Circulation and Gas Exchange How have different organisms evolved to move oxygen in and carbon dioxide out? Diffusion alone is not adequate for transporting substances over long distances. Circulatory system ensures that no substance must diffuse very far to enter or leave a cell.

Circulation in Invertebrates Gastrovascular Cavities –All cells are bathed by a suitable medium and diffusion distances are short (2 cells thick) so no circulatory system is necessary –Ex: hydra, planaria (flatworm)

Circulation in Invertebrates Open and Closed Circulatory Systems –Both contain circulatory fluid (blood) a set of tubes through which blood flows (blood vessels) A muscular pump (the heart)

Open and Closed Circulatory Systems Blood pressure is the driving force for fluid movement in the circulatory system Open systems –No distinction between blood and interstitial fluid –General body fluid = hemolymph –One or more hearts pump hemolymph into interconnected sinuses surrounding organs and hemolymph exchanges material with cells –Examples: insects and arthropods

Open and Closed Circulatory Systems, cont’d Closed System –Blood is distinct from interstitial fluid –Blood is confined to vessels –One or more hearts pump blood into large vessels that branch into smaller ones –Examples Earthworms Squid, octupus Vertebrates

Figure 42.2 Open and closed circulatory systems

Vertebrate Cardiovascular Systems Atria receive blood returning to heart Ventricles pump blood out of the heart Arteries, capillaries, and veins transport blood Arteries carry blood away from the heart Veins carry blood to the heart Capillaries are where gas exchange takes place Arteries – arterioles-capillaries-venules- veins Do not describe arteries as carrying oxygenated blood!

Figure 42.3 Generalized circulatory schemes of vertebrates

Vertebrate Cardiovascular Systems, cont’d The higher the metabolic rate, the more complex the circulatory system Fish heart – 1 atrium, 1 ventricle Frogs/amphibians/most reptiles – 2 atria, 1 ventricle –Some mixing of oxygenated and deoxygenated blood in ventricle Birds and Mammals – 2 atria, 2 ventricles –Separate systemic and pulmonary circulation routes

Pathway of Blood Flow Through the Mammalian Heart And we’re off…………. And we’re off…………. superior and inferior vena cava → right atrium → atrioventricular valve → right ventricle → semilunar valve → pulmonary artery → capillary bed of lungs → pulmonary vein → left atrium → atrioventricular valve → left ventricle → semilunar valve → aorta → arteries → arterioles → systemic capillary beds → venules → veins → vena cava – as above superior and inferior vena cava → right atrium → atrioventricular valve → right ventricle → semilunar valve → pulmonary artery → capillary bed of lungs → pulmonary vein → left atrium → atrioventricular valve → left ventricle → semilunar valve → aorta → arteries → arterioles → systemic capillary beds → venules → veins → vena cava – as above Note: valves help prevent back flow of blood heart murmur = leaky valves heart murmur = leaky valves

Figure 42.4 The mammalian cardiovascular system: an overview

Figure 42.5 The mammalian heart: a closer look

Cardiac Cycle Equals one complete sequence of pumping blood including (systole/contraction) and refilling (diastole/relaxation)

Cardiac Output Equals volume of blood pumped per min Can pump entire volume of blood (~5.3L) in a minute or less Depends on two factors: –heart rate, number of beats per minute typically 70 beats per minute resting and –stroke volume, amount of blood pumped in a single contraction 75 mL is typical stroke volume in humans

Figure 42.7 The control of heart rhythm

Control of Heart Rhythm Some cardiac muscle cells are self-excitable (ie, do not need to be excited by the nervous system) The sinoatrial (SA) node, located in wall of right atrium, is the “pacemaker” / sets the tempo of the heartbeat Electrical impulse from SA node spreads rapidly throughout the atria and they contract in unison Next impulse reaches relay point, the atrioventricular (AV) node, located in wall between right atrium & right ventricle After a very brief delay to allow atria to empty, impulses spread throughout ventricles via Bundle branches & Purkinje fibers

Control of Heart Rhythm SA node can be influenced by –two sets of nerves that affect heart rate (one speeds up, one slows down) or by –hormones such as epinephrine –body temperature (↑ temp ↑heart rate)

Figure 42.8 The structure of blood vessels

More on Arteries, Veins, and Capillaries Similarities –Both arteries and veins are composed of 3 similar layers Outer layer of connective tissue with elastic fibers Middle layer of smooth muscle Inner layer of smooth endothelium

More on Arteries, Veins, and Capillaries Differences Capillaries have endothelium only –Only vessels with walls thin enough to permit the transfer of substances between blood and interstitial fluid Arteries have thicker walls than veins and operate with high velocity and pressure Veins are thinner than arteries and operate at low velocity & pressure

Blood Flow & Blood Pressure Blood Flow –Cross-sectional area determines blood flow –Blood flows faster through arteries than capillaries due to greater total cross-sectional area of capillaries compared to arteries Blood Pressure –Typically 120 systolic / 80 diastolic –Determined by cardiac output and peripheral resistance

Exchange of Substances Between Blood and Interstitial Fluid Nerve signals and hormones can divert blood to where it is most needed in the body Movement of substances across the capillary endothelium is regulated by net differences between blood pressure (outward pressure higher at arterial end and lower at venous end) and osmotic pressure (relatively constant pressure inward due to higher [ ] of solutes in the blood) 85% of fluid that leaves blood at the arterial end reenters at the venous end. Remaining 15% is returned via lymphatic system.

Figure The movement of fluid between capillaries and the interstitial fluid

Lymphatic System Returns lost fluid and blood proteins to the blood Composition of lymph is similar to that of interstitial fluid Lymphatic system drains into the circulatory system near the junction of the venae cavae with the right atrium Contraction of skeletal muscles helps squeeze lymph towards heart

Lymphatic System Valves present to prevent backflow Lymph nodes filter lymph and contain numerous white blood cells that help remove viruses and bacteria Lymphatic system also helps transport fats from digestive tract to circulatory system

Cardiovascular Disease Heart attack - death of cardiac muscle due to lack of oxygen usually due to thrombus or embolus Stroke - death of nervous tissue in brain due to lack of oxygen due to rupture or blockage of arteries in the head Atherosclerosis – growth of plaques on inner wall of arteries

Cardiovascular Disease Arteriosclerosis – “hardening of the arteries”/ atherosclerotic plaques hardened by calcium deposits Hypertension –high blood pressure/damages endothelial layer of arteries –Often associated with high cholesterol levels/high LDLs and low HDLs