Respiratory Care: Assessment and Management

Chapter 1

A quick look at anatomy and physiology

To assess and manage a patient with a respiratory problem, you need to have a full understanding of the anatomy and physiology of that system. This chapter will discuss the role of the respiratory system, concentrating on its major anatomical structures. It will then look at the science of respiration. There is also a reader activity at the end of the chapter.

The respiratory system

The respiratory system carries out the activity of breathing or inhalation. This is the movement of air into the lungs to supply the body with oxygen. The respiratory system is also responsible for the movement of air out of the lungs, to expel carbon dioxide, known as exhalation.

The major anatomical structures in the respiratory system are: the nose, pharynx, larynx, trachea, the two bronchi and the lungs. The lungs include the bronchioles and the alveoli. The lungs themselves are surrounded by the pleurae. There is a thin layer of tissue called the visceral pleura, which covers the lungs. This layer also covers the chest wall and surrounds the heart and is called the parietal pleura. There is a thin layer of fluid between the visceral and parietal pleurae, which allows movement between them. The space between the visceral and parietal layers is called the pleural space.

The respiratory system is divided into two parts. One is the conducting part, which passes air into the lungs. This includes the nasal passages and the pharynx, larynx, trachea, bronchi and larger bronchioles. The second part is called the respiratory part, and this is where gas exchange occurs in the smaller bronchioles and alveoli.

The nose is the start of the respiratory system. The nasal cavity is hollow so the air passes through it, while being heated and moistened. The cavity is lined with hair and mucus, which acts a filter, trapping foreign particles. The other opening is the mouth or oval cavity. The nose is generally used for the activity of breathing, as the mouth lacks a filter system. This explains why people who breathe through their mouths have a higher incidence of oral infections (Abreu et al. 2008, Gulati, Grewal & Kaur 1998). The air is then passed to the pharynx.

The pharynx is a fibromuscular tube situated behind the nasal cavity, the oral cavity and the larynx. It is usually called the throat and extends from the base of the skull level to C6 or the cricoid cartilage. Its function is to deliver food products from the mouth to the oesophagus. It also warms, moistens and filters the air we inhale.

The nasopharynx is found behind the nasal cavity and the soft palate. When you swallow food, it passes through the oropharynx and the laryngopharynx. The soft palate rises, allowing the pharyngeal wall to pull forward and form a seal over the nasopharynx. The oropharynx lies between the soft palate and the base of the tongue. The narrower laryngopharynx extends from the hyoid bone and the start of the oesophagus.

The two laryngeal cartilages that can be felt during a neck examination are the laryngeal cartilage (or ‘Adam’s apple’) and the slightly lower cricoid cartilage. The distance between the laryngeal cartilage and the sternal notch is used to assess lung hyperinflation (Bickley 2003).

The warmed air then leaves the pharynx and enters the larynx (voice box). This is an area of ligaments and muscle. The trachea is attached to the cricoid cartilage of the larynx. It is roughly 2.5cm in diameter and 10–12cm long. The walls are surrounded by cartilage to protect the airway from collapsing. In the disorder tracheomalacia, there can be a weakness in the longitudinal fibres of the trachea or impaired cartilage integrity which can cause trapping and collection of secretions that can lead to infection (Carden et al. 2005). Interestingly the shape of the adult trachea varies even without disease – as some remain circular and others a more ovoid shape (Tewfik & Gest 2015).

The trachea divides into the right and left main bronchi at the keel-like partition called the carina. It is situated to the left of the median line. However, the right bronchus appears to be more central than the left, making it look like a direct continuation of the trachea (Tewfik & Gest 2015). It then branches into the lobar, segmental and sub-segmental bronchi. It divides a further 25 times into the pulmonary alveoli at the terminal ends of the respiratory tree where the gas transfer occurs.

The first seven divisions are called the larger airways. These contain:

Ciliated epithelium, which contains goblet cells. These glandular cells secrete a gel-forming mucin found in mucus, called airway surface liquid (ASL). ASL has a mucus component that traps the inhaled particles and a soluble layer, which keeps mucus at an optimum distance from the underlying epithelia, preventing easy clearance (Tarran 2004).

Mucus-secreting cells in the surface epithelium.

Endocrine cells.

Cartilage and smooth muscle in the branch walls.

The remaining 16–18 branches are called the small airways. They have cells that produce surfactant, which reduces the surface tension of fluid in the lungs. They contain fewer goblet cells and there is no cartilage present.

Box 1.1: Cystic fibrosis

In the genetic disorder cystic fibrosis, the goblet cells in the mucosa produce thick, sticky mucus. It cannot be transported out of the respiratory system by the cilia that line the tract. The smaller passages become blocked with this dense mucus, leading to significant infection and poor gas exchange.

Air flow

The movement of air in the respiratory system depends on the difference in air pressure between the oral cavity and the pulmonary alveoli. When you breathe in, the intrathoracic pressure drops below the atmospheric pressure. The air is therefore forced into the alveolar tree. When you breathe out, the muscles in the lung and chest wall recoil like elastic. This causes the intrathoracic pressure to rise above atmospheric pressure. In Alpha-1 antitrypsin (A1AT) deficiency there is a lack of the elasticity needed to allow the airways to recoil.

The rate at which air can move along the airways therefore depends on any resistance in the airway. Any problem with the lumen of the airway can potentially increase this resistance. Such problems can include:

Contents blocking the lumen of the airway

Internal or external pressure

Lack of muscle tone

The thickness of the epithelial lining.

Obstruction within the airways can be due to smooth muscle spasm, as in asthma and chronic obstructive pulmonary disorder (COPD), increased airways secretions or sputum.

The alveoli

There are over 150 million small air-filled sacs in each lung. These are the alveoli, which are the terminal ends of the bronchi tree at the end of the bronchioles. Each alveolus contains type I and II alveolar cells, which have a thin cell wall to facilitate gas exchange. Type I is mainly a squamous cell; type II has a secretory function, producing surfactant. Each alveolus has a complicated network of hundreds of capillaries. These also have a thin membrane, facilitating gas exchange at the large alveolar capillary interface or membrane.

Lobes of the lungs

There are two lungs in the thoracic cavity. The right lung has three lobes, while the left lung has two lobes. This is important to remember when performing a physical assessment.

The respiratory muscles

The inspiratory muscles consist of three types of muscle. They are:

The diaphragm

The intercostal muscles

The accessory muscles.

The diaphragm

The diaphragm is a dome-shaped muscle at the bottom of the lungs that separates the thoracic cavity from the abdominal pelvic cavity. It consists of two parts: the peripheral muscle which has radial muscle fibres extending from the ribs, sternum and the spinal column; and the central tendon. The central tendon is situated behind the xiphisternal joint and extends from ribs 4–7 and the anterior surface of the lumbar vertebrae. Being made of dense collagen fibres, it is a strong insertion point for the muscles.

When you inhale air into your lungs, the muscles in the diaphragm contract and pull the central tendon into the abdominal cavity. This allows the space within the thorax to increase, allowing more room for the air-filled lungs. You can feel this if you take a deep breath in, and place your hands on your abdomen.

The intercostal muscles

The intercostal muscles consist of the external and intercostal groups of muscles that run between the ribs and help form and move the chest wall. The external muscles start at the inferior border of each rib, and their role is to elevate the rib during inspiration. The internal muscles start at the superior border of each rib, and their role is to depress the ribs after exhalation.

The accessory muscles

The accessory muscles are found in the neck and shoulder area. These include: the sternomastoid, scalenus anterior, medius and posterior, pectoralis major and minor, inferior fibres of serratus anterior and latissimus dorsi, serratus posterior anterior and the iliocostalis cervicis muscles. They are mainly used for inspiration if a patient is having difficulty inhaling oxygen into the lung using the usual muscles of respiration.

Figure 1.1: Diagram of the lungs

Males tend to use their diaphragms for breathing, whereas women tend to have greater thoracic movement.

Table 1.1: Muscles involved in breathing

The main function of the respiratory system

The main function of the lungs is to take oxygen from the air we inhale, and to exhale carbon dioxide. The air we breathe in (external respiration) is a mixture of gases, including nitrogen (N2) and oxygen (O2). Each gas contributes a partial pressure to become part of an atmospheric pressure of 760mmHg. The partial pressure of oxygen (PaO2) is 20.9% of the total atmospheric pressure. This decreases to 13.2% as the air passes through the warmed respiratory tract, across the respiratory membrane, into the alveoli in the lungs. The gases then diffuse into the pulmonary capillaries. The result is that the PaO2 levels in the bloodstream rise and the PaCO2 levels fall.

Internal respiration is the term used to describe this process, as the gases are diffused between the blood and interstitial fluid and the capillary cell membranes.

Figure 1.2: Cellular respiration

The process of respiration

Several functions are required to achieve respiration. To enable the lungs to take oxygen from the air we inhale and to exhale carbon dioxide:

The respiratory centre in the brain must regulate our speed and depth of breathing.

The muscles in the chest have to enable us to inflate or deflate the lungs.

The alveoli and capillaries in the lungs must transport oxygen and carbon dioxide.

The pulmonary circulation has to move blood in and out of the lungs.

Our immune system has to protect us against foreign particles.

Local control of respiration

The rate at which oxygen is delivered to the cellular tissue is regulated at a local level. Cells have a continuous need for oxygen, as they need to use the oxidative phosphorylation to generate their energy source known as adenosine triphosphate (ATP) (Pittman 2011). The cells also have an inbuilt ability to continuously monitor their metabolic requirements (Clanton, Hogan & Gladden 2013).

When the normal O2 uptake and delivery changes, the cells have an elaborate and diverse sensing and response system to compensate for the alteration. If the peripheral tissue becomes more active, the interstitial PaO2 drops and the levels of PaCO2 rise in response. The partial pressures in the tissue are altered, resulting in more oxygen being delivered to the tissue. The smooth muscles in the arteriole walls relax to aid this process. When PaCO2 levels increase, the bronchioles dilate, directing more O2 to the areas that require it.

Control of breathing

The activity of breathing relies on the following anatomical structures:

The respiratory centre in the brain

The spinal cord

The nerves

The neuromuscular junctions

The pleurae

The lungs

The muscles

The chest wall

The anterior horn cells.

The respiratory centre in the brain controls the involuntary component of breathing. Groups of dorsal neurons are based in the medulla oblongata and pons of the brainstem. These regulate the respiratory muscles and therefore control the frequency (rate) and depth of breathing. The dorsal neurons generate repetitive respiratory signals which build up over 2 seconds and result in the smooth contraction of the respiratory muscles. After inspiration, there is a 3-second time lag, allowing the muscles to relax and expiration to occur. The process then repeats itself. The cycle takes a total of 5 seconds.

The normal respiratory rate is therefore 12 respirations per minute.

The brain also detects any changes via sensory receptors such as the partial pressure of oxygen and carbon dioxide, or via mechanoreceptors in the stretch and relaxation of the muscles. The mechanoreceptors respond to any change to the volume of the lungs or the arterial blood pressure.

The baroreceptors are free nerve endings situated in the elastic tissue of the blood vessels. When the pressure changes the blood vessel wall contracts or dilates. The baroreceptors react immediately to this pressure. They are found in the carotid artery and the aorta.

There are chemoreceptors in the carotid and aorta arteries which monitor the oxygen and carbon dioxide concentration of the blood and the pH of the blood. When these levels drop, neurosignals are sent to the brain via the glossopharyngeal and vagal nerves. They respond much more sensitively to an increase of PaCO2. If there is a small increase in CO2 levels such as 0.13kPa, there can be an increase of 2–4L/min