1B.1 The Principles of Circulation PDF

1B.1 The principles of circulation Lesson objectives ❏ Know why many animals have a heart and circulation which act as a mass transport system to overcome the limitations of diffusion Key words Diffusion, concentration gradient, surface area to volume ratio (...

1B.1 The principles of circulation Lesson objectives ❏ Know why many animals have a heart and circulation which act as a mass transport system to overcome the limitations of diffusion Key words Diffusion, concentration gradient, surface area to volume ratio (sa : vol), vertebrates, mass transport system, single circulation system, double circulation system, systemic circulation, oxygenated blood, deoxygenated blood, pulmonary circulation Organisms need to transport (move) substances into and out of cells One way is by diffusion across the cell membrane: Movement is from area of high to low concentration (down a concentration gradient) This is sufficient for unicellular organisms but not fast enough for larger organisms. When organisms reach a certain size, diffusion alone is not enough. Transport in small organisms (uni-/multicellular) Nutrients, oxygen diffuse directly into the cell Waste substances diffuse out directly Reasons: - Short diffusion distances - Large surface area to volume ratio (large surface area in contact with outside environment as compared to volume of inside) - Low metabolic demands No need of specialised transport systems In multicellular organisms, exchange system (lungs/alveoli) and specialised circulatory system (heart/blood) is needed. Why is the transport of substances cross the outer surface not sufficient for larger organisms? sa:vol determines whether diffusion alone will Consider surface area to volume ratio allow the movement of substances : surface area in contact with outside environment (compared with the volume Not easy to calculate sa:vol of large organisms inside the organism) Scientists use models of cubes to show the real life situation The bigger the organism gets, the smaller the sa:vol becomes Less sa for absorption of nutrients and gases and secretion of waste products Greater vol - longer diffusion distance to the cells and tissues of the organism Mass transport system : structures in some animals (and all vertebrates) in which the flow of a fluid carries substances around the body Overcomes the limits of diffusion between internal and external environments Diffusion still happens locally Features of mass transport systems Mammalian circulatory system Exchange surfaces System of vessels that carry substances Making sure substances move in the right direction Making sure materials move fast enough Suitable transport medium A way of adapting rate of transport as needed Types of circulatory systems: open vs. closed Open circulatory system: blood circulating Closed circulatory system: blood contained within tubes in large open spaces (insects) Pressure can be increased to make blood flow No vessels to contain the blood, more quickly flows freely Flow can be directed more precisely to organs Types of circulatory systems: single vs. double circulation 1. Single circulation (e.g., fish) ○ Blood travels through the heart once during one cycle ○ Some organs get blood with more oxygen ○ Pressure drop further from the heart ○ Blood emerging from gills has low pressure, flows around the body much more slowly Compared to fish, why do birds and mammals need more oxygen? Heart pumps deoxygenated blood to gills → blood takes in O2, gives up CO2 → blood travels around the body → return to heart Types of circulatory systems: single vs. double circulation 2. Double circulation ○ Blood travels through the heart twice per cycle ○ Avoids mixing between oxygenated and deoxygenated blood ○ Fully oxygenated blood delivered to organs at high pressure Circuits ensure that two types of blood do not mix and each tissue receive as much oxygen possible Fully oxygenated blood delivered quickly to body tissues at higher pressure Systemic circulation Carries oxygenated blood from heart to body cells to be used as oxygen Pulmonary circulation Carries deoxygenated blood from the heart to the lungs to be oxygenated and then carries oxygenated blood back to heart