Heart and transport systems, gas exchange and digestion

• Exchange of substances

  • exchange between and organism and its environment relies on diffusion
  • amoeba diffuse through the outer surface where it will meet gas exchange requirements and allow the removal of heat produced during metabolic reactions
• Biological impact of a large surface area to volume ration

  • single-celled organisms have a large SA:V ration
  • provides a short diffusion pathway
  • satisfies gas exchange requirements for small organisms
  • as the size of the organisms increases, the SA:V ratio decreases
  • larger organisms have developed specialised exchange structures e.g. lungs and gills to maintain adequate rates of gaseous exchange
  • to maintain constant body temperature, changes in body shape or specialised structures have evolves
• Transport systems

  • in large organisms exchange systems work with transport systems
  • transport systems move substances to and from exchange surfaces
  • this prevents build-up of substances at the exchange surfaces and maintains concentration or diffusion gradients
  • mass transport involves the bulk movement of substances via a transport system
  • e.g. blood plasma is mainly water, carrying glucose, amino acids and carbon dioxide in solution
• The mammalian heart

  • vascular system circulates blood in vessels
  • blood is pushed through the vessels by the pumping action of the heart
  • the heart has right and left sides, each having an atrium and a ventricle
• The mammalian heart physiology: atria

  • have thin muscular walls and receive blood under low pressure returning to the heart in veins
• The mammalian heart physiology: ventricles

  • have thick muscular walls and pump blood at high pressure into arteries
• The mammalian heart physiology: right atrium

  • receives deoxygenated blood from the body (except the lungs) via the vena cava
• The mammalian heart physiology: right ventricle

  • pumps deoxygenated blood into the pulmonary artery leading to the lungs
• The mammalian heart physiology: left atrium

  • receives oxygenated blood from the lungs via the pulmonary veins
• The mammalian heart physiology: left ventricle

  • pumps oxygenated blood into the aorta leading to the rest of the body
• The mammalian heart physiology: atrio-ventricular valves

  • open to allow blood into the ventricles
  • they close as the ventricles contract, preventing back-flow of blood into the atria
• The mammalian heart physiology: tendinous cords

  • prevent atrio-ventricular vales 'turning inside out' as the ventricular pressure increases
• The mammalian heart physiology: semilunar vales

  • open to allow blood into the pulmonary artery and aorta as the ventricles contract
  • they close as the ventricles relax, preventing the backflow of blood into ventricles
• Blood flows in one direction through the heart and blood vessels
• Cardiac cycle

  • sequence of contraction and relaxation of the heart chambers and opening and closing of valves during one heart beat
  • volumes and pressures in he heart chambers change during the cycle- so does the pressure in the aorta
  • contraction of the heart of the chamber is systole and relaxation is diastole
• Cardiac cycle

  1. left ventricle contracts. pressure increases above that of the atrium and the bicuspid valve closes
  2. semilunar valve opens when the pressure in the ventricle is greater than that in the aorta. blood flows into the aorta
  3. the ventricle relaxes and the semilunar valve closes as pressure in the aorta is greater than that in the ventricle
  4. pressure inside the ventricle falls below that in the atrium. the atrio-ventricular valve then opens, allowing blood to flow into the ventricle
• The heartbeat is myogenic- the muscular contraction of the heart originate from within the heart muscle itself
• Control of the heartbeat (steps 1-4)

  • sinoatrial node (SAN) is modified muscle cells in the wall of the right atrium
  • produces regular bursts of electrical impulses called waves of depolarisation across the atria causing them to contract together
  • impulses don't pass directly to the ventricles but reach the atrioventricular node (AVN) between atria and the ventricles
  • there's a delay before the AV node reacts to ensure that the ventricles contract after the atria allowing time for them to fill with blood
• Control of the heartbeat (steps 5-6)

  • impulses from the AV node travel rapidly through Purkyne fibres organised into the Bundle of His to all parts of the ventricle
  • the ventricles are stimulated to contract together starting at the bottom to push the blood up and out into the arteries
• Cardiac output

  • the volume of blood pumped out of the ventricle per minute
  • stroke volume is the amount of blood expelled from the left ventricle of the heart per contraction
  • heart rate is the number of contractions per minute
  • CO = SV x HR
• Control of heart rate

  • controlled by the cardiac center in the medulla of the brain
  • divided into cardioacceleratory centre and a cardioinhibitory centre
  • control of the heart beat is by the autonomic nervous system
• Cardioacceleratory centre

  • sends nerve impulses via the sympathetic nervous system (e.g. noradrenaline)
  • 'fight or flight'
  • increases heart rate
• Cardioinhibitory centre

  • nerve impulses via the parasympathetic nervous system (e.g. acetylcholine)
  • 'rest and digest'
  • decreases heart rate
• Exercise and heart rate

  • heart rate increases to supply more blood carrying oxygen and glucose to respiring muscles
  • increases production of carbon dioxide
  • carbon dioxide dissolves in the blood to produce carbonic acid lowering the pH
  • chemoreceptors in the medulla and in the aortic and cartoid bodies are stimulated and transmit impulses to the respiratory centre to increase the rate of ventilation and cardioacceleratory centre
• Venous return during exercise

  • muscles contract strongly, pressing on the veins and increasing the rate at which blood returns to the heart in veins
  • increased venous return causes the cardiac muscle to contract more strongly pumping out an increased volume of blood
• Blood pressure during exercise

  • increase in blood pressure is detected by pressure receptors in the wall of the aorta and the carotid artery
  • if blood pressure increases too much the pressure receptors send more impulses to the medulla stimulating the cardioinhibitory centre and inhibiting the cardioacceleratory centre
  • more impulses are sent from the cardiac centre along parasympathetic neurones to the SAN causing a decrease in heart rate thereby reducing blood pressure
• The circulatory system

  • consists of the heart and blood vessels
  • mammals have a double circulatory system- blood pumped from the heart to the lungs and returns to the heart before being pumped to the rest of the body
  • double circulatory system ensures blood is pumped at high pressure to the body after the pressure has been reduced after passing through the lungs
• The circulatory system

  • consists of the heart and blood vessels
  • mammals have a double circulatory system- blood pumped from the heart to the lungs and returns to the heart before being pumped to the rest of the body
  • double circulatory system ensures blood is pumped at high pressure to the body after the pressure has been reduced after passing through the lungs
• Blood vessels

  • blood flows away from the heart in arteries which branch into smaller arteries and then arterioles
  • arterioles branch into capillary beds where exchange substances with body tissues takes place
  • capillaries merge into venules and then into veins which carry blood towards the heart
  • arteries carry blood rapidly under high pressure. as blood flows into arterioles and then capillary beds resistance to the flow of the blood increases which causes blood and rate of flow to fall
  • blood flows into the venules and veins and back to the heart under low pressure
• Arteries

  • carry blood away from the heart at high blood pressure
  • the aorta has a large amount of elastic tissue and when the left ventricle contracts the aorta distends, retaining some of the blood forced out of the ventricle
  • most of the blood is forced along the aorta to the body tissues
  • helps to provide a smooth flow of blood and maintain a high pressure when the ventricle relaxes
• Physiology of arteries

  • thicker wall and a smaller lumen than veins
  • contain more elastic fibres and smooth muscle tissue
  • don't possess valves except the aorta and pulmonary artery
  • transport blood at a higher pressure than veins
  • carry oxygenated blood expect for the pulmonary artery
  • aorta and larger arteries have a higher ratio of elastic fibres to smooth muscle fibres for high pressure
• Physiology of arterioles

  • don't have to stand high pressure and are found in main arteries
  • possess a higher proportion of smooth muscle than elastic fibres
  • regulate flow of blood to different tissues or organs by contraction or relaxation of smooth muscles in their walls
  • smooth muscle is under control of the automatic nervous system
  • vasoconstriction reduces blood flow to the capillaries
  • vasodilation increases blood flow
• Veins

  • carry blood under low pressure to the heart
  • walls are thinner and contain less elastic fibres and smooth muscle
  • veins expect pulmonary vein carry deoxygenated blood
  • lumen is larger than arteries to reduce resistance to blood flow
  • pressure from heart contracting isn't enough to return blood along the veins from lower body to the heart
  • contracting muscles in the legs and body press on the vein and squeeze blood along
  • veins have semi-lunar valves at intervals preventing back-flow ensuring blood travels in one direction to the heart
• Capillaries

  • walls are one endothelial cell thick giving short pathway for exchange of substances
  • large number of capillaries giving a large SA for exchange with tissues
  • total cross-sectional area is very high producing large frictional resistance, reducing blood flow, allowing more time for exchange
  • no cell is very far from a capillary
  • small diameter
• Transport systems- exchange

  • blood consists of liquid blood plasma with suspended blood cells and dissolved substances
  • blood plasma contains glucose, amino acids, fatty acids, hormones, urea, mineral ions and proteins
  • substances enter and leave blood capillaries at exchange surfaces
• Exchange of materials: oxygen

  • where it enters the blood: alveoli of lungs
  • where it leaves the blood: tissue of body
• Exchange of materials: carbon dioxide

  • where it enters the blood: body tissues
  • where it leaves the blood: alveoli of lungs
• Exchange of materials: glucose, amino acids, fatty acids, mineral ions

  • where it enters the blood: epithelium of villi of small intestine
  • where it leaves the blood: tissues of body
• Exchange of materials: hormones

  • where it enters the blood: endocrine glands
  • where it leaves the blood: target organs/tissues
• Exchange of materials: urea

  • where it enters the blood: liver
  • where it leaves the blood: kidney