Lecture 14: Respiration

The Respiratory System

• The human respiratory system allows us to take in oxygen and remove carbon dioxide from our bodies.
• In a lifetime, we take around 600 million breaths, and if we laid out the 750 million little tubes that make up our lungs, they would cover the size of a tennis court.

Gas Exchange

• Gas exchange, also known as respiration, is the uptake of oxygen from the atmosphere and the discharge of carbon dioxide back into the environment.
• A particular gas within a mixture of gases exerts a pressure, known as partial pressure.
• Gases always diffuse from a region of high partial pressure to a low partial pressure.

Breathing

• When we take a deep breath, air rushes in through the nose or mouth, the chest expands, the abdomen expands slightly, and the shoulders may lift up.
• This is due to negative pressure breathing, where the pressure must be lower in the lungs to pull air in.
• The chest expands due to muscle contraction, which creates a lower pressure inside the lungs.

Lung Expansion

• The lungs expand when the chest contracts due to a double membrane surrounding the lungs, which creates a surface tension that easily sticks together but doesn't easily pull apart.

Expiration

• Breathing out, or expiration, is a passive process that does not require muscle contraction.
• When the diaphragm and rib cage relax, the volume of the thoracic cavity reduces, driving air out of the lungs.

Lung Volumes and Capacities

• We can alter the volume within our lungs.

Gas Exchange at the Alveoli

• The human lungs contain millions of alveoli, creating a huge surface area for gas exchange.
• Oxygen diffuses rapidly across the membrane into the surrounding capillaries for dispersal around the body.

Gas Transport in the Blood

• Oxygen is predominantly transported around the body bound to haemoglobin within red blood cells (98%).
• Carbon dioxide is transported by three different mechanisms: 7% dissolved in plasma, 23% bound to haemoglobin, and 70% converted to bicarbonate.

Oxygen Transport by Haemoglobin

• Each haemoglobin molecule can carry four molecules of oxygen.
• The blood's saturation depends on how much oxygen is bound, with 100% saturation occurring when all binding sites are occupied.

Oxyhaemoglobin Saturation Curves

• At normal PO2 levels, haemoglobin is 98% saturated.
• Minor changes in PO2 do not have a significant effect on the saturation.

When Things Go Wrong

• Impaired diffusion of gases between the alveoli and blood or altered oxygen transport in the blood can result in hypoxia.
• Two general categories of dysfunction: obstructive lung disease and restrictive lung disease.
• Additional conditions affecting respiratory function include diseases affecting diffusion of oxygen and carbon dioxide, reduced ventilation, failure of adequate pulmonary blood flow, and ventilation/perfusion abnormalities.

Involuntary Responses

• Sneezing: receptors in the nose send a signal to the brain to close off the mouth and force air out of the lungs through the nose to expel irritants.
• Coughing: receptors in the respiratory tract send a signal to the brain to close off the glottis and vocal cords, building pressure in the lungs and forcing air out when muscles contract.
• Hiccups: a trigger leads to involuntary contraction of the diaphragm, closing off vocal cords briefly, causing air to "bounce" off them, creating the "hic" sound.