4. Mass transport

what do dissociation curves show

how saturated the haemoglobin is with oxygen at any given partial pressure

what is the Bohr effect

• when cells respire they raise the pCO2

• this increases the rate of oxygen unloading so the dissociation curve shifts right

  • the saturation of blood with oxygen is lower for a given pO2 so more oxygen is being released

characteristics of arteries

• carry blood from the heart to the rest of the body

• thick and muscular walls

• elastic tissue to stretch and recoil as the heart beats

  • which helps maintain the high pressure

• the inner lining is folded, allowing the artery to stretch

  • helps to maintain high pressure

• all arteries carry oxygenated blood

• EXCEPT pulmonary arteries which take deoxygenated blood to the lungs

characteristics of arterioles

• form a network throughout the body

• blood is directed to areas of demand in the body by muscles inside the arterioles

• muscles contract to restrict blood flow or relax to allow full blood flow

characteristics of veins

• take blood back to the heart under low pressure

• wider lumen

• very little elastic or muscle tissue

• veins contain valves to stop back flow of blood

• contraction of body muscles surrounding veins helps blood to flow

• carries deoxygenated blood

• EXCEPT for the pulmonary vein which carries oxygenated blood to the heart from the lungs

characteristics of capillaries

• found near cells in exchange tissues so there’s a short diffusion pathway

• walls are one cell thick, shortening diffusion pathway

• large number to increase surface area for exchange

• networks of capillaries in tissue are called capillary beds

what is pressure filtration

• at the start of the capillary bed, nearest the arteries, the hydrostatic pressure inside the capillaries is greater than the hydrostatic pressure in the tissue fluid

• this difference in hydrostatic pressure means an overall outward pressure forces fluid out of the capillaries and into the spaces around the cells, forming tissue fluid

• as fluid leaves, the hydrostatic pressure reduces in the capillaries so the hydrostatic pressure is much lower than the vellum end of the capillary bed

• due to the fluid loss and an increasing concentration of plasma proteins, the water potential at the venue end of the capillary bed is lower than the water potential in the tissue fluid

• this means that some water re-enters the capillaries from the tissue fluid at the venue end by osmosis

where is excess tissue fluid drained

into the lymphatic system which transports this excess fluid from the tissues and returns it to the circulatory system

what do the atrioventricular valves (AV) do

they link the atria to the ventricles and stop blood flowing back into the atria when the ventricles contract

what do semi-lunar valves (SL) do

they link the ventricles to the pulmonary artery and aorta and stop blood flowing back into the heart after the ventricles contract

what do the cords do

they attach the atrioventricular valves to the ventricles to stop them being forced up into the atria when the ventricles contract

why does blood only flow in one direction through the heart

the valves only open one way and whether they’re open or closed depends on the relative pressure of the heart chambers.

what is the cardiac cycle

an ongoing sequence of contraction and relaxation of the atria and ventricles that keeps the blood continuously circulating around the body. The volume of the atria and ventricles changes as they contract and relax. Pressure changes also occur, due to the changes in chamber volume. The cardiac cycle can be simplified into three stages

what is the first stage of the cardiac cycle

ventricles relax and atria contract

• ventricles relax and atria contract

  • decreasing the volume of the chambers

  • increasing pressure inside the chambers

• slight increase in ventricular pressure and chamber volume as the ventricles receive the ejected blood from the contracting atria

what is the second stage of the cardiac cycle

ventricles contract and atria relax

• atria relax and ventricles contract

  • decreasing volume

  • increasing pressure

• the pressure becomes higher in the ventricles than the atria, which forces the AV valves shut to prevent back-flow

• the pressure in the ventricles is also higher than the aorta and pulmonary artery

  • this forces open the SL valves and blood is forced into these arteries

what is the third stage of the cardiac cycle

ventricles relax and atria relax

• ventricles and atria both relax

• the higher pressure in the pulmonary artery and aorta closes the SL valves to prevent back-flow into the ventricles

• blood returns to the heart and the atria fill again due to the higher pressure in the vena cava and pulmonary vein

  • this starts to increase pressure of the aorta

• as ventricles continue to relax, their pressure falls below the pressure of the atria and so the AV valves open

• this allows blood to flow passively into the ventricles from the atria

• the atria contract and the whole process begins again

what are the two types of disease that affect the arteries

aneurysm and thrombosis

what are atheromas

when damage occurs in the endothelium, white blood cells and lipids from the blood clump up together under the lining to form fatty acids which hardens to form a fibrous plaque called an atheroma

what are the two types of disease that affect the arteries and atheromas increase the risk of

aneurysm and thrombosis

what is aneurysm

balloon-like swelling of the artery

what is thrombosis

formation of a blood clot

what causes myocardial infraction (heart attack) and what are its effects

• if a coronary artery becomes completely blocked, an area of heart muscle will be totally cut off from its blood supply, receiving no oxygen which causes myocardial infraction

• it can cause damage and death of heart muscle

• symptoms include pain in the chest and upper body, shortness of breath and sweating

• if large areas of the heart are affected, complete heart failure can occur which is often fatal

what are the factors that increase the risk of coronary heart disease

• high blood cholesterol and poor diet

• cigarette smoking

• high blood pressure

structure of the xylem

very long, tube-like structures formed from dead cells joined end to end

cohesion-tension theory of water transport

• water evaporates from the leaves at the top of the xylem

  • this creates tension which pulls more water to the leaf

• water molecules are cohesive so when some are pulled into the leaf, others follow

  • the whole column of water in the xylem therefore moves upwards

• water enters the stem through the roots

what are the four factors that effect transpiration

• light - lighter = faster rate, stomata open when its light to let CO2 in for photosynthesis.

• temperature - higher = faster rate, molecules have more energy so they evaporate from cells inside the leaf faster, increasing the concentration gradient between the inside and outside of the leaf

• humidity - lower = faster rate, if air around the plant is dry, concentration gradient between the leaf and air increases

• wind - high = faster rate, lots of air movement blows away water molecules around the stomata which increases the concentration gradient

structure of the phloem

• contains sieve tube elements and companion cells

• sieve tube elements - living cells that form the tube for transporting solutes and have no nucleus and few organelles

• there’s therefore a companion cell for each sieve tube element - they carry out living functions for sieve cells.

what is the mass flow hypothesis

1) source

• active transport is used to actively load the solutes (sucrose) from companion cells into the sieve tubes of the phloem at the source (leaves)

• this lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from the xylem and companion cells

• this creates a high pressure inside the sieve tubes at the source end of the phloem

2)

• at the sink end, solutes are removed from the phloem to be used up

• this increases the water potential inside the sieve tubes so water also leaves the tubes by osmosis

• this lowers the pressure inside the sieve tubes

3) sink

• the result is a pressure gradient from the source end to the sink end

• this gradient pushes solutes along the sieve tubes towards the sink

• when they reach the sink, the solutes will be used (respiration) or stored (starch)

evidence supporting mass flow

• a radioactive tracer such as 14C can be used to track the movement of organic substances in a plant

• if a ring of bark is removed, a bulge forms above the ring. The fluid from the bulge has a higher concentration of sugars than the fluid from below the ring - evidence that there’s downwards flow of sugars

• using aphids, sap flows out quicker nearer the leaves than further down the stem - evidence that there’s a pressure gradient

• if a metabolic inhibitor is put into the phloem, then translocation stops - evidence that active transport is involved

evidence against mass flow

• sugar travels to many different sinks, not just to the one with the highest water potential, as the model would suggest

• the sieve plates would create a barrier to mass flow, a lot of pressure would be needed for the solutes to get through at a reasonable rate