The cardiac cycle

1. When both the atria and the ventricles are relaxed, blood flows into the atria from the major veins
2. the blood flows through the atrioventricular valves into the ventricles
3. the atria contract simultaneously, pushing blood into the ventricles
4. blood fills into the atrioventricular valves, causing them to snap shut to prevent blood from flowing back into the atria
5. when the pressure in the arteries is higher than the pressure in the ventricles, the semilunar valves remain shut
6. the walls of the ventricles contract,starting from the bottom
7. when the pressure in the ventricles is higher than the pressure in the arteries, the semi-lunar valves open and blood is pushed out of the heart, the contraction only lasts for a short time
8. the ventricles relax
9. when the pressure of the ventricles is lower than the atria, the atrioventricular valves open up
10. when the pressure in the ventricles drops below the pressure in the artery’s the semilunar valves close shut again

 

• If the chambers of the heart contract out of sequence, it will lead to inefficient pumping

Filling phase

• When the atria and ventricles relax the internal volume increases and blood flows into the heart from the major veins
• This phase is called the DIASTOLE

Atrial contraction

• The heart beat starts when the atria contract
• both right and left atrium contract together
• the pressure created by this contraction helps to push blood into the ventricles
• This stretches the walls of the ventricles and ensures that they are full of blood
• Contraction of the atria is called atrial systole
• Once the ventricles are full they begin to contract
• blood fills the atrioventricular valve flaps causing them to shut. This prevents blood from returning to the atria

Ventricular contraction

• There is a short period when all 4 of the heart valves are closed
• the walls of the ventricles contract
• this is called ventricular systole
• this raises the pressure in the ventricles very quickly
• contraction starts at the apex of the heart
• this pushes the blood upwards towards the arteries
• the semilunar valves open and blood is pushed out of the heart
• the ventricle walls then relax allowing the heart to fill up again

 

 

 

You may have noticed that I have effectively talked about the same thing twice, one is a summary and one is in detail, they are both effective in their own ways…..

 

 

Tissues in the lungs

The trachea, bronchi and bronchioles are airways that allow passage of air into the lungs and out again. To be effective, these airways must meet certain requirements:

• the larger airways must be large enough to allow sufficient air to flow through without obstruction
• they must also divide into smaller airways to reach the alveoli
• the airways must be strong enough to prevent it from collapsing when the air pressure inside is low (which occurs during inhalation)
• they must be flexible to allow movement
• they must also be able to stretch and recoil

The trachea and the Bronchi

These both have a similar structure. They differ only in size. The Bronchi are narrower than the trachea They have relatively thick walls that have several layers of tissue

• much of the wall consists of cartilage
• the cartilage is in the form of incomplete C shaped rings in the trachea, however it is less regular in the bronchi
• on the surface of the cartilage is a layer of glandular tissue, connective tissue, elastic fibres, smooth muscles and blood vessels. It is often called the “loose tissue”
• The inner lining is an epithelium layer that has two types of cell. Most of the cells have cilia. This is called the ciliated epithelium. Among the ciliated cells are goblet cells.

Bronchioles

They are much narrower than the bronchi. The larger Bronchioles may have some cartilage, but smaller ones do not have cartilage. the wall is made mainly of smooth muscle and elastic fibres. The smallest bronchioles have clusters of alveoli at their ends

 

The role of each tissue

Cartilage

• It plays a structural role
• it supports the trachea and the bronchi, this prevents the collapse when the air pressure inside is low during inhalation
• it does not form a complete ring, so there is some flexibility, so you can move your neck without constraining the airways
• It allows the oesophagus to expand during swallowing

Smooth muscle

• it can contract
• when it contracts, it constricts the airways
• when the airway is constricted, the lumen of the airway becomes narrower
• constricting the lumen can restrict the movement of air to and from the alveoli, this is most common in the bronchioles

Elastic fibres

• when the airway contracts, the diameter of the lumen is reduced
• the smooth muscle cannot reverse this effect
• when the airway constricts, it deforms the elastic fibres in the loose tissue
• as the smooth muscle relaxes, the elastic fibres recoil and return to their original shape, this dilates the airway

goblet tissues and glandular tissues

• these secret mucus
• the mucus traps tiny particles from the air, these particles may include pollen and bacteria
• trapping the bacteria so that they can be removed can reduce the risk of infection

 ciliated epithelium

• these consists of ciliated cells, they have numerous tiny, hair like structures projecting from their membrane, they are known as cilia
• Cilia moves in a synchronised pattern to waft mucus up the airway to the back of the throat. Once there the mucus is swallowed and the acidity in the stomach would kill any bacteria.

The lungs as an organ of exchange

The lungs

these are a large pair of inflatable structures lying in the chest cavity. Air can pass through the nose and along the trachea, Bronchi and Bronchioles. Each part of this airway is adapted to its function of allowing the passage of air. Finally the air reaches the surface of the  Alveoli. The walls of the Alveoli are the surface where exchanges of gases take place.

The lungs are protected by ribs. Movement of both the ribs and the diaphragm helps to produce breathing movements known as ventilation

Gaseous exchanges/how the lungs are adapted

Gases pass both ways through the thin walls of the Alveoli. Oxygen passes from the air in the Alveoli to the blood in the capillaries  Carbon dioxide pass from the blood to the air in the Alveoli.

• the large surface area provides more space for molecules to pass through. The individual Alveoli are very small (about 100-300Micrometers across). They are so numerous that the total surface area is larger than our skin. The total surface area of the lung exchange surface is about 70m(squared)
• A barrier permeable to oxygen and carbon dioxide means that plasma membranes that surround the thin cytoplasm form the barrier for exchange. These allow the diffusion of co2 and o2
• The thin barrier to reduce the diffusion distance has a number of adaptations to reduce the distance that gases have to diffuse

1. The alveolus wall is one cell thick
2. the capillary wall is one cell thick
3. both walls consist of squamous cells- this means flattened or very thin cells
4. The capillaries are in close contact with the Alveolus walls
5. The capillaries are so narrow that the blood cells are squeezed against the capillary wall, making them closer to the air in the alveoli, reducing the rate that it flows past in the blood
6. the barrier of diffusion is only 2 flattened cells thick, and is less than 1 micrometer thick.

A thin layer of moisture surround the alveoli. This moisture passes through the cell membranes from the cytoplasm of the alveolus cells. As we breathe out, it evaporates and is lost. The lungs must produce a substance called a surfactant, that reduces the cohesion between the water molecules. Without the surfactant, the alveolus would collapse due to the cohesive forces between the water molecules lining the air sac.

 

Maintaining a diffusion gradient

For diffusion to be rapid, a steep diffusion gradient is needed. This means having a high concentration of molecules on the supply side of the exchange surface, and a low concentration on the demand side.

To maintain the diffusion gradient, a fresh supply of molecules on one side is needed to keep the concentration high, and a way of removing the molecules from the other side is needed to keep the concentration low

This is achieved by the action of the blood transport system and the ventilation movements

The blood brings carbon dioxide from the tissue to the lungs. This ensures that the concentration of carbon dioxide in the blood is higher than in the alveoli air space. It also carries oxygen away from the lungs. This ensures that the concentration of oxygen in the blood is kept lower than the concentration than the air inside the alveoli.

The heart pumps blood along the pulmonary artery to the lungs. In the lungs the artery divides up to form finer and finer vessels. These eventually carry blood in tiny capillaries that are only just big enough for a red blood cell to squeeze through. These capillaries lie over the surface of the alveoli.

The breathing movements of the lungs helps to ventilate the lungs. They replace the used air with fresh air. This brings more oxygen in the lungs and ensures that the concentration of oxygen in the air of the alveoli remains higher than the concentration of oxygen in the blood.

The constant supply of gas to one side of the exchange surface and the removal from the other side ensures that diffusion, and therefore exchange can continue.

 

Inhaling

1. The Diaphragm contracts and moves upwards
2. The rib cage external intercostal muscles contract to move ribs upwards and outwards
3. The volume of the thorax increases
4. The pressure in the lungs falls below atmospheric pressure so air moves in

Exhaling

1. The Diaphragm relaxes and moves upwards
   
2. the ribcage external intercostal muscles relax so that the ribs move down and inwards
3. the volume of the thorax decreases
4. the pressure in the lungs rises above that of atmospheric pressure so air moves out