Module 2 Exchange and Transport 1

• The alveolus wall is an efficient exchange surface as it is only one cell thick (squamous cell). It is moist and is highly folded for a large surface area.
• Alveoli are supplied with a rich network of capillaries (walls one cell thick) which carry blood close to the alveolus wall (the surface exchange).
• Intercostal muscles on the ribs :
  • Contract to raise the ribs.
  • Relax which makes the ribs fall
• Muscle fibres in alveolus wall contract/relax to move air in and out of the lungs (ventilation). Along with the blood supply, this keeps up a concentration gradient of oxygen and carbon dioxide.
• These features increase the rate of diffusion of oxygen into the blood from the alveolus and carbon dioxide out of the blood into the alveolus.

• Inspiration:
  • pressure in chest cavity drops blow atmospheric pressure
• Expiration :
  • pressure in lung increases and rises above atmospheric pressure.
• The trachea and bronchi have rings of cartilage which keep them open for airflow during ventilation.
• Smooth muscle contracts and narrows the bronchi and bronchioles and elastic tissue opens these airways. This controls airflow.
• Goblet cells in the lining of the trachea, bronchi and bronchioles secrete mucus which traps particles (e.g. pollen and bacteria).
• Ciliated epithelial cells in the lining beat upwards. This removes any swallowed mucus or particles, keeping the lungs clean.

• Large multicellular animals have a small surface area for their volume, resulting in a large distance for diffusion of gases.
• They need a special transport (blood) system to suppy oxygen and remove carbon dioxide, especially if very active. (e.g. birds, mammals and fish).
• Single system (fish): heart - gills - body - heart.
• Double system (mammels): Left heart - body - right heart - lungs - left heart.
• Cosed circulatory system (fish, birds and mammals) :
  • blood stays in blood vessels.
• Open system (insects):
  • blood leaves vessels.

• The left ventricle wall (thick/high pressure) pumps blood around the body. The right ventricle wall (thinner/low pressure) only have to pump blood to the lungs.
• Atrial wall are very thin since they only have to pump blood a short distance into the ventricles.
• Cardiac cycle:
  • chambers fill (diastole) ; ventricles contract (ventricular systole)(Atrio-ventricular valves close, "lub" sound); atria contract (atrial systole)(semi-lunar valves close, "dub" sound).
• The Sino-artial node (in right atrium) maintains beat rhythm. The AtrioVentricular Node and Purkyne fibres pass the beat on to ventricles.
• Heart cells respire fatty acids so needs oxygen for aerobic respiration.
• Coronary Atrteries lie over the surface of the heart. They carry oxygenated blood to the heart itself.
• Veins carry blood to the heart.
• Arteries carry blood away from the heart.
• String-like tendinous cords attach valves to the walls of the venricles and prevent the flimsy valves turning inside out.

• Why low pressure ?
  • Blood from the right ventricle does not need to travel far
  • The lungs contain a lot of very fine capillaries which are not supported and could easily burst.
  • So pressure of blood must be kept low.
• Heart muscle is myogenic it can initiate its own contractions.
• P- shows the excitation of atria.
• QRS- indicates the excitation of ventricles.
• T- shows diastole.

• Blood contains cells, plasma proteins and dissolved substances.
• Tissue fluid is blood minus cells and plasma protiens.
• Harmoglobin (Hb) in red blood cells (erythrocytes) picks up oxygen easily at the lungs where oxygen pressure is high (Hb dissociation curve is to the left and S-shaped) and releases it at tissues where oxygen pressure low. (down a diffusion gradient)
• Carbon dioxide from tissues combines with Hb, making it release oxygen ( Carbon dioxide makes the dissociation curve move to the right). This Bohr effect means more oxygen for active tissues. The carbon dioxide diffuses into plasma.

Foetal Haemoglobin

• Has a higher affinity for oxygen than adult haemoglobin.
• Can "pick up" oxygen from an environment that makes adult haemoglobin release oxygen.

• Arteries:
  • carries blood away from the heart
  • small lumen to maintain high pressure
  • wall is relatively thick ; contains collagon, a fibrous protien to give it strength to withstand high pressure.
  • contains elastic tissue thats allows wall to stretch and recoil. The recoil maintains the high pressure while heart is relaxed.
  • contains smooth muscle which contract to reduce blood flow to certain organs.
  • folded endothelium so can unfold when the artery is stretched.
• Veins
  • large lumen to ease the flow of blood
  • thinner layers of collagon, smooth muscle and elastic tissue. They do not need to stretch and recoil and do not contrict blood flow.
  • Main feature valves that prevent blood flowing in the wrong direction.

• Capillaries
  • has very thin walls. This allows exchange of materials between the blood and cells of tissue via the tissue fluid.
  • Lumen is very narrow. Red blood cells are squeezed this helps them gve up oxygen.

How tissue fluid is formed :

• At the arterial end of a capillary, the blood is under high pressure due to the contractions of the heart muscle.
• This is know as hydrostatic pressure
• The fluid can leave through tiny gaps in the capillary wall.
• The fluid that leaves the capillary is known as the tissue fluid.
• Fluid surrounds the body cells so exchange can occur by diffusion/ facilitated diffusion.

Lymph

• Main difference between tissue fluid and lymph is that it contains lymphocytes.

Formation of Lymph

• Some tissue fluid is drained away in the lymphatic system.
• It consists of a number of capillary like vessels.
• Fluid starts in the tissue and drains the excess in a large vessel which rejoins the blood system in the chest cavity.

• Carbon dioxide diffuses from body cells to red blood cells.
• Enzyme (carbonic anhydrase) in red blood cells combine carbon dioxide and water

CO2 + H2O > H2CO3

Carbonic acid

• Hydrogen carbonate ions diffuse out of red blood cell.

H+ + HCO3-

• H ions combine with Hb (in plasma) to form HHb ( Haemoglobinic acid )
• Chloride ions (Cl -) move from plasm into red blood cell to maintain charge (chloride shift)

• Oxyhaemoglobin dissociates under the influence of hydrogen ions

HbO8 -> Hb + 4O2

• Oxygen is released into the blood plasma