The Circulatory System

1. The Circulatory System Jenny McInerney

2. Circulation video

3. The Need for a Circulatory System • Organisms need to exchange materials with the external environment • This must be done on a cellular level • In most multicelllular organisms, direct exchange with the environment is not possible • Diffusion is inefficient in large organisms

4. The Need for a Circulatory System cont’d • The circulatory system solves this problem by providing a link between the tissues of a body and the organs that exchange gases, absorb nutrients, & dispose of wastes • The circulatory system solves the diffusion problem by taking substances where they need to go so that the distance they have to diffuse into or out of a cell is never very great

5. Circularcanal Radial canal Mouth 5 cm Figure 42.2 Invertebrate Circulation: Gastrovascular Cavities • Hydras & other cnidarians don’t need a true circulatory system due to the simplicity of their body plan • Instead, they have a gastrovascular cavity: a body cavity filled with fluid that is continuous with the external environment that both digests and distributes nutrients

6. Invertebrate Circulation: Definition of a Circulatory System • More complex organisms require at least some kind of circulatory system • A ciculatory system has 3 main parts: a circulatory fluid (blood), a set of tubes (blood vessels), and a muscular pump (a heart) • The heart circulates blood by increasing the hydrostatic pressure of the blood on the blood vessels, causing it to move down a pressure gradient through a circuit and back to the heart

7. Heart Hemolymph in sinusessurrounding ograns Ostia Anterior vessel Lateral vessels Tubular heart Invertebrate Circulation: Open Circulatory Systems • Found in insects, arthropods, and most molluscs • One or more hearts pump a fluid called hemolymph into a network of sinuses - body cavities where the hemolymph directly bathes the organs - and then draw the blood back through pores called ostia • No distinction between blood and interstitial fluid • Evolutionary advantages: lower hydrostatic pressure requires less energy to run; lack of extensive blood vessels requires less energy to build and maintain; can serve as hydrostatic skeleton in molluscs

8. Heart Interstitialfluid Small branch vessels in each organ Dorsal vessel(main heart) Ventral vessels Auxiliary hearts Invertebrate Circulation/Vertebrate Circulation: CLosed Circulatory Systems • One or more hearts pump blood through a network of blood vessels, and nutrients diffuse through capillary beds into the interstitial fluid • Blood is distinct from interstitial fluid and is confined to blood vessels • Found in earthworms, squids and octopuses, and all vertebrates • Evolutionary advantages: more effective at meeting the metabolic requirements of larger and more complex animals

9. An Overview of Vertebrate Circulation • Heart comprised of one or two atria (singular, atrium) connected to one or two ventricles • Three kinds of blood vessels: arteries (carry blood away from heart), veins (carry blood to heart), and capillaries (sites of diffusion between blood and interstitial fluid) • Blood vessels classified based on blood direction, not content

10. An Overview of Vertebrate Circulation cont’d • Ateries branch out to become aterioles • Venules converge to become veins • The circulatory systems of the different taxa are variations on this theme

11. Fish • 2-chambered heart • Single curcuit of blood flow

12. Amphibians • 3-chambered heart • Double circulation (blood gets pumped again after getting oxygen form the lungs or skin) • Mixing of oxygen-rich and oxygen-poor blood in the single central ventricle • 2 circuits of blood flow • Pulmocutaneous (to the lungs/skin) • Systemic (to the rest of the body)

13. Reptiles (excluding birds) • 3-chambered heart • Double circulation • Presence of s septum in the middle of the ventricle decreases mixing of oxygen-rich and oxygen-poor blood • 2 circuits of blood flow • Pulmonary (to lungs) • Systemic

14. Mammals • 4-chambered heart • Double circulation • No mixing of blood - two separate ventricles • 2 circuits of blood flow • Pulmonary • Systemic

15. Capillaries of head and forelimbs Anterior vena cava Aorta Pulmonary artery Pulmonary artery 9 6 3 3 7 8 Capillaries of right lung Capillaries of left lung 2 4 11 Pulmonary vein Pulmonary vein Left atrium 5 1 Right atrium 10 Left ventricle Right ventricle Aorta Posterior vena cava Capillaries of abdominal organs and hind limbs Pathway of Mammalian Circulation • Right ventricle • Pulmonary artery • Capillary beds in lungs • Left atrium • Left ventricle • Aorta • Arterioles and capillaries of head and forelimbs • (at same time as) arterioles and capillaries of trunk and hind limbs • Anterior (superior) vena cava • (at same time as) posterior (inferior) vena cava • Right atrium • Right ventricle • And so on . . .

16. A Closer Look At the Mammalian Heart • The heart contracts and relaxes in a rhythmic cycle: it pumps blood on the contractions and fills with blood on the relaxations • The contraction phase is called the systole • The relaxation phase is called the diastole • One complete sequence of pumping and filling = the cardiac cycle

17. A Closer Look at the Mammalian Heart cont’d • Cardiac output: volume of blood the left ventricle pumps into the systemic circuit in a minute • Based on heart rate (in beats per minute) • And on stroke volume - the amount of blood the left ventricle pumps in each contraction

18. Atrioventricularvalve Semilunarvalve Semilunarvalve Atrioventricularvalve A Closer Look at the Mammalian Heart cont’d • Sets of valves prevent the blood from leaking into the wrong place at the wrong time • Atrioventricular (AV) valves located inbetween each atrium and its connected ventricle are forced shut with each contraction so that the blood can’t leak from the ventricle back into the atrium • Semilunar valves located at the exits of each ventricle (the pulmonary artery and the aorta) are forced shut with each relaxation so that the blood can’t leak back from the blood vessel into the ventricle

19. Maintaing the Heartbeat • Sinoatrial (SA) node/pacemaker: a region of specialized tissue within the heart that controls the rate and timing of all cardial muscle cell contractions • Cardial muscle cells are self-excitable - they contract without direction from the central nervous system, and each have their own intrinsic pulse/rhythmn - the SA node just makes sure they all beat together

20. Maintaing the Heartbeat cont’d • Because the pacemaker is located within the heart, vertebrate heats can be called myogenic • Invertebrate hearts are called neurogenic because their pacemakers are located in motor nerves outside the heart

21. Signals spread Throughoutventricles. Signals pass to heart apex. Pacemaker generates wave of signals to contract. Signals are delayed at AV node. Bundlebranches AV node SA node(pacemaker) Purkinjefibers Heartapex ECG 1 3 2 4 Maintaining the Heartbeat cont’d • Signals pass from the SA node to the atrioventricular (AV) node, which delays them for .1 of a second so that the atria can completely drain • Then they are passed to bundle bunches and Purkinje fibers at the apex of the heart that force the ventricles to contract

22. Vein Artery Valve Endothelium Basementmembrane Smoothmuscle 100 µm Endothelium Connectivetissue Endothelium Smoothmuscle Capillary Connectivetissue Artery Vein Venule Arteriole Blood Vessel Structure and Function • Ateries and veins are composed of 3 layers • Outer layer (connecting, elastic muscle), middle layer (smooth, elastic muscle), inner layer (endothelium, smooth, flat cells designed to reduce resistance to blood flow) • Arteries have thick, elastic walls because they handle the most blood pressure; veins have thinner walls to conduct blood back to the heart at low velocity and pressure, and capillaries have only two thin walls because they are the sites of diffusion between the blood and the interstitial fluid

23. Blood Velocity • Blood velocity is governed by the law of continuity, which is used to describe the movement of fluid through pipes • As pipe diameter decreases, fluid velocity increases • However, velocity in the capillaries is over 1,000 times greater than that in the capillaries - why? • Diameter is measured in total cross-sectional area - the total cross sectional area of the capillaries is greater than that of any other single vessel in the circulatory system

24. Blood Pressure • Fluids exert pressure on the surfaces they contact; this pressure is what drives fluids through pipes • Fluids flow from areas of high pressure to low pressure

25. Blood Pressure cont’d • Blood pressure is higher in arteries than in veins, and is highest during ventrical systole (systolic pressure) • Blood pressure is lowest during ventrical diastole (diastolic pressure)

26. Blood Pressure cont’d • Peripheral resistance: the sudden decrease of velocity that occurs when the arteries hit the arterioles • Pressure builds up as the heart continues to exert pressure of the blood to move forward, but it cannot proceed through the smaller arterioles quickly enough to relieve the pressure on the arteries

27. Blood Pressure cont’d • Blood pressure is determined by cardiac output and peripheral resistance (measured in systolic pressure over diastolic pressure)

28. Blood Pressure cont’d • Cardiac output increases to maintain blood pressure when arterioles dilate to allow for increased blood flow during heavy exercise • Gravity also affects blood pressure - it takes extra force to pump blood above the level of the heart to the head and brain

29. Blood Pressure cont’d • By the time blood gets to the veins, most of the blood pressure has dissipated in the arterioles and capillaries • Blood in the veins is moved partly by contractions of the smooth muscle in the vein walls and mostly by skeletal muscle contractions during physical activity which serve to squeeze the blood through the veins

30. Thoroughfare channel Precapillary sphincters Arteriole Venule Capillaries Venule Arteriole 20 m Capillary Function • Blood can be diverted to wherever in the body it is needed most • Two mechanisms regulate the distribution of blood to the capillaries • Contraction or relaxation of the smooth muscles of the arterioles to constrict or increase blood flow to the capillaries • Precapillary sphincters: rings of smooth muscle located at the entrance to the capillaries that can cut off or redirect blood flow (a) Sphincters relaxed (b) Sphincters contracted (c) Capillaries and larger vessels (SEM)

31. Capillary Function cont’d • Transfer of substances from blood to interstitial fluid can occur through • Simple diffusion through the endomethial cells or through the clefts between adjoining cells • Or through the movement of vesicle made by endocytosis on one side of the cells and exocytosis on the other side

32. Tissue cell INTERSTITIAL FLUID Net fluid movement out Net fluid movement in Capillary Red blood cell Capillary 15 m At the venule end of a capillary, blood pressure is less than osmotic pressure, and fluid flows from the interstitial fluid into the capillary. At the arterial end of a capillary, blood pressure is greater than osmotic pressure, and fluid flows out of the capillary into the interstitial fluid. Direction of blood flow Blood pressure Osmotic pressure Inward flow Pressure Outward flow Venule end Arterial end of capillary Figure 42.14 Capillary Function cont’d • Blood pressure forces fluid through the capillary clefts and out of the capillaries at the arterial end of the capillary bed, resulting in a net loss of fluid on that end of the capillary bed • Large solutes and plasma proteins remain in the capillaries to create a relatively constant osmotic pressure, while the blood pressure decreases substantially at the venule end of the capillary bed • This causes ~ 85% of the lost fluid to flow back into the capillaries from the interstitial fluid at the end of the capillary bed

33. Fluid Return by the Lymphatic System • Fluid enters the lymphatic system through tiny lymphatic capillaries interspersed throughout the interstitial fluid alongside the capillary beds • Once it enters the lymphatic system, it becomes known as lymph, though there is little difference in composition between it and the interstitial fluid • Lymph is conducted through the lymphatic system the same way blood is conducted through the veins - muscle contractions • The lymphatic system drains into the circulatory system near the junction of the venae cavae with the right atrium, rthius returning the remaining 15% of fluid to the circulatory system

34. Blood Composition and Function • Blood consists of several kinds of cells - erythrocytes, leukocytes, and platelets - suspended in a plasma matrix • The cells can be separated from the plasma by using a centrifuge to spin out a sample of blood • When the sample is done spinning, the cells will settle to the bottom in a dense red pellet while the plasma will float above it - transparent and straw-coloured

35. Plasma • Composed of 90% water and various other solutes • Dissolved ions - electrolytes - collectively act as buffers and help maintain the osmotic balance of blood; moreover, the functioning of muscles relies on stable corresponding concentrations of these ions in the interstitial fluid

36. Plasma cont’d • Plasma also contains plasma proteins: collectively act as buffers against pH changes (the average pH of human blood is 7.4), they help to maintain osmotic balance between the blood and the interstitial fluid, and they contribute to the blod’s visccosity (thickness) • Specific classes of plasma proteins also act as escorts for lipids • Other classes of plasma proteins include the imunoglobulins, or antibodies, and fibrinogens, clotting factors which form clots when blood vessels have been injured • Plasma from which these clotting factors have been removed is called serum

37. Plasma • Plasma also includes other nutrients, metabolic and respiratoy wastes, and hormones • Plasma is about the same composition as interstitial fluid, but with a higher concentration of proteins

38. Erythrocytes • Also known as red blood cells, erythrocytes are by far the most numerous of the blood cells • Their function is to provide for the storing and rapid diffusion of oxygen • Reflected in their structure - a small, round, biconcave (thinner in the middle than at the edges) disk - which maximizes surface area

39. Erythrocytes cont’d • Lack nuclei - the space is instead taken up by hemoglobin, an iron-containing protein that transports oxygen • Generate ATP through anaerobic respiration so as to preserve the stores of oxygen they carry

40. Leukocytes • Also known as white blood cells, there are five types of leukocytes: • Monocytes, neutrophils, basophils, esinophils, and lymphocytes • Their collective function is to fight infection • Spend the most time in the interstitial fluid and lympatic system, where they “wage most of their battles” against pathogens, but their numbers in the blood increase temporarily when fighting infection

41. Platelets • Fragments of cells that aid in the clotting process • No nuclei; originate as pinched-off fragments of large cells in bone marrow

42. Stem Cells • All of the cells in the bloodstream wear out and are recycled and replaced throughout the lifetime of the individual • Replacement cells all develop from a common population of pluripotent stem cells located in the red marrow of bones (particularly the ribs, vertebrae, breastbone, and pelvis)

43. Stem Cells cont’d • In a negative-feedback system, the body produces erythropoietin (EPO) which stimulates the production of erythrocytes when tissues are not getting enough oxygen • When the tissues signal that they are getting too much oxygen, EPO production slacks off • EPO is used by physicians to treat people with lower-than-normal hemoglobin levels, as in anemia

44. Stem Cells cont’d • Because of its ability to deliver extra oxygen to muscles and theoretically improve performance, some athletes inject themselves with EPO in a practice called “blood doping” • This has been deemed illegal by the International Olympic Committee and several other sports federations

45. Stem Cells cont’d • Recently, researchers have made advances in using stem cells to treat certain diseases, such as leukemia • The healthy stem cells would be removed from the patient, set to grow in a culture, then the cancerous stem cells would be removed and replaced with the healthy stem cells

46. Blood Clotting • Blood contains a self-sealing material called fibrinogen (in its inactive form) • Fibrinogen is converted into fibrin (its active form) when clotting factors are released from platelets • The fibrinogen aggregates into fibrous mesh networks that for the basic framework of the clot • The series of chemical reactions through which clots form are still not fully understood

47. Blood Clotting • Hemophilia: a disease caused by a genetic mutation in any step of the clotting process • Characterized by excessive bleeding even from small cuts and bruises

48. Connective tissue Smooth muscle Plaque Endothelium (a) Normal artery (b) Partly clogged artery Cardiovascular Disease • Atherosclerosis: accumulation of fatty deposits (plaque) on the interior walls of arteries causing them to harden and lose elasticity, resulting in high blood pressure and inccreased risk of heart attack or stroke • Heart attack: death of heart tissue due to lack of oxygen • Stroke: death of brain tissue due to lack of oxygen • Both can be caused by a thrombus, or clot, that clogs an artery and block blood flow downstream from it • A thrombus that originates in one area and then is carried somewhere else by the circulatory system is called an embolus

49. Cardiovascular Disease cont’d • Hypertension: high blood pressure; can cause or be caused by atherosclerosis, increases risk of heart attack and stroke • Arrhythmia: irregular heartbeat; often congenital and the result of a heart defect

50. Cardiovascular Disease cont’d • Hypercholesterolemia: excessive amounts of “bad” cholesterol - low-density lipoproteins (LDLs) - as opposed to “good” cholesterol - high-density lipoproteins; also increases risk of heart attack and stroke, associated with obesity/overweight-ness