Biomedical Science - Ventilation in the respiratory system

Ventilation

Ventilation is the continuous movement of inspiration followed by expiration in a repeating cycle

• Most of the time ventilation is automatic

• We can override it by consciously changing our breathing pattern or holding breath for a short time

• The ventilation pattern also changes during speech - increased rate of breaths and volume of air inspired4

Most important structures in ventilation

diaphragm, ribs, intercostal muscles

What does ventilation depend on?

changes in volume and pressure in the thoracic cavity

structures needed to allow continuous ventilation

The ribs and the sternum

The vertebrae (spine) - holds the ribs in place

Muscles

The intercostal muscles -between the ribs

The diaphragm

The abdominal muscles

Accessory muscles of the chest and neck

The sternum

Flat; midline of the anterior thorax

Attachment point for ribs by costal cartilage

Ribs

12 pairs

Ribs are fairly mobile, through their hinge joints with the spinal vertebrae and cartilage joints with the sternum

This allows ribs to move up and out for inspiration and down and in forexpiration

The vertebrae - spinal column

33 vertebrae (singular - vertebra)

5 divisions:

1.Cervical (neck region) C1- C7

2.Thoracic (thorax) T1-T12 (ribs attach)

Lumbar - L1- L5

Sacral - S1 - S5 (fused)

Coccygeal (coccyx region - 4 fused)

Separated by fibrocartilage disc

Thoracic vertebra

12 thoracic vertebrae (T1-T12) form the middle section of the vertebral column.

Attachment of the Ribs:

•Ribs 1-10: Attach to thoracic vertebrae via costovertebral joints.

•Ribs 11 and 12: Floating ribs, attach only to the vertebral bodies, not to the transverse processes or sternum.

•Movement: Thoracic vertebrae and ribs enable limited rotational and flexion/extension movements to support respiration.

Diaphragm

Primary muscle of inspiration

Attaches anteriorly to the ribcage and posteriorly to the vertebral column

Has openings to allow for the descending aorta, ascending inferior vena cava and oesophagus

Contraction of the diaphragm causes it to flatten

Contraction is caused by the phrenic nerve

intercostal muscles

-Each rib is connected to the rib below by one external and internal intercostal muscle

• Contraction of the external intercostal muscles elevates the ribs and spreads them apart

• Contraction of the internal intercostal muscles depresses the ribs and pulls them closer together

• Internal intercostals not used in quiet breathing - but are used in forced expiration

inspiration

The muscles between the ribs (intercostal) contract

Ribs are pulled up and out

Diaphragm is pulled down

Lower pressure in lungs; air moves in

Expiration

The muscles between the ribs(intercostal) relax

Ribs move down

Diaphragm relaxes up

Higher pressure in lungs; air moves out

Pause - then inspiration starts again

Accessory muscles

Inspiration: accessory muscles of chest and neck - sternocleidomastoid, scalenes

Expiration: the abdominal muscles (active breathing)

Lung capacity

the amount of air the lungs can hold (around 5-6l in a healthy adult)

Total Lung Capacity (TLC)

maximum amount of air in the whole respiratory tract

calculated by measuring lung volumes at different phases of the respiratory cycle

Tidal volume

Amount of air that moves in and out of the lungs during each cycle of quiet breathing (Approx 500ml)

Inspiratory reserve volume

The largest volume of air that have be inspired into lungs during forced inspiration after the tidal volume (approx 3L)

Expiratory reserve volume

the largest volume of air that can be expelled from the lungs during forced expiration after the tidal volume (approx 1.5L)

Residual volume

volume of air in the lungs after forced expiration (approx 1L)

respiratory centres in the brain

Brainstem: Medulla oblongata, pons

Medulla oblongata and the 2 regions

The main respiratory control centre

Has 2 regions that control respiration:

The dorsal respiratory group (DRG) stimulates inspiration by activating the external intercostals and the diaphragm

The ventral respiratory group (VRG) mostly stimulates expiration by stimulating the internal intercostal muscles and abdominal muscles. (mainly forceful/active breathing)

The pons and 2 regions

Located just above the medulla

Controls the rate and depth of breathing -fine tunes rate and volume

Has 2 regions:

1.The apneustic area - stimulates the inspiratory centre, prolonging the contraction of inspiratory muscles

The pneumotaxic area - inhibits the inspiratory centre, limiting the contraction of the inspiratory muscles, and preventing the lungs from overinflating

pulmonary stretch receptors in lung

As the lungs inflate with air, receptors detect the increase in pressure

Pressure communicated to the respiratory centres in the brainstem

This inhibits the apneustic area of the pons (and also the DRG directly)

In turn this inhibits the inspiratory neurons (in the DRG), thus allowing expiration (passive recoil)

peripheral chemoreceptors

Aortic bodies in the arch of the aorta

Carotid bodies in the carotid arteries

Detect changes in O2 ,CO2 and pH

Usually when O2 is low, CO2 is high and pH is low

The chemoreceptors then send signals via neurons to the brainstem (ending in the medulla oblongata in the tractus solitarius)

This activates the DRG to increase breathing rate so getting rid of CO2 more quickly

central chemoreceptors

Situated in the medulla - detects pH changes in the cerebrospinal fluid (due to CO2 fluctuations)

This activates the nearby DRG to increase inspiration

It also activates the VRG to increase expiration

Together this also increases the breathing rate so getting rid of CO2 more quickly

other factors affecting breathing rate

Emotional stress/anxiety

Pain

Emotion

Air resistance e.g. in asthma

Fever increases body's oxygen demand - increases breathing rate

COPD - Chronic Obstructive Pulmonary Disease (COPD) i.e. emphysema, chronic bronchitis