The Circulatory System

1. The Circulatory System A&P1 Tutor: Eleshia Howell (c) Eleshia Howell, 2012

2. The circulatory, or cardiovascular system, consists of the heart, the blood vessels and the blood. It also correlates closely with the lymphatic system, but this will be studied separately. • The heart is the muscular organ that pumps the blood through the circulatory channels • Arteries • Veins • Capillaries • There are two anatomically distinct systems of blood vessels... • Pulmonary circulation • Systemic circulation (c) Eleshia Howell, 2012

3. Pulmonary & Systemic Circulation (c) Eleshia Howell, 2012

4. The right side of the heart pumps blood to the lungs (the pulmonary circulation) where gas exchange occurs. • The left side of the heart pumps blood into the systemic circulation, which supplies the rest of the body. Tissue wastes are passed into the blood for excretion; nutrients and oxygen are extracted for use by body cells. • The circulatory system ensures a continuous flow of blood to all the cells throughout the body. (c) Eleshia Howell, 2012

5. Blood Vessels • As we have previously noted, there are 3 types which vary in size, structure and function: • Arteries – carry blood away from the heart • Veins – carry blood to the heart • Capillaries – provide a network between the arteries and veins and the localised tissue they infiltrate. (c) Eleshia Howell, 2012

6. Fig: 5.2 (c) Eleshia Howell, 2012

7. There are 3 layers to the structure of the vessel walls: • Tunica externa (or adventitia) – the outer layer of fibrous tissue • Tunica media – middle layer of smooth muscle & elastic tissue • Tunica intima – inner layer of squamous epithelium (endothelium) (c) Eleshia Howell, 2012

8. Comparison of a typical artery and a typical vein. (c) Eleshia Howell, 2012

9. Arteries • The amount of muscular and elastic tissue varies in the arteries depending upon their size and function. • Gradually change in size & composition as they get further away from the heart. • Largest and more elastic are proximal to heart, gradually becoming smaller and more muscular, distally become arterioles. • The elasticity of the larger vessels allow the walls to stretch, absorbing the waves of pressure created with each heart beat. (c) Eleshia Howell, 2012

10. (c) Eleshia Howell, 2012

11. Elastic arteries – are large vessels, eg aorta, pulmonary trunk. Tunica media has many elastic fibres and few muscle cells. • Muscular arteries – medium sized vessels. Tunica media has many muscle cells. • Resistance vessels – arterioles, consist almost entirely of smooth muscle which enables their diameter to be precisely controlled, regulating the pressure within them. • The change in diameter of the blood vessels is controlled by the ANS. (c) Eleshia Howell, 2012

12. Capillaries • The smallest arterioles break up into a number of minute vessels called capillaries, which form a vast network of tiny vessels that link the smallest arterioles to the smallest venules. • Capillary walls consist of a single layer of endothelial cells sitting on a very thin basement membrane, through which water and other small molecules can pass. They are approx 1 RBC in diameter. • Blood cells and large molecules such as plasma proteins do not normally pass through capillary walls. (c) Eleshia Howell, 2012

13. The capillary bed is the site of exchange of substances between the blood and the tissue fluid, which bathes the body cells. • Entry to capillary beds is guarded by rings of smooth muscle that direct blood flow. Hypoxia (low levels of oxygen in the tissues), or high levels of tissue wastes, dilate the sphincters and increase blood flow through the affected beds. • A number of different types of capillaries exist, eg fenestrated, sinusoid, continuous, allowing them to perform specific functions for the tissue they are associated with, eg allowing larger molecules to permeate. (c) Eleshia Howell, 2012

14. Capillary network (c) Eleshia Howell, 2012

15. (c) Eleshia Howell, 2012

16. Veins • Are blood vessels that return blood at low pressure to the heart. • Walls of the vessels are thinner than arteries but are larger in diameter. They have the same 3 layers of tissue. • Less muscle and elastic in tunica media creates less pressure in the vessels. • Some veins have valves to prevent backflow of blood – these are mainly present in the limbs, abundant in lower limbs to enable blood to flow to the heart against gravity. (c) Eleshia Howell, 2012

17. The valves are formed by folds of the tunica intima and strengthened by connective tissue. They form a semilunar shape. p75. (c) Eleshia Howell, 2012

18. Veins are capacitance vessels because they are distensible, and therefore have the capacity to hold a large proportion of the body's blood. At any one time, about two-thirds of the body's blood is in the venous system. This allows the vascular system to absorb (to an extent) sudden changes in blood volume, such as in haemorrhage; the veins can recoil, helping to prevent a sudden fall in blood pressure. • The smallest veins are called venules. (c) Eleshia Howell, 2012

19. Control of blood vessel diameter • The smooth muscle in the tunica media of veins and arteries is supplied by sympathetic nerves of the ANS, which arise from the vasomotor centre of the medulla oblongata. • Change in the diameter of the lumen (inside space of a tubular structure) of blood vessels controls the volume of blood flowing through. • Arterioles contain more smooth muscle than other vessels, so are the most regulated by this ANS mechanism. (c) Eleshia Howell, 2012

20. Sympathetic activity constricts the smooth muscle of the blood vessels (vasoconstriction), increasing the pressure inside. A degree of resting sympathetic activity maintains a constant baseline tone in the vessel wall and prevents pressure falling too low. • Decreased nerve stimulation relaxes the smooth muscle, enlarging the lumen (vasodilation). • Arterioles for skeletal muscle and the brain experience a lesser response to stimulation to prevent these important tissues from being deprived of oxygen during fight or flight response. (c) Eleshia Howell, 2012

21. Resistance to flow of fluids along a vessel is determined by 3 factors: • The diameter of the vessel • The length • The viscosity of the fluid • The diameter of the vessel, providing peripheral resistance, is a major factor in blood pressure regulation. (c) Eleshia Howell, 2012

22. Local regulation of blood flow • Requirements for oxygen and nutrients in the tissues vary depending on their activities, therefore it is important that regulation occurs locally so that blood flow = needs. • Auto-regulation = ability of an organ to control its own blood flow. • Blood flow is increased or decreased to maintain homeostasis and function via these local control mechanisms: (c) Eleshia Howell, 2012

23. Release of metabolic waste products, eg carbon dioxide, lactic acid, by active tissue increases blood flow to the area • Tissue temperature – increased metabolic activity increases tissue temp = vasodilation • Hypoxia – stimulates vasodilation • Release of vasodilator chemicals by inflamed or metabolically active tissue, eg histamine during inflammatory response. • Vasoconstrictor substances, eg adrenaline, angiotensin, released from adrenal medulla during sympathetic response. (c) Eleshia Howell, 2012

24. Capillary exchange Exchange of Gases ~ • The exchange of gases between capillary blood and the local cells is called internal respiration. • Oxygen is carried from the lungs to the tissues in combination with haemoglobin (oxyhaemoglobin). • Exchange occurs between the blood at the arterial end of the capillaries and the interstitial fluid, then between the fluid and the cells. (c) Eleshia Howell, 2012

25. Oxygen diffuses down from the oxygen-rich arterial blood into the tissues where oxygen levels are lower (constant tissue consumption). • Oxyhaemoglobin, being an unstable compound, breaks up easily to free the oxygen molecules. • Carbon dioxide, a waste product of cell metabolism, diffuses into the venous blood to be transported to the lungs for excretion (c) Eleshia Howell, 2012

26. Exchange of other Substances ~ • Nutrients required by the cells are transported in the blood plasma. They pass through the semi-permeable capillary walls into the tissue fluid, then through the cell membrane into the cell. • This mechanism of transfer depends mainly on diffusion and osmosis. • Diffusion allows small molecules to pass through into the tissue fluid, retaining large molecules in the blood.Diffusible substances include: O2, CO2, glucose, amino acids, fatty acids, vitamins, mineral salts and water. (c) Eleshia Howell, 2012

27. Osmosis draws substances from a low concentration to a high concentration. The main substances responsible for the osmotic pressure between blood and tissue fluid are plasma proteins, especially albumin. • The 2 main forces determining overall fluid movement across the capillary wall are hydrostatic pressure (blood pressure, pushes fluid out of the bloodstream), and osmotic pressure (pulls fluid back in). This is a dynamic process, producing constant change. (c) Eleshia Howell, 2012

28. Not all the water and cell waste products return to the blood capillaries. Of the 24litres of fluid that moves across the capillaries each day, only about 21litres return to the venous bloodstream • The excess is drained away from the tissue spaces by the lymphatic capillaries. (c) Eleshia Howell, 2012

29. The Heart • A hollow muscular organ, approx the size of person’s fist, weighing 225-300g (heaviest in men). • The heart is situated in the thoracic cavity, in the mediastinum, laying obliquely to the left, roughly between 2nd – 5th ribs. It has a base and an apex. • The heart wall is composed of three layers of tissue – pericardium, myocardium & endocardium. (c) Eleshia Howell, 2012

30. p79 (c) Eleshia Howell, 2012

31. p79 (c) Eleshia Howell, 2012

32. Pericardium • The outermost layer, made up of 2 sacs – • the outer sac consists of fibrous tissue, is continuous with the tunica externa of the great blood vessels above (eg aorta) and adheres to the diaphragm below. Its non-elastic nature prevents over-distension of the heart. • the inner is a continuous double layer of serous membrane – the outermost is known as the parietal pericardium and lines the fibrous sac; the inner layer or visceral pericardium (aka epicardium) is adherent to the heart muscle. (c) Eleshia Howell, 2012

33. The serous membrane consists of epithelial cells which secrete fluid into the space between the visceral and parietal layers, allowing smooth movement between them when the heart beats. (the space between the two layers is only a potential space) (c) Eleshia Howell, 2012

34. Myocardium • Composed of specialised, involuntary muscle tissue only found in the heart – cardiac muscle. • Cardiac muscle is striated, like skeletal muscle, and each fibre has a nucleus and one or more branches. The cells and their branches are in very close contact with the ends / branches of adjacent cells, giving the appearance of a ‘sheet’ of muscle, rather than a large number of individual cells. • When an impulse is initiated, it spreads from cell to cell via the branches and cells and causes a contraction of the entire muscle. (c) Eleshia Howell, 2012

35. Running through the myocardium is also the network of specialised conducting fibres responsible for transmitting the heart’s electrical signals. • The myocardium is thickest at the apex, and thickest in the left ventricle, which has the greatest workload (pumping the blood through to aorta to serve the body). (c) Eleshia Howell, 2012

36. Endocardium • The innermost layer, lines the chambers of the heart and the valves. • A thin, smooth epithelial membrane, continuous with the lining of the blood vessels; that permits the smooth flow of blood inside the heart. (c) Eleshia Howell, 2012

37. Surface Anatomy of the heart (c) Eleshia Howell, 2012

38. Internal anatomy of the heart (c) Eleshia Howell, 2012

39. The heart is divided into a right and left side by a partition of myocardium called the Septum. • There are 4 chambers of the heart, 2 on each side. The atrium is in the upper section, the ventricle in the lower section. • The atrium & ventricle is separated by a valve (atrioventricular valve – AV) which are made of double folds of endocardium tissue, strengthened with fibrous tissue. • The right AV is known as the tricuspid valve ~ it has 3 flaps of tissue (cusps) and the left AV is called the mitral valve ~ it only has 2 cusps. (c) Eleshia Howell, 2012

40. Blood flow is one way ~ blood enters the heart via atria and passes into ventricle before moving into the circulation. • Right atrium: • collects blood from systemic circuit • Right ventricle: • pumps blood to pulmonary circuit • Left atrium: • collects blood from pulmonary circuit • Left ventricle: • pumps blood to systemic circuit • AV’s open & close passively according to changes in pressure in the chambers. (c) Eleshia Howell, 2012

41. Blood flow through the heart • The 2 largest veins of the body – superior & inferior vena cava – empty into right atrium. • From there it passes into right ventricle before being pumped into the pulmonary artery (the only artery in the body to carry deoxygenated blood). The pulmonary valve, formed by 3 semilunar cusps, prevents backflow of blood into the ventricle. The pulmonary artery divides into left and right to serve each lung, sending the blood into pulmonary circuit. (c) Eleshia Howell, 2012

42. When blood is in the lungs, exchange of gases take place – CO2 is excreted and O2 is absorbed. • 2 pulmonary veins from each lung carry oxygenated blood back to the left atrium of the heart, passing through AV valve into left ventricle. • From there it is pumped into the Aorta, the major artery which begins the systemic circuit. The opening of the aorta also has a valve (aortic valve) formed by 3 semilunar cusps to prevent backflow. (c) Eleshia Howell, 2012

43. Special Notes: • Both atria contract at the same time • Followed by simultaneous contraction of the ventricles • Blood flow through atria is assisted by gravity, whereas ventricles have to forcefully pump the blood ‘upwards’ to their corresponding vessels. • This is why the muscular walls of the ventricles are thicker & stronger than those of the atria. (c) Eleshia Howell, 2012

44. Coronary circulation Arterial Supply: • The right and left coronary arteries branch from the aorta just distal to the aortic valve to supply the heart itself. The arteries traverse the heart, forming a vast network of capillaries. • They receive about 5% of the oxygenated blood pumped from the heart ~ for a relatively small body area, the heart must really be important!! (c) Eleshia Howell, 2012

45. Venous Supply: • Most of the venous blood is collected into a number of cardiac veins that join to form the coronary sinus, which opens into the right atrium. • The remainder drains directly into the heart via small venous channels. (c) Eleshia Howell, 2012

46. p82 (c) Eleshia Howell, 2012

47. Conduction System • The heart generates its own electrical impulses and beats independently of nervous or hormonal control ~ autorhythmicity. • it is, however, supplied with both sympathetic and parasympathetic nerve fibres, which increase and decrease the intrinsic heart rate. • Circulating hormones, such as adrenaline and thyroxine, can also alter the rhythm of the heart. (c) Eleshia Howell, 2012

48. Small groups of specialised neuromuscular cells in the myocardium infiltrate and conduct impulses, causing co-ordinated and synchronised contraction of the heart • Sinoatrial and atrioventricular nodes • Atrioventricular bundle / Purkinje fibres. Sinoatrial Node (SA Node): • is the impulse-generating (pacemaker) tissue located in the right atrium ~the generator of normal sinus rhythm. (60-80 beats/m) • The cells are modified cardiac myocytes. Though they possess some contractile filaments, they do not contract. (c) Eleshia Howell, 2012

49. Atrioventricular Node (AV Node): • Situated in the wall of the atrial septum, near the AV valve. • It helps to transmit the electrical signals from the atria into the ventricles, creating a slight delay of 0.1 second, allowing the atrium to finish contracting before the ventricular contraction begins. • The AV node also acts as a secondary pacemaker if there is a problem with the SA Node. Its firing rate is slower though (40-60 bpm). (c) Eleshia Howell, 2012

50. Atrioventricular Bundle (Bundle of His) • A mass of specialised fibres originating from the AV node which crosses the fibrous ring separating the atria and ventricles before dividing into left and right bundles. • Within the ventricular myocardium the branches break up into fine fibres, known as Perkinje Fibres. • The AV bundle and Perkinje’s transmit electrical impulses from the AV node to the apex of the heart where ventricular contraction begins, before sweeping upwards and outwards to pump the blood into vessels (c) Eleshia Howell, 2012