The Human Respiratory System

• The respiratory system's function is to supply O₂ to the blood and remove CO₂ from it

The Human Respiratory System

• The respiratory system creates the path for air to travel, from the nose to the lungs
• The primary function of the respiratory system is the exchange of gases during internal and external respiration
• Inhaled air is also filtered, warmed, and humidified by the system
• Sound is produced by the system's vocal cords
• Body pH levels are controlled by the lungs
• The olfactory bulb aids in smelling
• Human cells need a constant supply of oxygen for proper function.
• Carbon dioxide (CO2) is eliminated as a metabolic waste product.
• Gas exchange between the atmosphere and the human body happens during the respiration process

Anatomy

• The anatomy of the respiratory tract can be divided into two groups "structurally": The Upper and Lower Respiratory Tracts.
• Upper Respiratory Tract organs include the: Nose, Nasal cavity, Sinuses, and Pharynx
• Lower Respiratory Tract organs include the: Larynx, Trachea, Bronchial Tree, and Lungs.
• The organs of the respiratory tract can be divided into two groups "functionally": The Conducting and Respiratory Portions
• The conducting portion is made up of interconnecting cavities and tubes that conduct air into the lungs: Nose, Pharynx, Larynx, Trachea, and Bronchi
• The respiratory portion is where respiratory gases are exchanged: Bronchioles, Alveolar Ducts, and Alveoli

Anatomy Details

• The nose and mouth filter, warm, and humidify air
• Mucus traps bacteria and foreign debris in the nose and mouth
• Cilia sweeps mucus toward the throat, where it is digested by the stomach
• The pharynx is the throat and a passageway for food and air
• Tonsils are clusters of lymphatic tissue in the pharynx
• The larynx contains the vocal cords
• The epiglottis covers the larynx during swallowing, preventing liquids and food from entering
• The trachea is the windpipe lined with C-shaped cartilage.
• The bronchi are branches extending to the lungs
• Bronchioles are smaller branches of the bronchi
• Lungs contain alveoli, air sacs for gas exchange

The Nose

• The nose is located between the forehead and upper lip
• The nose is the first organ of the respiratory tract
• The nose provides an entrance for inhaled air
• The nose performs the process of warming, moistening, and filtering inhaled air
• The nose has external and internal portions (nasal cavities)
• The nose is a bony and cartilaginous structure
• The bony part is made up of the frontal, nasal and maxillae bones
• The cartilaginous part is made up of septal, lateral, and alar cartilages.
• The internal portion is a large cavity in the skull merging with the external nose anteriorly and communicating with the throat posteriorly
• The nose is lined with highly vascular ciliated columnar epithelium
• The nose contains mucus-secreting goblet cells

The Nasal Cavity

• Two nasal cavity openings are separated by the nasal septum, which contains septum cartilage
• The anterior openings are called nostrils (anterior nares) through which air enters
• The anterior nares are the openings of the nasal cavity and contain small hairs.
• The posterior nares are the openings of the nasal cavity into the pharynx

Paranasal Sinuses

• Sinuses are air-filled spaces.
• They are present within the maxillary, frontal, ethmoid, and sphenoid bones of the skull.
• These spaces open to the nasal cavity
• The space is lined with mucus membrane
• The sinuses reduce the weight of the skull

Functions of Nasal Cavity

• The nasal cavity helps air pass through it during respiration
• Air is conditioned, converting air between -5°C to 55°C to 31-37°C in the nasopharynx
• Turbinates make the inhaled air water-saturated so that the lungs receive 100% humid air
• Inhaled air is filtered, warmed and humidified in the nasal cavity
• The mucociliary system holds back microorganisms and foreign particles
• Antibodies and enzymes present in the mucus are active against bacteria.
• The irritants are forcefully expelled from the nose through sneezing
• The olfactory system recognizes the environment and food
• Vocal cords help with voice generation

The Pharynx

• The throat is a "funnel-shaped tube" that is posterior to the nasal cavity, oral cavity, and larynx, and anterior to the cervical vertebra.
• The pharynx has three components: the nasopharynx (uppermost portion), the oropharynx (middle portion), and the laryngopharynx (lowermost portion).
• The pharynx serves as a common passageway for both air and food and provides a resonating chamber for speech sounds
• The pharynx functions as a passageway for food and air, supports taste, warming and humidifying, hearing, protection, and speech

The Epiglottis

• The epiglottis is a large, leaf-shaped piece of cartilage found at the entrance of the larynx
• The epiglottis closes the tracheal opening during swallowing of food
• The epiglottis prevents food from entering the trachea
• During swallowing, there is elevation of the larynx.

The Larynx

• The larynx is the triangular chamber located in the front, upper part of the neck
• It features a prominent elevation in front of the larynx, known as the Adam's apple
• The larynx joins the pharynx with the trachea
• The larynx comprises cartilage: 3 single and 3 paired cartilages
• Single cartilages encompass the thyroid cartilage, cricoid cartilage, and epiglottis
• Paired cartilages include the arytenoids, corniculates, and cuneiforms

The Vocal Cords

• Inside the larynx are two pairs of folds made of muscle and connective tissues covered in a mucous membrane: the vocal cords
• Changing the tension on the vocal cords controls pitch, while increasing loudness depends upon increasing the force of air vibrating the vocal cords
• When vocal cords are opened, this is abduction
• When vocal cords are closed, this is adduction
• The larynx contains the vestibule (false vocal cord), ventricle, and the infraglottic cavity (from the vocal cords to the trachea cavity)
• The vocal cords are relaxed, and the glottis, during normal breathing, is a triangular sli
• During swallowing, the false vocal cords and epiglottis close off the glottis.

The Trachea

• The trachea is a windpipe
• It is measured around 10-11 cm
• It extends downwards up to the 5th thoracic vertebra.
• At the end, it divides into the right and left bronchi, which enter their respective lungs.
• It is composed of 16-20 C-shaped rings of hyaline cartilage.
• Connective tissues and involuntary muscles join the cartilage and form the posterior wall.
• The trachea's cartilage is covered by 3 tissue layers: the outer, middle, and inner layer
• The trachea helps in the cough reflex.

The Bronchi

• The bronchi are the air passageways into the respiratory tract carrying inhales air into the lungs
• There are right and left bronchi
• The primary bronchi split into three secondary bronchi
• They enter into the superior, middle, and inferior lobes of the right lung
• Secondary bronchi further divide into tertiary bronchi, which then divide into smaller branches

The Bronchioles

• Bronchioles are the finest branches of the bronchi
• Bronchioles are converted into terminal bronchioles, which are further subdivided into two or more respiratory bronchioles
• Bronchioles terminate at alveolar sacs

The Lungs

• The lungs are in the thoracic cavity as two cone-shaped lobes separated by the heart and other structures
• They are present from the clavicle to the diaphragm
• The paired soft, spongy, cone-shaped lungs, separated medially by the mediastinum, are enclosed by the diaphragm and thoracic cage
• The lungs have two layers of serous membrane, collectively known as the pleural membrane, that enclose and protect them
• The outer layer, the parietal pleura, is attached to the thoracic cavity
• The visceral pleura is the inner layer covering the lung itself
• The left lung is smaller than the right lung due to the heart in the thoracic cavity
• The lungs have an apex, base, costal surface, and medial surface
• The lung has its round apex extending up to the root of the neck
• The base has a concave and semilunar shape and is associated with the thoracic surface of the diaphragm
• The Costal surface is convex and associated with the costal cartilages, ribs, and intercostal muscles
• The Medial surface has a concave shape
• A triangular shaped hilum at the 5-7th thoracic vertebra, where all artery veins and nerve supply pass through this surface

The Mediastinum

• The mediastinum is the area between the lungs, occupied by the heart, great vessels, trachea, right and left bronchi, esophagus, lymph nodes, lymph vessels, and nerves.

Pleura and Pleural Cavity

• Each lung is enclosed with a doubled-layered serous pleural membrane
• The parietal pleura is the outer layer that lines the thoracic cavity wall.
• The visceral pleura is the deep layer that lines the lungs.
• The pleural cavity is the space between the two layers which contains pleural fluid
• Pleural fluid secreted by the membranes lubricating the fluid that prevent the friction between the layers during breathing

Lobes/Fissures and Lobules

• Each lung is separated into lobes
• The left lung is divided into two lobes, and the right into three lobes
• The lobes are further divided into numerous lobules which contains alveoli
• The lungs facilitate respiration, alter blood pH, filter out small blood clots, alter drug concentrations, convert angiotensin-I to angiotensin-II, and provide protection via Ig-A

Alveoli

• Alveoli are hollow cavities within mammalian lungs and spherical projections of respiratory bronchioles
• Alveolar membranes are the major sites of gas exchange with the blood
• A human lung features around 300 million alveoli composed of an epithelial layer, 70% occupied with blood capillaries
• Alveoli facilitate external respiration through gas exchange with blood and defense against microbes by diffusion

Mechanism of Respiration

• The process by which the respiratory organs allow the air to move in and out of the lungs is called breathing
• Oxygen-rich air is taken in from the atmosphere, and carbon dioxide-rich air is expelled in exchange
• Inhalation/Inspiration is the process of air flowing into the lungs
• Exhalation/Expiration is the process of air leaving the lungs
• A single breath comprises one inhalation and one exhalation
• Breathing rate is the number of times an individual breaths in one minute
• Breathing rate increases while walking fast, running, or after heavy exercise.
• Breathing rate decreases when in a relaxed state
• Average breathing rate of an adult is 15-18 times per minute

Inspiration and Expiration

• During inspiration, the Diaphragm contracts and flattens, external intercostals lift the rib cage, and the lungs stretched to have a larger size.
• During inspiration, air pressure inside the lungs decreases and air is sucked into the lungs.
• During expiration, inspiratory muscles relax, the rib cage descends, the lungs recoil, and gases are forced out
• The average breathing rate has 3 phases
• Inspiration lasts about 2 seconds
• Expiration lasts about 3 seconds
• Pause

Lung Volumes

• Tidal Volume (TV): volume of air inspired/expired during normal respiration (~500 ml average).
• Inspiratory Reserve Volume (IRV): additional air volume a person can inspire after forcible inspiration (2500-3000 ml average).
• Expiratory Reserve Volume (ERV): additional air volume a person can expire after forcible inspiration (1000-1100 ml average).
• Residual Volume (RV): air volume remaining in lungs after a forcible expiration (1100-1200 ml average).

Lung Capacity

• Inspiratory Capacity (IC): total air volume a person can inspire after normal expiration (TV+IRV, ~3500 ml average)
• Expiratory Capacity (EC): total air volume a person can expire after normal inspiration (TV+ERV, ~1600 ml average).
• Functional Residual Capacity (FRC): air volume remaining in lungs after normal expiration (ERV+ RV, ~2300 ml average).
• Vital Capacity (VC): maximum air amount a person can breathe out after forced inspiration (ERV, TV & IRV, ~4600 ml average).
• Total Lung Capacity (TLC): total air volume accommodated in the lungs after forced inspiration (RV, ERV, TV, &IRV/VC+RV, ~5800 ml average), meaning TLC=VC

Transport of Respiratory Gases

• Respiration involves inhalation of oxygen and exhalation of carbon dioxide
• During respiration, O2 and CO2 gases are transported by the blood.
• Oxygen enters the lungs to reach the alveoli
• The alveoli and close capillaries are one cell thick, with an air and blood barrier of about one micrometer.
• Oxygen passes easily from alveoli to capillaries and reaches the blood
• Carbon dioxide to be exhaled in the same manner enters the alveoli from the blood
• The oxygenated blood travels to the heart and is distributed throughout the body