Circulatory and Gas Exchange System Revision Notes

A steep diffusion gradient 

• Blood circulation carries oxygenated blood away from the alveoli, and brings deoxygenated blood to the alveoli 
• Ventilation brings air rich in oxygen into the alveoli, and air with increased carbon dioxide is removed.
• Capillaries surrounding the alveoli are narrow, slowing down the bloodflow and allowing plenty of time for efficient gas exchange. 

How alveoli are adapted for efficient gas exchange? 

• Dense capillary network is in close contact with alveoli
• Movement of blood through the capillaries maintains the steepness of the gradient
• Narrow width of capillaries means that erythrocytes are pressed close to the capillary wall and so close to the alveolar wall = reduces distance = faster process

Lungs - cartilage in the trachea and bronchi are important in keeping airways open 

Is two pumps side by side: right side = recieves deoxygenated blood that's been all around the body and pumps it to the lungs.

Left side = recieves oxygenated blood from the lungs and pumps it all around the body

Blood cycle: Atria -> pushed into ventricles -> pump blood out the heart 

First two atria contract together then the two ventricles contract 

First stage: Diastole 

• Heart muscle is relaxed
• Atria, then ventricles are filling with blood 
• Atrio-ventricular valves open and allow blood to go into ventricles

Next stage: Atrial Systole 

• Muscular wall of atria contracts
• Atria are emptied
• Atrio-ventricular valves are fully open 

Last stage: Ventricular Systole 

• Muscular walls of the ventricles contract
• Atrio-ventricular valves are forced shut (prevented from going inside out by tendons = chordae tendinae)
• Pressure in ventricles forced blood out the ventricles

• Blood in right ventricle -> pumped through semi-lunar valves -> into pulmonary artery 
• Blood in left ventricle -> pumped tgriygg semi-lunar valves -> into the aorta

The pressure of the blood in the ventricles pushes these valves open. These valves prevent any backflow into the heart. 

Following ventricular systole the heart muscle relaxes this is diastole. The chambers of the heart refill. The cycle repeats itself about 75 times every minute. 

Cardiac Muscle: Myogenic 

• Sino-atrial node (SAN) initates an electrical impulse in the right atrium 
• Which moves to the left atrium causing the atrial walls to contract
• This is atrial systole 
• The electrical impulse passes from the the atrium to the atrio-ventricular node (AVN)
• There is a short delay to allow the blood to empty from the atria
• The electrical impulse passes from the AVN down the septum through the purkyne fibres
• To the bundle of His located at the bottom of the heart called the apex
• The Purkyne fibres split into two 
• So the electrical impulse spreads up the muscular walls 
• The impulse causes the muscular ventricle walls to contract
• This is ventricular systole 

Used to measure heart function:

P wave = Represents the impulses passing from the SAN to the AVN through the walls of the atria, leading to atrial systole. 

QRS wave = shows the electrical activity in the ventricles that results in ventricular systole. (electrical impulses passing down bundle of His along Purkyne fibres) 

T wave = short phase that occurs as the ventricles recover.

If there's no P, QRS wave = ventricular fibrillation = myocardial infarction (heart attack) and needs urgent medical attention or they will die 

Small and unclear P wave = atrial fibrillation 

Deep S wave = hypertrophy = increase in muscle thickness

Exercising muscles require more oxygen and glucose to be delivered to fuel the increase of respiration required (link to heart).

A trained athlete will have a greater stroke volume than a non-athlete when their heart rates are the same because everyones heart rate and stroke volume will increase when they exercise. By training the thickness of the left ventricle will increase so that the stroke volume at rest will be higher in a trained athlete. This means it will take fewer beats to deliever the same cardiac output so resting heart rate will be lower.

• Babies 0-12 = 100-160 bpm
• Children aged 1-10 = 60-140 bpm
• Children aged 10+ and adults = 60-100 bpm
• Highly trained athletes 40-60 bpm 

Heart -> arteries -> arterioles -> capillaries -> venules -> veins -> back to the heart

Arteries: 

• Lumen - narrow, maintains high blood pressure and fast flow 
• Endothelium or tunica intima - reduces friction 
• Elastic fibres or tunica media - is thick, allows artery wall to stretch and recoil = elastic recoil (pulses) 
• Smooth muscle - thick allowing fast speed and high pressures
• Collagen fibres or tunica externa - protects from damage as we move

Veins: also contain valves to prevent backflow, and blood flowing in one direction only

• Lumen - wide, maintains low blood pressure and slow flow
• Endothelium - reduces friction 
• Elastic fibres - much thinner
• Smooth muscle - much thinner
• Collagen fibres - protects from damage 

Capillaries: 

• Lumen - very narrow, red blood cells are squeezed through
• Endothelium - very thin with tiny gaps between the cells allowing substances to be exchanged between the capillaries and tissues.

Arterioles and Venules:

The arterioles have a thin wall, mainly of muscle fibres but with some elastic fibres. When this muscle contracts, it makes the lumen of the arteriole narrower. When it relaxes, the lumen becomes wider. This means that arterioles can increase or decrease the flow of blood to particular tissues, this is also one means of regulating blood pressure. 

Venules have a very thin wall of muscle and elastic tissue. They are like small veins and carry blood from the capillaries back to the veins. 

Materials such as oxygen are transported round the body via mass transport = when everything is moving in a stream in one direction: e.g. cells, plasma and dissolved substances

Very small organisms, with very few cells, do not need a circulatory system as each cell in the organism is very close to the medium in which they live so oxygen and nutrients can be absorbed over their whole body surface by diffusion. 

Humans (larger organisms) wouldn't be able to do this because the diffusion path would be so long that substances wouldn't move fast enough: blood system carries oxygen and glucose to respiring cells. 

Closed system: the blood stays in the blood vessels at all times unless body is injured. 

Double circulation: 'Two circuits' - Pulmonary circulation goes from the heart to the lungs and back to the heart

Systemic circulation goes from the heart to the body organs and then back again. = when blood passes round once it goes through the heart twice. 

Closed advantages: 

• high pressure can be maintained
• Lower volume of transport fluid (blood) is needed
• Allows a more complete separation of function between organs

Double advantages: 

• Blood pressure can be maintained at high level
• If blood had to travel directly to the body from the lungs without first returning to the heart then the resistance to flow in the lungs would mean the blood pressure round the body would be much lower if it the heart was pumping as hard as it could 
• Oxygenated and deoxygenated blood don't mix
• Oxygen delivery to respiring cells is optimised

Deoxygenated blood that has come from the tissues returns to the right atrium of the heart via the vena cava. It passes into the right ventricle, and gets pumped to the lungs through the pulmonary artery. Oxygenated blood from the lungs returns to the left atrium of the heart. It passes into the left ventricle. From here, it is pumped to the body in the aorta. 

Exchanging materials: 

• Blood at arteriole end of capillary network is under much higher pressure than at venule end
• Hydrostatic pressure forces water and small soluble molecules out of the plasma
• Loss of materials = loss of pressure in capillaries
• Water, with dissolved nutrients and oxygen = tissue fluid 
• Brings oxygen and nutrients e.g. glucose and amino acids .. removes waste e.g. co2 
• Materials are exchanged between the tissue fluid and the body cells by diffusion across the plasma membranes
• Red blood cells and larger proteins remain in the capillary (more hydrostatic push)
• Venule end: water from the tissue fluid returns to the blood capillary by osmosis down a water potential gradient along with solutes

• Some of the tissue fluid drains into a separate set of vessels - the lymph vessels
• Lymph is not pumped through the lymph vessels
• Instead they have valves, to prevent backflow
• Lymph is squeezed along the vessels as skeletal muscles contract 
• Eventually the lymph is returned to the blood at a vein in the neck region. 

Blood pressure: Is the force exerted by the blood against the artery wall. It is also called hydrostatic pressure. It depends upon the force generated by contraction of the ventricle and the diameter of the lumen of the blood vessel. 

Measured by sphygmomanometer. mmHg

Top figure = systolic pressure (pressure in artery when the left ventricle is contracting)

Lower figure = (pressure generated by the elastic recoil 'between beats') 

120/80 - optimal 

Too high = hypertension             Too low = hypotension

The sounds you hear in the stethoscope during a blood pressure measurement are called the Korotkov sounds. 

We breathe in -> Trachea -> Two bronchi -> Branch into smaller Bronchioles -> Cluster of air sacs alveoli 

The Alveoli: 

• Actual site of gas exchange 
• Tiny hollow sacs made of thin flattened squamous epithelium (lining tissue) cells
• Thin and flat = short distance between air in the alveoli and the blood in the capillary, so gas exchange is efficient as possible

The Trachea:

• Rings of cartilage make sure it stays open when breathing in and out
• Lined with ciliated epithelium and goblet cells.
• Goblet cells produce mucus which is a glycoprotein 
• Ciliated epithelium have tiny hairs called cilia ... work together to trap bacteria and dirt and move it to the back of the throat

• Consist of many alveoli which are lined with squamous epithelium cells
• Contains many blood capillaries

The lung is an organ = made up of many different types of tissues .. such as squamous epithelium and elastic tissue. 

Surface area = the number of cells in contact with the enviroment

Volume = the space occupied by all the cells that need to be supplied with molecules. (as the number of cells increases, the volume increases) 

A good exchange surface has a large surface area, a thin surface and steep diffusion gradient.

A large surface area 

•  Bronchioles are highly branched, giving a large number of pathways for air to enter and leave the lungs
• Millions of alveoli in each lung
• Alveoli are highly folded, giving an even greater surface area

A thin surface

• Squamous epithelium cells are only 0.1-0.5um thick = rapid diffusion across them
• Capillary wall is also made of a single layer of thin, flattened cells. 

A steep diffusion gradient 

• Blood circulation carries oxygenated blood away from the alveoli, and brings deoxygenated blood to the alveoli 
• Ventilation brings air rich in oxygen into the alveoli, and air with increased carbon dioxide is removed.
• Capillaries surrounding the alveoli are narrow, slowing down the bloodflow and allowing plenty of time for efficient gas exchange. 

How alveoli are adapted for efficient gas exchange? 

• Dense capillary network is in close contact with alveoli
• Movement of blood through the capillaries maintains the steepness of the gradient
• Narrow width of capillaries means that erythrocytes are pressed close to the capillary wall and so close to the alveolar wall = reduces distance = faster process

Lungs - cartilage in the trachea and bronchi are important in keeping airways open 

When a person stops breathing but their heart still beats.

Causes:

• A respiratory disorder, e.g. severe asthma or pneumonia
• Obstruction in the trachea or bronchi e.g. caused by choking on food or a child putting a small object into their mouth 
• Overdosing on drugs e.g. heroin, barbituarates. 

Respiratory arrest may also occur following cardiac arrest (when the heart stops beating) 

Whenever breathing has stopped or the pulse is weak, cyanosis occurs.

Cyanosis = bluish apperance of the skin especially around the lips, due to build up of deoxygenated haemoglobin. 

The volume of air breathed in and out of the lungs depends on a number of factors e.g. level of activity, size of our lungs and how healthy we are.

Lungs volumes can be measured used a spirometer 

Measuring the rate at which air can be expelled from the lungs when a person forcibly breathes out can help diagnose and monitor conditions such as asthma using a peak flow meter.