3.2 - Transport in animals Flashcards
Control of cardiac cycle
- SAN initiates wave of electrical excitation
- which spreads over atrial wall
- causing the atria contract / atrial systole
- simultaneously
- a band of fibres between the atria and ventricles stops the wave of excitation passing directly
to the ventricular walls - the wave of excitation reaches the atrioventricular node (AVN) on the septum
- the AVN delays the wave of excitation for 0.1s to allow the atrial sysolte to complete before ventricular systole
- wave of excitation spreads down the septum to the bundle of His and then to the Purkyne fibres
- ventricles contract simultaneously
- ventricles contract from apex upwards to pump blood upwards into arteries to completely empty the ventricles
Structure and function of blood vessels
Arteries - carry blood away from the heart at high pressure
- lumen is small - maintains high pressure
wall:
* is thick and contains collagen to give strength to withstand high pressure
* has elastic tissue - allows stretch when heart pumps and then allows recoil to maintain high
pressure when heart relaxes
* has smooth muscle - can contract and constrict the artery to narrow the lumen (e.g. in vasoconstriction to redirect blood flow)
Veins - carry blood back to the heart at low pressure
- Lumen is large to make flow of blood easier
- Walls are thinner (have thinner layers of collagen, elastic tissue and smooth muscle) as they do not need to withstand high pressure and are not used to constrict blood flow
- Contain valves - stop blood flowing in wrong direction and help it back to the heart
Capillaries - allow exchange of materials between blood and cells
- thin walls of flattened endothelial cells (squamous epithelial) to reduce diffusion distance
- lumen is narrow to squeeze RBCs up next to the wall to reduce
diffusion distance further
Formation of tissue fluid
- Due to the contraction of the heart, blood is at high hydrostatic pressure at the arteriole end
of the capillaries - Between the cells of the capillary walls there are many small gaps
- The hydrostatic pressure is greater than the osmotic pressure
- This forces fluid out of the capillaries carrying plasma and dissolved substances e.g. oxygen
and glucose (and some small WBCs - neutrophils) with it - this is the tissue fluid - RBCs, proteins and some WBCs can’t leave the capillaries because they are too large
- Hydrostatic pressure is lower at the venule end
- Osmotic pressure (in direction of capillary) is now greater
- due to presence of plasma proteins in the blood (lowers ψ)
- Fluid moves back into the capillary taking dissolved waste e.g. CO2 with it
Formation of lymph
- Not all the tissue fluid returns to the capillaries
- Pores allow fluid to leave the tissue fluid and enter lymph vessels
- It will remove proteins (made by cells) out of the tissue fluid
- It will remove neutrophils from tissue fluid
- Low in O2 and glucose (used by cells)
- More CO2 and waste (made by cells)
- A lot of fats absorbed from intestines
- Contains lymphocytes (WBCs produced in lymph nodes) which engulf and digest bacteria in
the lymph fluid - part of the immune system
Contents of blood, tissue fluid and lymphs
Blood - Erythrocytes, Neutrophils, Platelets, Large proteins, Glucose, Amino acids, Oxygen
Tissue Fluid - Neutrophils, Glucose ( less respired), Amino acids, ( less cells use) Oxygen (less respired) carbon dioxide (more released)
Lymph - Neutrophils, Lymphocytes, Fats, Glucose ( little) amino acids ( few) oxygen (little), carbon dioxide
Oxygen dissociation curve - explaining the shape of the curve
- At low pO2 - low saturation of haemoglobin with oxygen
> haem group is at centre - makes it difficult to associate - As pO2 increases - faster increase in saturation
> higher conc of O2, steeper gradient for diffusion of O2 into haemoglobin.
> when one O2 associated - conformational change in shape of haemoglobin makes it easier for
O2 to diffuse in and associate - At high pO2 - saturation is high but levels off as unlikely to reach 100%
> when 3 O2 associated, difficult for 4th molecule to diffuse in and associate to reach 100% even
at highest pO2
Releasing O2 from haemoglobin – why it is important between 2-5kPa is the steepest part of curve
- At low pO2 oxygen dissociates from haemoglobin
- This happens in the respiring tissues.
- The steepest part of the curve is bewteen 2-5kPa - this drop in pO2 gives a large drop in saturation and releases a lot of O2
- This corresponds to the pO2 in the respiring tissue as they need a lot of oxygen for aerobic respiration.
Why it is important that the fetal and adult haemoglobin are different
- fetus gains O2, for respiration, from mother across placenta
- pO2 in placenta is low (2-4kPa)
- maternal haemaglobin releases O2
- fetal haemoglobin has higher affinity for O2
- this maintains a diffusion gradient towards fetus
Why it is important after birth, that thee adult haemoglobin replaces the fetal haemoglobin
- Affinity of fetal haemoglobin for oxygen would be too high
- So would not release oxygen readily enough
- Pregnant mothers would need a difference between the affinity of their haemoglobin and that of their foetus for oxygen
Carbon dioxide carriage in the blood
- Small amounts dissolve in plasma or combines with haemoglobin to form
carbaminohaemoglobin
The rest is carried as HCO3-:
- CO2 diffuses into red blood cells
- CO2 reacts with water
- This reaction is catalysed by carbonic anhydrase (enzyme)
- To form carbonic acid (H2CO3)
- Carbonic acids dissociates to form H+ ions and HCO3- ions
- H+ ions make conditions acidic
- The presence of H+ ions in the red blood cells could make them very acidic.
- To stop this, H+ ions combine with haemoglobin in the red blood cells to form haemoglobin acid (HHb)
- Haemoglobin acts as a buffer (maintains constant pH)
Releasing more oxygen e.g. during exercise – the Bohr effect
- In low pO2, e.g. in the respiring cells, oxyhaemoglobin dissociates and releases oxygen
- When CO2 is present (respiring tissues), there is more carbonic acid to dissociates and form more H+ ions.
- H+ ions displace oxygen molecules on haemoglobin and form more haemoglobinic acid
- As a result, in the presence of CO2, more oxygen is released - this is the Bohr effect
- The Bohr effect results in oxygen being more readily released when more CO2 is produced from respiration - releasing more CO2 will mean they need more O2 for aerobic respiration
