Introduction to the Respiratory System

Hello, in this post we'll begin discussing the next topic in our Veterinary Physiology 1 unit - the respiratory system. In this post we'll take a look at the anatomical structures of the respiratory tract and the conducting and respiratory zones. We'll cover the structure of alveoli and anatomical and functional features of the bronchial and pulmonary circulations. We'll discuss pressure gradients and the muscle mechanics of expiration and inspiration. We'll finish off by discussing the differences between avian and mammalian respiratory systems. 

Before we begin, it is important to remember that there are two types of respiration : internal and external.

• Internal Respiration: is also known as cellular respiration. It is an intracellular process where oxygen is used by the mitochondria to generate ATP through oxidative phosphorylation.
• External Respiration: This involves four processes:

1. Pulmonary Ventilation
2. Gas Exchanges (lungs)
3. Transportation of oxygen and carbon dioxide
4. Gas exchange (tissues)

Anatomy of the Respiratory System

The respiratory system can be divided into two main areas: the upper and lower respiratory tracts.

The Upper Respiratory Tract (URT) includes:

• the nasal cavities: they act as a coarse hair filter and clean the air. The air also makes contact with a large surface area and mucous membranes in the nose and this warms the air. The air also becomes humidified through saturation with water vapour when it passes through the nasal cavities.
• oral cavity: The oral cavity allows an alternate passage of airflow. The oral cavity is used in breathing during exercise and when the nasal cavities are obstructed, they decrease resistance and allow more airflow.
• pharynx
• larynx: this provides a non-collapsible (patent) airway, prevents food and liquid entering the trachea during swallowing and enables voice production.

The Lower Respiratory tract can be broken down into two zones:

• The conducting zone, this is a passage for air to enter and exit the respiratory zone, it contains goblet cells (which secrete mucous) and cilia and it acts to warm, humidify and filter the air. It is not involved in gas exchange: The conducting zone includes:
• The trachea: this is a flexible tube consisting of cartilage rings joined by connective tissue. It is lined with ciliated epithelia and goblet cells. This functions as a mucous escalator which gets rid of debris. In most animals the cartilage rings are incomplete and form a C shape. This allows for expansion of the oesophagus. However, in birds the rings are complete.
• Bronchi: the trachea branches into primary bronchi which also contain cartilage plates. They work to prevent collapsion.
• Bronchioles: These contain small branches of smooth muscle and contain no cartilage. They have diameter that is less than 1mm and are able to collapse.
• Terminal Bronchioles: these are the last anatomical structures present in the conducting zone. We've been told that people often forget to include these in exam answers!
• the respiratory zone: this is the site of gas exchange which is maximised by a large surface area, thin walls, no cartilage, no goblet cells and sparse or absent cilia. It includes:
• respiratory bronchioles: which contain alveoli out-pockets
• alveolar ducts: which end in a cluster of alveoli
• alveoli: which are tiny sac-like structures involved in gas exchange. There are several types of alveolar cells, including:
• Type 1 alveolar cells: these form the respiratory membrane and occupy 95% of the alveolar space
• Type 2 alveolar cells: these synthesise surfactant. Surfactant acts as a detergent and decreases the surface tension of the fluid layer in the alveoli, making it easier for them to expand during breathing.
• Alveolar Macrophages: these patrol alveoli and phagocytose small particles. 

Blood Supply to the Lungs

The lungs are unique in that they receive blood from two different sources. They receive blood from the pulmonary circulation which is the total output of deoxygenated blood from the right ventricle via the pulmonary artery. The lungs also receive bronchial circulation which arises from the aorta and provides oxygenated blood to the lung tissue (to the level of the terminal bronchioles. The alveoli receive oxygen etc. from gas exchange).

Gas Laws

Airflow in and out of the lungs is driven by a pressure gradient between the atmospheric pressure (Patm) and alveolar pressure (Palv). Air flows from regions of high pressure to low pressure and is proportional to the pressure difference. During inspiration Palv is less than Patm, causing air to flow into the lungs. During expiration Palv is greater than Patm causing air to flow out the lungs.

Boyle's Law states that pressure is inversely related to volume, that is when the volume decreases pressure increases. This can be shown in the equation below:

P1V1=P2V2

Pulmonary Pressures

Four very important pressures are present in the lungs, they are:

1. Atmospheric Pressure (Patm): this is 760mmHg but because all the other pressures are stated relative to Patm it is referred to as 0 mmHg.
2. Intra-alveolar Pressure (Palv): this is the pressure inside the alveoli. It changes during breathing due to changes in lung volume and the airflow into and out of the lungs.
3. Intrapleural Pressure (Pip): This is the pressure inside the pleural cavity. It varies with respiration but is always less than Palv and negative. This is due to the inward recoil of the lungs and the outward recoil of the rib cage. Pip = 756 mmHg (or -4mmHg)
4. Transpulmonary Pressure (Ptp): This is the difference between Palv and Pip. ie. Palv - Pip = 760 - 756 = 4mmHg. It always remains positive under normal conditions and is the force that keeps the lungs inflated. 

The Mechanics of Pulmonary Ventilation

Inspiration

Inspiration is an active process and the diaphragm and external intercostals are the primary muscles that are involved. The somatic motor neurons which control inspiratory muscles fire more action potentials, this causes the diaphragm and external intercostal muscles to contract. The diaphragm pulls caudally and the ribs move forwards and outwards, this expands the lungs and increases their volume. This causes a decrease in the intrapleural pressure which leads to an increase in transpulmonary pressure. This makes a 'suction' which pulls the lungs outward, causing the alveolar pressure to decrease. This sets up a pressure gradient because the alveolar pressure is lower than the atmospheric pressure, this causes air to flow into the lungs until both pressures are equal.

Expiration:

There are two types of expiration, passive and active. Passive doesn't require any energy and involves the relaxation of muscles that were involved in inspiration. It occurs during normal, quiet breathing. Active requires energy and involves the contraction of internal intercostal and abdominal muscles. It produces stronger and faster contraction of the lungs and is important for respiration during exercise and disease. 

The motor neurons that control the respiratory muscles start to fire less action potentials. This causes the diaphragm and external intercostals to relax causing the thorax, lungs and diaphragm to return to their natural states via elastic recoil. This causes the lung volume to decrease. A decreased lung volume leads to a higher alveolar pressure, setting up another pressure gradient. This time, however, the Palv is higher than the Patm causing the air to flow out the lungs until the two pressures are equal.

Avian vs Mammalian Respiration

In birds the lungs communicate with air sacs which are divided into cranial and caudal groups. Birds exhibit unidirectional airflow whereas mammals have bidirectional flow. Gas exchange does not occur in the air sacs, they act more as bellows for moving air within the respiratory system. A bird's lungs are rigid and so the volume changes very little during ventilation. Parabronchi also replace alveoli and birds do not have diaphragms. 

Avian respiration occurs in two cycles. During cycle 1 inspiration causes air to move to the caudal air sacs and expiration causes air to move from the caudal air sacs to the lungs. During the second cycle inspiration causes air to move from the lungs to the cranial air sacs while expiration causes air to be expired from the cranial air sacs out of the nostrils and mouth. Cycle 1 and 2 occur at the same time. The use of two cycles to move one packet of air through the respiratory system maximises oxygen uptake at high altitudes where oxygen levels are low. 

The Avian Respiratory System During Inspiration and Expiration

That's it for this post, see you next time :)