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The blood system is a mass flow system which moves substances from one part of the body to another. It is linked with exchange surfaces

  • Heart pumps blood through arteries that branch into smaller arterioles, capillaries, then from a network of venules to veins and back to the heart
  • During exercise, supply of blood to muscle and skin increases; blood to digestive system decreases / middle layer muscles of smaller arterials and arterioles change their diameter to adjust blood supply

  • Smallest, most numerous blood vessels
    • Carry blood from arteries to veins
    • Blood flows from arterioles into the capillaries, then from them into venules
    • Size of lumen is roughly equal to diameter of erythrocytes
    • Thin wall is composed of endothelium (single layer of overlapping flat cells)
  • Function: exchange of materials between blood and tissue cells (e.g. O2, CO2, nutrients, wastes)
  • Capillary distribution varies with metabolic activity of body tissues
    • Skeletal muscle, liver, and kidney have extensive capillary networks
    • They are metabolically active and require an abundant supply of oxygen and nutrients
    • Connective tissue have less abundant supply of capillaries
    • Epidermis of skin and lens, and cornea of eye completely lack a capillary network
  • Flow of blood is controlled by structures called precapillary sphincters
    • Located between arterioles and capillaries
    • Muscle fibres allow them to contract
  • Hydrostatic pressure is created by the heart which pumps blood into arteries
  • At the arteriole end
    • Hydrostatic pressure > water potential
    • As plasma proteins lower water potential
    • H2O, small molecules, fluid are forced out through the permeable capillary wall
    • Plasma proteins are not forced out as they are too large
  • At the venule end
    • Water potential > hydrostatic pressure (due to lower volume)
    • Fluid tends to flow back into the blood with waste products produced by cells

Diffusion and gas exchange
  • All organisms exchange food, waste, gases, heat with their surroundings by diffusion
  • Rate of diffusion is given by Fick's law and depends on
    • Thickness of the membrane the molecules must diffuse across,
    • Surface area for gas exchange
    • Mass and solubility of molecule
    • Rate of diffusion is proportional to (surface area x conc. difference) / distance
  • Large organisms have a small surface area : volume ratio
    • Decreases the rate of diffusion
    • More difficult to exchange materials (e.g. waste) with surroundings
  • Organisms also need to exchange heat with their surroundings
    • Large animals lose less heat than small animals
    • Small mammals lose heat very readily → have a high metabolism to maintain body temp
    • Large mammals feed once every few days while small mammals must feed continuously
  • Plant cells respire all the time, chloroplasts causes photosynthesise / plants exchange gases
    • Main gas exchange surfaces in plants are spongy mesophyll cells in leaves
    • Leaves have large surface area / loosely-packed spongy cells further increase area

Structure of the lungs
  1. Air is
    1. filtered in nostrils with small hairs
    2. moistened and warmed by nasal cavities
    3. mucus traps foreign particles while cilia propels particles towards the throat
  2. Air passes into the pharynx → larynx → trachea
    1. The epiglottis is found within the larynx
    2. Breathing: epiglottis projects upwards → larynx is open
    3. Swallowing: larynx pulled up / epiglottis blocks larynx / prevents food from entering airway
  3. Trachea
    1. Contains C-shaped cartilage rings / prevents collapsing of tube
    2. Trachea divides into 2 tubes with smaller diameter called bronchi
    3. To prevent microorganisms, a bronchus is supported with ciliated epithelia
    4. Right bronchus is bigger than the left one → common site for inhaled foreign objects
  4. Bronchi further divide into bronchioles
    1. Their diameter can be controlled by smooth muscles
    2. Bronchial tubes form a system called alveoli (100µm in diameter)

Alveolar Gas Exchange
  • Greater partial pressure of O2 in alveolar air / more O2 dissolves in blood (Henry's Law)
  • Alveoli walls are composed of endothelium → gases diffuse through 2 thin cells
    • Alveoli is constantly moist
    • O2 can dissolve and diffuse through the cells into the blood
    • It is then taken up by haemoglobin
  • Alveoli contain phagocytes to kill bacteria that have not been trapped by mucus
  • O2 diffuses down its conc. gradient from air to blood; CO2 diffuses from blood to air
  • Ventilation: Flow of air in and out of alveoli

  • Tidal volume, VT, volume of air inhaled and exhaled in a normal single breath (≈0.5 L)
  • Functional residual capacity, FRC, volume remaining in lungs after exhalation of tidal volume (≈2.5 L)
  • Expiratory reserve volume, ER, volume of a maximal exhalation (≈1.5 L)
  • Residual volume, RV, volume remaining in lung after maximal exhalation (≈1L)
  • Inspiratory reserve volume, IR, additional volume that can be inhaled after inhalation of tidal volume
  • Vital capacity, VC, maximum volume of exhalation after lungs are maximally filled
    • best clinical indicator of breathing
  • Minute ventilation is the overall flow of air into lungs (analogous to cardiac output)
    • Minute Ventilation = Tidal Volume x Respiratory Rate
    • (0.5 litre/breath * 10 breaths/min = 5 litres per minute)
  • "Dead space" - not all O2 available in air is available to alveoli
    • Fresh air mixes with exhaled air during inspiration
    • Alveolar ventilation takes dead space into account
    • Alveolar ventilation = (Tidal Volume - Dead Space) x Respiratory Rate
    • (350 ml x 10 breaths per minute = 3500 ml or 3.5 litres)

Measurements of Ventilation
  • A spirometer is used to measure expired breath
  • Restrictive disorders, such as pulmonary fibrosis, reduce compliance and vital capacity
  • Four measures are called respiratory volumes
    • Tidal volume
    • Inspiratory reserve volume
    • Expiratory reserve volume
    • Residual volume
  • Others, called respiratory capacities, are calculated by adding 2 or more of the respiratory volumes

Coordination of breathing
Breathing centre in the medulla of the brain consists of
  • Inspiratory centre (dorsal respiratory group, DRG)
  • Expiratory centre (ventral respiratory group, VRG)

Inspiration (inhalation)
  • Inspiratory centre sends impulses to intercostal muscles and diaphragm via intercostal and phrenic nerves
  • Muscles contract, ribs raise, diaphragm moves down
  • Volume of alveoli increases / pressure decreases below atmospheric / air flows in
  • Inhalation requires muscular effort, thus burning calories and ATP

Normal expiration
  • Stretch receptors are stimulated, send impulses to expiratory centre via vagus nerve
  • Diaphragm (moves upwards) and external intercostal muscles relax
  • Volume of alveoli decreases / pressure increases above atmospheric / air flows out

Forced expiration (exercise)
  • Abdominal muscles contract, pushing diaphragm upwards
  • Internal intercostal muscles contract, pulling ribs downward
  • Gives larger and faster expiration

The purpose of respiration is to maintain pH, oxygen, and carbon dioxide levels in the blood within homeostatic limits. Brainstem respiratory centers monitor these conditions in the blood by various means

Control of breathing - Respiratory control
Breathing rate is monitored by
  • CO2 conc. - increases when more CO2 is produced as a waste product
  • O2 conc. - decreases as it is used in respiration to produce ATP
  • More sensitive to changes in CO2

Sensory nerves send information to the medulla via cranial nerves
  • Chemoreceptors, aortic and carotid bodies, are located in the aorta and carotid arteries
  • Primarily monitor pH and CO2 level (homeostasis control)
  • Aortic bodies send signals via vagus nerves about breathing reflexes, blood pressure and cardiac activity
  • Carotid bodies send signals about sensations of breathing and blood pressure

Motor nerves send commands to muscles or organs
  • Phrenic nerve innervates diaphragm
    • Originate from cervical plexus, high in neck
    • Stimulate breathing by carrying messages from medulla
  • Intercostal nerves enter intercostal muscles and run along the rib cage

References and Further Reading
AQA (2006) GCE Biology/Biology (Human) 2006 specification, [PDF]