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PHYSIOLOGY LECTURE OUTLINE:
RESPIRATORY PHYSIOLOGY
The Primary Functions of the Respiratory System
1. Ventilation (Inspiration - Expiration) to exchange air with the body and environment.
2. To exchange gases, oxygen and carbon dioxide, between lungs and blood.
3. To homeostatically maintain pH of body fluids.
4. For vocalization and sound production.
Respiratory System
The Respiratory System includes structures involved in ventilation and gas exchange. The respiratory system divided into upper and lower tracts as well as conduction and respiratory portions. Below is a basic outline of the anatomical structures:
Upper tract
Nose, nasal cavity, mouth, pharynx (throat) and larynx (voice box).
Lower tract
Trachea (wind pipe), left and right primary bronchi, secondary and tertiary bronchi, bronchioles, terminal and respiratory bronchioles, alveolar duct, alveolar sac and alveoli - the end where gas is exchanged).
The conducting portion consists of the airways from environment that lead to the exchange surface of lungs (from nose to terminal bronchioles). The respiratory portion is from the respiratory bronchioles to the alveoli (the surface area for gas exchange). The diameter of airways get progressively smaller, but total cross-sectional surface area increases. Velocity of air flow is therefore highest in trachea, lowest in terminal bronchioles.
Air moves down its pressure gradient caused by the changes in volume of the thoracic cavity. The lungs are located in the thoracic cavity and as the volume of the thoracic cavity increases, the pressure inside this cavity (and thus in the lungs) decreases. As the volume of the thoracic cavity decreases, the pressure increases. Thus, movement of thorax creates alternating conditions of high and low pressure within lungs. This creates air exchange in response to pressure gradients.
The relationship between the pressure and volume of a gas is described by Boyle's Law (P1 V1 = P2 V2). As volume decreases, pressure increases and vice versa. Changes in volume of chest cavity during ventilation cause pressure gradients. Increase chest volume - decrease pressure - air moves in from atmosphere. Decrease chest volume - increase pressure - air moves out from body.
The Bones and Muscles of the Thorax Surround the Lungs
The thoracic cage is created by the bones and muscles bounding the thorax: The ribs and spine; chest wall; and the diaphragm, a dome-shaped skeletal muscle forming thoracic floor.
The internal and external intercostal muscles connect the 12 rib pairs. The sternocleidomastoids and scalenes connect the head and neck to the first 2 ribs.
Because the muscles of the thorax are skeletal muscles, they are innervated by somatic motor neurons. Control of those neurons originates in a network of respiratory neurons in the medulla oblongata. The thorax is functionally a sealed cavity with 3 membranous bags. The pericardial sac contains the heart and there are 2 pleural, each sac containing one lung.
Pleural Sacs Enclose the Lungs
Lungs are light, spongy tissue mostly occupied by air-filled spaces.. The narrow apex of each lung is at the top of the thoracic cavity, under the upper ribs and clavicle. The bronchi connect lungs to trachea to the atmosphere. Each lung is divided into lobes: the right lung into three lobes, and the left lung into two (tri before you bi). Because of the space occupied by the heart, the left lung is smaller than the right. Each lung is contained in double-walled pleural sac. It is like an air-filled balloon surrounded by a water-filled balloon. The pleura contains elastic connective tissue and capillaries. The pleural fluid holds opposing pleural layers together, creating a slippery surface allowing movement of membranes as lungs move. It holds lungs tight and stretched against thoracic wall due to fluid's cohesiveness.
The Alveoli are the Site of Gas Exchange
An alveolus has a single layer of thin exchange epithelium and is the site of gas exchange.
There are 3 types of cells in alveoli:
1. Type I alveolar cells: Very thin, allowing gas exchange.
2. Type II alveolar cells: Thicker, secrete surfactant to ease lung expansion.
3. Alveolar macrophages: Protect and defend
Alveoli do not contain muscle fibers and cannot contract. There are elastin fibers between alveoli and these do contribute to elastic recoil when lung tissue is stretched. Capillaries cover 80-90% of alveolar surface forming an almost continuous blood-air contact. Gas exchange occurs by simple diffusion. The single endothelial cell of the capillary and the single squamous epithelium of the alveoli have a fused basement membrane in between them, this arrangement allows for a rapid diffusion of gases.
The Pulmonary Circulation is a High-Flow, Low-Pressure System
At rest, the pulmonary circulation of the cardiovascular system contains about 0.5 L of blood (10% of total blood volume) with 75 ml in capillaries for gas exchange. The conducting portions of the respiratory tract that are not involved in gas exchange receive oxygenated arterial blood from the systemic circulation. But the deoxygenated venous blood leaving these regions does not return to the systemic circulation and the vena cava. Instead, it flows into the pulmonary veins along with freshly oxygenated blood from the alveoli. The addition of deoxygenated blood slightly lowers the oxygen content of the blood before it ever reaches the left side of the heart.
The rate of blood flow through the lungs is greater than that of other tissues. The lungs receive the entire volume of right ventricle cardiac output (5 L/min). The pulmonary circulation has low pressure in, with an average pressure = 25/8 mm Hg compared to 120/80 mm Hg systemic blood pressure. This is correlated with low pulmonary resistance.
The pulmonary arteries are much more compliant (distensibile, easier to stretch) than the aorta and systemic arteries. Also, the total length of pulmonary blood vessels is shorter. As we already know, this means that the right ventricle doesn't have to pump as hard to overcome peripheral resistance. This allows for low pulmonary blood pressure. In turn, there is low net hydrostatic pressure, yielding low fluid flow into interstitial space. Lymphatic system removes filtered fluid, a small volume of interstitial fluid, less than 0.5 L/day of fluid is lost from the pulmonary capillaries, compared to the loss of 3 L/day from the systemic capillaries.
GAS LAWS
Air flow is similar to blood flow in that the driving force for air flow is a pressure gradient and it is opposed by resistance.
Air Flow = DP/R
Air is a Mixture of Gases
The atmosphere is mixture of gases and water vapor. In many parts of the world, pollutant gases such as ozone, sulfur dioxide, and carbon monoxide are also present in varying trace amounts.
Dalton's Law
The total pressure of a gaseous mixture is the sum of individual gas (partial) pressures. Individual gases move down partial pressure gradients. The sum of all partial pressures gives the total pressure for a mixture of gases (Dalton's Law). This is how individual gas exchange occurs: gases move from areas of higher partial pressures to areas of lower partial pressures.
Partial pressures = Patm x % of gas in atmosphere. Partial pressures vary with amount of water vapor.
e.g., air is a mixture of gases.
N2 = 79%
O2 = 21%
CO2 = 0.03%
If atmospheric pressure of air at sea level is 760 mmHg (a standard value, see section below) and air is a mixture of the above gases (N2, O2 and CO2), then we can calculate the partial pressure exerted by each gas in this mixture. The partial pressure of N2 is symbolized by PN2 and partial pressure of O2 is PO2, etc.
Calculating Partial Pressures of gases in air at sea level:
1) PN2 = 760 mm Hg x % of gas in mixture (79%, = 0.79)
= 760 mm Hg x 0.79
= 600 mm Hg
2) PO2 = 760 mm Hg x % of gas in mixture (21%, = 0.21)
= 760 mm Hg x 0.21
= 160 mm Hg
3) PCO2 = 760 mm Hg x % of gas in mixture (0.03%, = 0.003)
= 760 mm Hg x 0.003
= 0.24 mm Hg (which is negligible)
Boyle's Law Describes Pressure-Volume Relationship of Gases
Gas pressure in a sealed container is created by the collision of gas molecules with the walls of the container and each other. The smaller the container, the more frequent the collisions, resulting in higher pressures. Boyle's Law states: P1 V1 = P2 V2. As volume decreases, pressure increases and vice versa. Changes in volume of chest cavity during ventilation cause pressure gradients, which create air flow. An increase in chest volume causes a decrease in pressure - air moves in from atmosphere. A decrease inchest volume causes an increase in pressure - air moves out from body.
The Solubility of a Gas in Liquid Depends on Pressure, Solubility and Temperature
Where air and water meet, a particular gas flows from the medium with higher partial pressure to medium with lower partial pressure. Movement of a gas into a liquid is directly proportional to 3 factors:
1. The Partial Pressure gradient of that gas.
The greater the partial pressure gradient, the more soluble the gas in liquid.
2. Temperature of liquid and surroundings.
The warmer the liquid, the less soluble the gas in it! Think of the soda can example.
3. Solubility of the gas in that liquid.
Gases have different solubilities in liquids depending on their molecular chemistry.
Solubility: The more soluble a gas is, the less partial pressure needed to force it into solution. Poorer solubility requires higher partial pressures to move even small amounts of gas. Oxygen is about 20 times less soluble in water than carbon dioxide is. This is why there is such a large partial pressure gradient for O2 (60 mm Hg) compared to the more soluble CO2 (6 mm Hg).
Gases move between phases until equilibrium is reached. Partial pressure for a gas in the air phase at equilibrium = Partial pressure of that gas in liquid phase. This does not mean that concentrations are equal! Concentrations depend on solubility. Thus O2 needs oxygen-carrying compounds in blood, such as hemoglobin (Hb).
VENTILATION
Ventilation = Movement of air between environment and lungs, the first exchange.
The Airways Warm, Humidify, and Filter Inspired Air
The upper airways condition inspired air before it reaches alveoli in three main ways:
1. Warm air to body temperature (37 °C) to avoid alveolar damage.
2. Humidify to 100% to keep exchange epithelium moist.
3. Filter out foreign material, to protect delicate lung tissue.
Most of this conditioning is done in the nasal cavity, that is why breathing through your nose has a different effect than breathing through your mouth. Heat and water from the mucosal lining of airways warms and humidifies the inspired air. Filtration occurs in the nasal cavity, trachea and bronchi. These areas are lined with ciliated epithelium that secrete mucus and dilute saline, which traps inhaled particles larger than 2 mm and has immunoglobulins to disable microorganisms.
The cilia of the epithelial lining is referred to as the mucus escalator. The mucus is continuously moved toward pharynx, then swallowed or spat out! The stomach acids will further destroy microorganisms. Secretion of the watery saline layer beneath the mucus is a critical step in mucus escalator. Cilia would be trapped in mucus without it. Cystic fibrosis is a genetic disorder wherein there are defective Cl- channels which inhibit saline production, so the mucus is thick and hampers cilia movement.
During Ventilation, Air Flows Because of Pressure Gradients
The lungs are held to the thoracic cage by pleural fluid and contraction of thoracic muscles creates the pumping action and the pressure gradients. Air moves in response to pressure gradients. The primary muscles involved in quiet breathing (eupnea) are: the diaphragm, intercostals and scalenes.
Forced breathing is termed hypereupnea (e.g., exercise, wind instrument, blowing up a balloon): Other chest and abdomen muscles assist.
Air flow in respiratory system obeys the same rules as blood flow: Air flow = DP/R. Flow increases as the pressure gradient increases and decreases as resistance (R) increases.
Various Respiratory Pressures
1. Atmospheric pressure is the weight of the column of air above you. This remains relatively constant, At sea level, this value is 760 mmHg. The pressure in the lungs must be higher or lower than atmospheric pressure for air flow to be created.
2. Intra-alveolar pressure (PA): The pressure inside the alveoli, where gas is exchanged.
3. Intra-pleural pressure: The pressure within the pleural fluid, which is always less than intra-alveolar.
Inspiration Occurs When Alveolar Pressure Decreases
Inspiration: Somatic motor neurons trigger contraction of diaphragm, inspiratory muscles. Diaphragm contraction - compression of dome shape, drops toward abdomen, causing 60-75% change of thoracic cavity volume. External intercostal and scalene contraction to move rib cage upwards and out, causing 25-40% change of thoracic cavity volume. As thoracic cavity volume increases, pressure decreases and air moves into lungs.
Typically, very small changes in alveolar pressure are required for ventilation. When thoracic cavity volume increase, inter-alveolar pressure drops about 2 mm Hg below atmospheric (to about 758 mm Hg) and air begins to flow into alveoli. Air flow continues until pressure inside lungs equals atmospheric pressure (760 mm Hg). At the end of inspiration, the somatic motor neurons to the diaphragm and external intercostals stop firing, causing relaxation. This allows for passive expiration, due to elastic recoil of lungs, not muscle contraction. Expiration occurs when intra-alveolar pressure exceeds atmospheric pressure (reaches about 762 mm Hg).
Active expiration happens during exercise or forced heavy breathing. This occurs during voluntary exhalations and when ventilation exceeds 30-40 breaths/min. Uses internal intercostals and abdominals muscles, expiratory muscles. Diseases afflicting skeletal muscle can adversely affect ventilation. Myasthenia gravis is a disease in which ACh receptors on motor end plates are destroyed. Also, polio is caused by a virus that can paralyze respiratory muscles.
Intrapleural Pressure Changes during Ventilation
The lungs are "stuck" to the thoracic cage by pleural fluid and this aids in lung expansion and contraction. Intrapleural pressure is sub-atmospheric and sub-intra-alveolar (usually ranging from 754 to 758 mm Hg). Puncturing the pleural cavity leads to pneumothorax - a loss of the pressure gradient that keeps lungs in their stretched state. The afflicted lung will collapse under elastic recoil. Air must be removed from the intra-pleural space and puncture sealed to correct pneumothorax.