A-Level生物：Transport in the Human Body

 

　　Gas exchange & transport

 

　　Lungs have squamous epithelium, alveoli and constant supply of deoxygenated blood to ensure rapid transport of gases. If more O2 is needed, breathing rate is increased by chemo receptors sends impulse to breathing centre.

 

　　Blood circulation achieved through elastic fibers in middle layer of arteries stretching (enables wall to bulge outwards). Veins return deoxygenated blood through valves to prevent backflow, muscles surrounding them squeeze and breathing creates a negative pressure, drawing blood back in.

 

　　Effect of water potential and hydrostatic pressure ensures water forced out of capillary, and the reabsorbed along with waste products at venule end of capillary.

 

　　Haemoglobin, an oxygen-transporting pigment. When first O2 molecule binds, causes a change in molecular shape so that other 3 can bind more easily. Saturated at high pp of oxygen (lungs), unsaturated a low pp of oxygen (muscles).

 

　　Cardiac cycle - atrial/ventricular systole and diastole allows pumping of blood around body. (Senatorial noed initiates heartbeat).

 

　　Redistribution of blood flow away from digestive system to muscles during exercise.

 

　　Cells from malignant tumors can break off and spread to other parts of the body via the blood or lymph system.

 

　　Fetal Circulation; e.g. umbilical vein constricts just before birth so no oxygen is transported (rapid rise in CO2 causes baby's first breath).

 

　　Hormones transported to target cells via bloodstream.

 

　　Nutrition

 

　　Nerve impulse to Milk transported to baby's mouth when baby suckles hypothalamus, oxytocin causes muscles to contract enabling milk ejection.

 

　　Absorption of digested food into epithelial cells; via microvilli to increase surface area. Active transport of glucose into gut. (cotransport proteins with Na+ ions at same time). Amino acids enter epithelial cells via facilitated diffusion into cotransport proteins.

 

　　Micelles enable products of triglyceride digestion to pass into epithelial cells.

 

　　Homeostasis and excretion

 

　　CO2 taken to gas exchange surface in form of H+ and hydrogencarbonate ions. Diffuse?In red blood cell has enzyme carbonic anhydrase so full of HCO3 ions out. (balance reached by Cl- ions moving into red blood cell)

 

　　Haemoglobin acts as buffer solution to ensure H+ ions keep pH constant. So either carried in plasma or haemoglobin according to levels.

 

　　Blood cholesterol transported in lipoproteins because it's insoluble.Transport of blood affected by temperature regulation; vasoconstriction and vasodilation.

 

　　Indigestible food excreted via faeces.

 

　　CO-ordination and movement

 

　　Transmission of nerve impulse; saltatory conduction ensures impulse travels faster.

 

　　Transmission across synapse achieved by vesicles fusing with presynaptic membrane and emptying contents into cleft.

 

　　Reflex arcs (in spinal cord).

 

　　Reproduction and growth

 

　　Sperm production; once shed into somniferous tubules carried passively to the epididymis. Surge of LH enables oocyte to be released down the fallopian tube.

 

　　Sperm can acquire the ability to swim up the female reproductive tract through capacitation (removal of certain proteins).

 

　　Placenta enables transport of O2 to fetus (has a lower affinity for O2 than fetal haemoglobin). Also it transports amino acids to fetus via active transport, and glucose via facilitated diffusion. Pinocytosis of antibodies.