Transport in Animals

• What is the cardiac cycle?

  • This is the sequence of contraction and relaxation of the heart chambers during one heart beat.

  • Both sides of the heart contract at the same time.

  • One cardiac cycle is followed by another in a continuous process, so there is no time for blood to stop flowing.
• What are the different stages of the cardiac cycle?

  Systole - when heart muscle is contracted.

  Diastole - when heart muscle is relaxed.
• What happens during diastole?

  • The ventricles and atria are relaxed.

  • The pressure in the ventricles decrease below that in the aorta and pulmonary artery, so the semilunar valves close.

  • Blood is continuing to enter the atria which increases the pressure in the atria.

  • Pressure is then higher in the atria than the ventricles, which means blood pushes open the atrioventricular valves, entering the ventricles.
• What happens during atrial systole?

  • The atrial walls contract causing a decrease in volume, and so an increase in pressure.

  • Therefore the pressure is higher in the atria than the ventricles, so blood forces the atrioventricular valves open.

  • Blood is forced into the ventricles, causing an increase in volume and pressure against the ventricles by the blood.
• What occurs at the same time as atrial systole?

  Ventricular diastole
• What happens during ventricular systole?

  • The ventricular walls contract, causing volume to decrease and therefore pressure to increase.

  • The pressure in the ventricles is therefore greater than that in the atria, forcing the atrioventricular valves to close, preventing the back flow of blood.

  • Pressure in the ventricles rises above that in the aorta and pulmonary artery, forcing the semilunar valves to open.

  • Blood is then forced into the arteries and out of the heart.

  • This continues until the pressure in the arteries falls below that of the ventricles.
• What is the definition of a double circulatory system?

  Blood passes through the heart twice during each complete circuit of the body.

  -> Includes a pulmonary and systemic circulation.
• What is the benefit of having a double circulatory system?

  1) The separation of oxygenated and deoxygenated blood allows or a steeper concentration gradient to be maintained, allowing a more efficient gas exchange.

  
  

  2) Blood only passes through one capillary network (rather than two in a single circulatory system), therefore maintains a higher blood pressure and speed of flow, so a steeper concentration gradient for efficient and faster exchange of nutrients and waste products from surrounding tissues; specifically beneficial for organisms with a higher metabolic rate.

  
  

  3) Blood is able to travel at a lower pressure to the lungs, as a higher pressure would damage the tissues in the lungs.
• What is a closed circulatory system?

  Blood is transported and enclosed within blood vessels.
• What is a single circulatory system?

  Blood passes through the heart once during a complete circuit.
• What is the circulatory system of a fish?

  SINGLE CIRCULATORY

  
  

  • The heart only has one atrium and one ventricle.

  • Deoxygenated blood is pumped from the heart to the gills, where gas exchange occurs.

  • The oxygenated blood than travels from the gills through the rest of the body, travelling through capillaries in organs, delivering nutrients and oxygen.

  • The blood, now deoxygenated, returns to the heart.
• What is a negative of a single circulatory system?

  The blood travels slower, so if the organism was larger nutrients and necessary molecules would not be delivered quickly enough, therefore there is a limit to the size an organism with a single circulatory system can be.
• What is an open circulatory system?

  • Blood is not contained within vessels, instead the fluid circulates though the body cavity so that tissues and cells are in direct contact with the blood.

  • The fluid moves around during movement as muscles help to circulate blood.
• What are the three major types of blood vessels?

  Arteries, veins and capillaries
• What is the role of an artery?

  Transport blood away from the heart.
• What is the role of arterioles?


  • Arteries branch into narrower blood vessels called arterioles, which transport blood into the capillaries.

  • This decreases the blood pressure from the arteries, so that when blood enters the capillaries they are not damaged.
• What is the role of capillaries?

  • Link arterioles to venules (then to veins).

  • Allow for the exchange of carbon dioxide and oxygen between tissues and blood.
• What is the role of veins?

  Transport blood back to the heart (at a low pressure)
• What is the general structure of a blood vessel?

  Tunica Externa - the outermost layer is made of collagen for strength.

  
  

  Tunica Media - a layer of smooth muscle (contracts to control blood flow) and elastic tissue (stretches and recoils to maintain blood pressure).

  
  

  Tunica Intima - a layer of squamous epithelial cells called the endothelium (reduce friction between blood flow and the wall of the vessel), which lines the lumen.

  
  

  Lumen - the space through which blood passes.
• What is the general structure of an artery?

  • The tunic externa contains collage, which provides strength and prevents the blood vessel bursting under high pressure.

  
  

  • The tunica media is thick, therefore it can withstand high pressure.

  -> the smooth muscle within this can contract and narrow the lumen to reduce the blood flow.

  -> elastic tissue maintains blood pressure as it stretches and recoils to balance fluctuations in pressure.

  
  

  •The tunica intima is lined with smooth epithelial cells to prevent friction.

  
  

  • The narrow lumen maintains blood pressure.

  
  

  • Contains a pulse.
• What is the role of elastic fibres?

  The stretch and recoil back to their original shape to help maintain the pressure of the blood.
• What is the role of smooth muscle?

  Contracts to monitor the blood flow and control the blood flow in the arteries.
• What is the role of collagen?

  Found in the tunic externa, providing strength to blood vessels.
• Describe how the structure of an arteriole is related to its function.

  • Muscular layer allows them to contract, so they are able to partially restrict blood flow to certain organs (eg. reduced blood flow to intestines during exercise so more blood can go to muscles).

  • Lower proportion of elastic fibres compared to arteries, as blood pressure is lower.

  • Larger number of muscle cells to contract and close lumen to control blood flow into the capillaries.
• How does the very small diameter of capillaries aid its function?

  The small lumen forces blood to travel slowly, providing more opportunities for diffusion to occur.
• How do capillaries increase surface area?

  • There are a large number of capillaries which branch between cells, and they also form networks called capillary beds.

  • This increase surface area to increase the efficiency of the exchange of substances.
• How does the wall of capillaries aid in the exchange of substances?

  • The wall is made solely from a single layer of endothelial cells, and as this is only one cell thick there is a short diffusion distance.

  • There a gaps between the cells called fenestrations, allowing blood plasma to leak out and form tissue fluid.

  • Very smooth to prevent friction between the blood and the capillary wall.
• How can white blood cells get to the infected tissues?

  White blood cells can combat infection by squeezing through intracellular junctions in the capillary walls.
• What is the structure of a vein?

  • Thinner tunica media as they do not have to withstand high pressures.

  • Larger lumen.

  • Contain valves.

  • No pulse.
• Why do veins have a larger lumen than arteries?

  • Ensures blood returns to the heart at an adequate speed, even at a lower pressure.

  • Reduces friction between blood and the endothelial layer of the vein.

  • The rate of blood flow is lower in veins, however a larger lumen ensures the volume of blood delivered per unit of time is equal.
• Why do veins contain valves?

  To prevent backflow, helping blood return to the heart.
• How can blood move through veins?

  As skeletal muscles contract, the blood is pushed back towards the heart.
• What is the role of venules?

  Transport the blood from the capillaries to the veins.
• What is the structure of venules?

  • Large lumen, and few or no elastic fibres.

  • As the blood is at low pressure after passing through the capillaries there is no need for a muscular layer.
• Why does tissue fluid form?

  • Humans have a closed circulatory system, so the blood cannot leave our capillaries.

  • In order for substances containing oxygen and nutrients to reach our cells, they have to leave our blood through tissue fluid.
• Why is plasma a good transportation medium?

  Plasma is largely composed of water (95%), and as water is a good solvent this means many substances are able to be dissolved in it and transported around the body.
• What is hydrostatic pressure?

  • The pressure exerted by a fluid eg. blood.

  • The hydrostatic pressure in this example is blood pressure, generated by the contraction of the heart muscle.
• What is oncotic pressure?

  • The osmotic pressure exerted by plasma proteins within a blood vessel.

  • Plasma proteins lower the water potential within the blood vessel, causing water to move into the blood vessel by osmosis.
• What happens at the arterial end during the formation of tissue fluid?

  • The hydrostatic pressure of the blood at the arterial end is great enough to force fluid out of the capillary.

  • Proteins remain in the blood as they are too large to pass through the pores in the capillary walls.

  • The increased protein content creates a water potential gradient (osmotic pressure) between the capillary and the tissue fluid.

  • At the arterial end the hydrostatic pressure is greater than the oncotic pressure, therefore there is net movement of water out of the capillaries.
• Which is greater at the arterial end: oncotic or hydrostatic pressure?

  At the arterial end hydrostatic pressure is greater than oncotic pressure, so fluid moves out of the capillary.