Respiration

Control of breathing

• Breathing is controlled by:

1. the Central Control ( the brain )

2. the Chemical Control.

• The Brain controls the breathing mechanism through the Medulla or the Respiratory Center.

• The Chemical control is by the chemical receptors or Chemoreceptors.

Respiratory Centers in the Brain

The 3 Medullary Centers Controlling Respiration:

1) Bilaterally, on either side of the reticular formation are the medullary respiratory centers; an inspiration center and an expiration center send nerve impulses to the diaphragm and intercostal muscles.

2) The next center up in the pons is the apneustic center. The apneustic center facilitates inspiration.  Generally, the apneustic center kicks the inspiratory center into gear if you go too long without breathing (for example, if you were very tired). 

3) The pneumotaxic center is located in the cranial part of the pons. This area controls the other 2 centers and cuts off inspiration at a certain point to make sure that inspiration does not continue too long. In other words it is a timer for duration of inspiration.

• The Chemical control is done through receptors in the body which is sensitive to CO2 and O2. . These receptors are known as Chemoreceptors.

• They are found in the:1. Carotid Bodies

2. Aortic Bodies.

Respiratory Chemoreceptors

1)Central Chemoreceptors are located on both sides of the medulla in the brain.  These chemoreceptors are primarily sensitive to pCO2 and the pH of blood.  As pH falls (gets more acidic) and pCO2 levels rise, these chemoreceptors provide stimulatory inputs to the inspiratory center; this increases ventilation or breathing in an attempt to reduce H+ and CO2 in the blood.

3)  Peripheral Chemoreceptors known as the Aortic Bodies in the aortic arch and the Carotid Bodies at the junction of the carotid arteries in the neck.

• Carotid and Aortic bodies are sensitive to:

– Decreasing O2 ( HYPOXIA )– Increasing CO2 ( HYPERCAPNIA)– Deceasing pH ( more acidic)

• When there is hypoxia or hypercapnia or when the blood becomes more acidic , nerve impulse are sent to the respiratory centers in the medulla in the brain which stimulates us to breathe more.

Summary:

BREATHING IS CONTROLLED by:

• 1. Respiratory centers in the Brain.

• 2. Chemoreceptors:

a. Peripheral chemorecetpors ( Carotid and Aortic Bodies)

b. Central chemoreceptors ( on surface of medulla of brain )

Mechanics of Breathing

Physiology of Respiration

Measurement of gases

1. Gases in atmosphere 78% N2, 21% O2, 0.04% CO2 2. Pressure developed by gas is % x total pressure (760mm Hg) (i.e., partial pressure)

78% x 760 = 600 mm Hg = PN2 21% x 760 = 160 mm Hg = PO2 0.04% x 760 = .3mm Hg = PCO2

pressures in alveoli and blood different partial pressures are independent so gases can diffuse in opposite directions

• Atmospheric Pressure (Patm) - pressure exerted by the air surrounding the body. At sea level its equal to 760mmHg. For our purposes, we'll assume it to be constant and assign it a value of 0mmHg.

• Intrapulmonary Pressure (Palv) - pressure exerted by the air within the alveoli. It rises and falls during inspiration and expiration, but it always equalizes with atmospheric pressure.

• Intrapleural Pressure (Pip) - pressure within the pleural cavity. It is always lower than both atmopsheric pressure and intrapulmonary pressure.

• FRC = Functional Residual Capacity • TV = Tidal Volume • VC = Vital Capacity • RV = Residual Volume • TLC = Total Lung Capacity • ERV = Expiratory Reserve Volume • IRV = Inspiratory Reserve Volume

Normal pulmonary function tests:

Pulmonary function tests (values shown as % of expected value):

• FEV1 61% • Vital Capacity 73%• FEV1/VC 0.61 (normally >0.72)• Residual volume 175% • Total lung capacity 105% • Diffusion Cap. CO 13%

Pulmonary function tests in COPD (emphysema):