Anatomy of the Pulmonary System

Overview of Anatomy and Physiology of the Pulmonary System and Thoracic Cage

• The pulmonary system involves the organs responsible for respiration.
• The thoracic cage encloses the lungs and other vital respiratory organs.

Anatomy of the Respiratory System

• The respiratory system consists of several organs: nose, mouth (oral cavity), pharynx (throat), larynx (voice box), trachea (windpipe), bronchi, and lungs.
• These organs are divided into two zones: conducting zone and respiratory zone.

Structures of the Respiratory Zone

• The respiratory zone consists of air-exchanging structures like respiratory bronchioles, alveolar ducts, and alveolar sacs.
• Alveoli are the tiny air sacs where gas exchange occurs.
• Alveoli are surrounded by capillaries for efficient gas exchange.
• The walls of alveoli consist of type I (thin) and type II (surfactant-producing) cells

The Trachea

• The trachea is a tube that descends into the mediastinum.
• It maintains an open airway due to C-shaped cartilage rings.
• The carina marks the division of the trachea into two primary bronchi.

Bronchi

• The bronchial tree starts with the primary (main) bronchi.
• Secondary (lobar) bronchi branch out from the primary bronchi, and then tertiary (segmental) bronchi branch into each lung segment.
• Bronchioles, smaller than bronchi, are further divided up into terminal bronchioles.

Gross Anatomy of the Lungs

• The major landmarks of the lungs include the apex, base, hilum, and root.
• The left lung has two lobes (superior and inferior).
• The right lung has three lobes (superior, middle, and inferior).
• There are oblique and horizontal fissures that separate the lobes.

The Pleurae

• The pleura is a double-layered sac that surrounds each lung.
• Parietal pleura lines the thoracic cavity.
• Visceral pleura adheres to the lungs.
• The pleural cavity is the potential space between the two layers.

Mechanisms of Ventilation

• Respiration involves two phases: inhalation (inspiration) and exhalation (expiration).

Inspiration (Inhalation)

• The primary muscles involved in inhalation are the diaphragm and intercostal muscles.
• The diaphragm flattens, and the intercostal muscles contract to increase the volume of the thoracic cavity.
• This decreases internal gas pressure, pulling air into the lungs.
• Accessory muscles like scalenes, sternocleidomastoid, pectoralis major, pectoralis minor, and erector spinae, assist with forceful inhalation.

Expiration (Exhalation)

• Quiet exhalation is mostly passive due to relaxation of the inspiratory muscles.
• The diaphragm moves superiorly, and the volume of the thoracic cavity decreases.
• Forced expiration, an active process, involves contracting internal and external oblique muscles and transverse abdominal muscles.

Neural Control of Ventilation

• The most crucial respiratory center is the ventral respiratory group (VRG) in the medulla oblongata.
• Neurons here generate the respiratory rhythm.
• Pontine respiratory centers in the pons refine respiratory patterns.

Lung Volumes and Capacities

• Lung volumes are the measures of air movement in and out of the lungs.
• Types of volumes include Tidal Volume (TV), Inspiratory Reserve Volume (IRV), Expiratory Reserve Volume (ERV), Residual Volume (RV).
• Lung capacities are combinations of volumes. Examples include Total Lung Capacity (TLC), Vital Capacity (VC), Functional Residual Capacity (FRC), Inspiratory Capacity (IC).

Respiratory Diseases

• Restrictive diseases hinder inhalation, decreasing vital capacity and lung volumes.
• Examples include fibrosis, muscular diseases, chest wall deformities, pleural effusion, and pneumothorax.
• Obstructive diseases impair exhalation, leading to reduced vital capacity but increased residual volume.
• Examples include emphysema, chronic bronchitis, and asthma.

The Diaphragm

• The diaphragm is a major respiratory muscle crucial for normal breathing.

• It's a dome-shaped, musculotendinous septum separating the thoracic and abdominal cavities.

• It accounts for a significant portion (approximately 75%) of tidal volume.

• Its contraction decreases intrapleural pressure and expands the rib cage.

• Important functions include blowing air, speaking, singing, whistling, crying, yawning, coughing, sneezing, vomiting, urination, and childbirth.

• Diaphragm muscle spindles are present, impacting its endurance.

Phrenic Nerve

• The phrenic nerve is the sole motor nerve supplying the diaphragm.
• It arises from the fourth cervical ramus and is part of the cervical plexus.
• Accessory phrenic nerves sometimes arise from C5 and C6.
• Damage or interruption of the phrenic nerve can impede respiratory function.

Diaphragmatic Fatigue

• Diaphragmatic fatigue can lead to difficulties maintaining normal lung volumes due to lactic acid accumulation.
• This ultimately results in shortness of breath (dyspnea).
• Fatigue can also stem from high breathing load and weakness in the respiratory muscles.

Important Characteristics of the Diaphragm

• The diaphragm primarily contains high oxidative slow twitch fibers (Type I), high oxidative, rapid twitch fibers (Type IIa) and low oxidative, rapid twitch fibers, (Type IIb).