxygen Transport

• Most oxygen (98.5%) is transported bound to hemoglobin in red blood cells
• A small amount of oxygen (1.5%) is transported dissolved in plasma
• Hemoglobin's affinity for oxygen is affected by temperature, pH, PCO2, and BPG

Carbon Dioxide Transport

• Carbon dioxide is transported in three forms: dissolved in plasma, bound to hemoglobin, and as bicarbonate ions
• About 7-10% of CO2 is dissolved in plasma
• Roughly 20% of CO2 is bound to hemoglobin
• The majority (70%) of carbon dioxide is transported in plasma as bicarbonate ions

Control of Respiration

• Respiratory centers in the brainstem (medulla and pons) control breathing
• The medulla oblongata sets the basic respiratory rate and rhythm
• The pons influences and modifies the activity of the medulla
• Chemical factors, such as CO2 and O2 levels, influence respiratory rate and depth

Lymphatic System Overview

• The lymphatic system consists of lymphatic vessels, lymph, lymph nodes, and lymphoid organs
• It returns fluids that have leaked from the blood vascular system back to the blood
• Lymphatic vessels collect tissue fluid (lymph) from loose connective tissue
• Lymph flows toward the heart

Lymphatic Vessels

• Lymphatic capillaries are highly permeable vessels that collect tissue fluid
• They are similar to blood capillaries but are more permeable
• Lymphatic capillaries drain into collecting lymphatic vessels
• Collecting lymphatic vessels have thinner walls, with more internal valves, and anastomose more frequently than veins
• Lymphatic trunks are formed by the union of the largest collecting vessels
• The lymphatic trunks drain into one of two large lymphatic ducts: the right lymphatic duct and the thoracic duct

Lymph

• Lymph is a clear fluid similar to plasma but with a lower protein content
• It contains lymphocytes and macrophages
• Lymph transports pathogens, cellular debris, and cancer cells

Lymph Nodes

• Lymph nodes are small organs clustered along lymphatic vessels
• They filter lymph and activate the immune system
• Lymph nodes contain macrophages that destroy pathogens and debris
• Lymphocytes in lymph nodes monitor the lymph for antigens and mount an immune response

Lymphoid Organs

• Lymphoid organs include the spleen, thymus, tonsils, and Peyer's patches
• These organs house lymphocytes and macrophages
• Lymphoid organs play a role in immunity

Spleen

• The spleen is the largest lymphoid organ
• It filters blood and removes aged or defective blood cells and platelets
• It also stores platelets and monocytes
• The spleen contains lymphocytes, macrophages, and erythrocytes

Thymus

• The thymus is most active during childhood
• It functions in T lymphocyte maturation
• The thymus does not directly fight antigens

Tonsils

• Tonsils are lymphoid tissues located in the pharynx region
• Palatine tonsils are located on either side of the posterior oral cavity
• Lingual tonsil is located at the base of the tongue
• Pharyngeal tonsil (adenoids) is located in the posterior wall of the nasopharynx
• Tonsils gather and remove pathogens entering the pharynx in food or air

Peyer's Patches

• Peyer's patches are clusters of lymphoid follicles in the wall of the small intestine
• They monitor intestinal bacteria and prevent the growth of pathogenic bacteria in the intestines

Functions of the Lymphatic System

• Fluid recovery: Returns excess tissue fluid to the bloodstream
• Immunity: Lymph nodes and lymphoid organs house lymphocytes and macrophages, which play essential roles in body defense
• Lipid absorption: Lacteals in the small intestine absorb dietary lipids