The Cardiovascular System

Question 1

Describe the structure of heart

Answer 1

• the heart is a cone-shaped, muscular, hollow organ situated in the mediastinum
• epicardium - outermost layer of the heart
  - -> part of the pericardium which is a double supportive layer which protects the heart
  - -> outermost layer of pericardium is composed of tough connective tissue which attaches to the diaphragm
  - -> inner layer, serous pericardium, is a thinner double membrane, the outer portion is attached to the fibrous pericardium and the inner layer forming the epicardium
  - -> between the two layers is a potential space (pericardial cavity) containing serous fluid which allows the two membranes to slide over one another as the heart contracts and expands
• myocardium - middle layer, consists of muscle
  - -> composed of cardiac muscle
  - -> the cells of this muscle are branched and striated and in close contact with adjacent muscle cells via intercalated discs
• endocardium - innermost layer
  - -> lines the heart, valves between the chambers and the blood vessels leaving the heart
• 4 chambers - 2 atria and 2 ventricles
• atria an ventricles separated by valves

Question 2

Describe the different valves and their function

Answer 2

• there are 4 valves, they prevent backflow of blood into the chambers
  - -> the valves only open one way, if there’s a higher pressure behind a valve its forced open, if there’s a higher pressure in front of the valve its forced shut
• tricuspid valve - situated between the right atrium and ventricle
• bicuspid valve - situated between the left atrium and the ventricle
• two semilunar valves - situated in the pulmonary artery and aorta

Question 3

Describe the varying thickness of the myocardium in the heart

Answer 3

• left ventricle of the heart is thicker and has more muscular walls than the right ventricle because it needs to contract powerfully to pump blood all the way round the body, whereas the right ventricle only needs to move blood into the lungs
• the ventricles have thicker walls than the atria because they have to push blood out of the heart whereas the atria only need to push blood into the ventricles

Question 4

Describe how blood moves through the heart

Answer 4

• the right atrium receives blood from the vena cava
• blood is then passed through the right ventricle and out of the heart via the pulmonary arteries to the lungs
• here it is oxygenated and returns through the pulmonary veins to the left atrium of the heart
• finally the blood exits the heart through the left ventricle into the aorta

Question 5

Describe the cardiac cycle

Answer 5

• the ventricles are relaxed
• the atria fill with blood, which decreases their volume and increases the pressure
• the higher pressure in the atria cause the tricuspid and bicuspid valves to open, allowing the blood to flow into the ventricles
• the atria then contract, decreasing their volume and increasing the pressure even further forcing the remaining blood out
• the ventricles then contact and the atria relax
• the pressure is higher in the ventricles than the atria so the bicuspid and tricuspid valves close to prevent backflow
• the high pressure in the ventricles opens the semilunar valves so blood is forced out of the pulmonary artery and aorta
• the ventricles and atria both relax increasing their volume and lowing the pressure in the heart chambers
• the higher pressure in the pulmonary artery and aorta causes the semilunar valves to close preventing backflow
• then the atria fill with blood again due to the higher pressure in the vena cava and pulmonary vein and the cycle starts again

Question 6

Explain the electrical conduction through the heart muscle to control the contraction of the heart

Answer 6

• myocardium is myogenic - it can contract and relax without receiving signals from nerves

Question 7

Describe the structure of heart

Answer 7

• the heart is a cone-shaped, muscular, hollow organ situated in the mediastinum
• epicardium - outermost layer of the heart
  - -> part of the pericardium which is a double supportive layer which protects the heart
  - -> outermost layer of pericardium is composed of tough connective tissue which attaches to the diaphragm
  - -> inner layer, serous pericardium, is a thinner double membrane, the outer portion is attached to the fibrous pericardium and the inner layer forming the epicardium
  - -> between the two layers is a potential space (pericardial cavity) containing serous fluid which allows the two membranes to slide over one another as the heart contracts and expands
• myocardium - middle layer, consists of muscle
  - -> composed of cardiac muscle
  - -> the cells of this muscle are branched and striated and in close contact with adjacent muscle cells via intercalated discs
• endocardium - innermost layer
  - -> lines the heart, valves between the chambers and the blood vessels leaving the heart
• 4 chambers - 2 atria and 2 ventricles
• atria an ventricles separated by valves

Question 8

Describe the different valves and their function

Answer 8

• there are 4 valves, they prevent backflow of blood into the chambers
  - -> the valves only open one way, if there’s a higher pressure behind a valve its forced open, if there’s a higher pressure in front of the valve its forced shut
• tricuspid valve - situated between the right atrium and ventricle
• bicuspid valve - situated between the left atrium and the ventricle
• two semilunar valves - situated in the pulmonary artery and aorta

Question 9

Describe the varying thickness of the myocardium in the heart

Answer 9

• left ventricle of the heart is thicker and has more muscular walls than the right ventricle because it needs to contract powerfully to pump blood all the way round the body, whereas the right ventricle only needs to move blood into the lungs
• the ventricles have thicker walls than the atria because they have to push blood out of the heart whereas the atria only need to push blood into the ventricles

Question 10

Describe how blood moves through the heart

Answer 10

• the right atrium receives blood from the vena cava
• blood is then passed through the right ventricle and out of the heart via the pulmonary arteries to the lungs
• here it is oxygenated and returns through the pulmonary veins to the left atrium of the heart
• finally the blood exits the heart through the left ventricle into the aorta

Question 11

Describe the cardiac cycle

Answer 11

• the ventricles are relaxed
• the atria fill with blood, which decreases their volume and increases the pressure
• the higher pressure in the atria cause the tricuspid and bicuspid valves to open, allowing the blood to flow into the ventricles
• the atria then contract, decreasing their volume and increasing the pressure even further forcing the remaining blood out
• the ventricles then contact and the atria relax
• the pressure is higher in the ventricles than the atria so the bicuspid and tricuspid valves close to prevent backflow
• the high pressure in the ventricles opens the semilunar valves so blood is forced out of the pulmonary artery and aorta
• the ventricles and atria both relax increasing their volume and lowing the pressure in the heart chambers
• the higher pressure in the pulmonary artery and aorta causes the semilunar valves to close preventing backflow
• then the atria fill with blood again due to the higher pressure in the vena cava and pulmonary vein and the cycle starts again

Question 12

Explain the electrical conduction through the heart muscle to control the contraction of the heart

Answer 12

• myocardium is myogenic - it can contract and relax without receiving signals from nerves
• the process starts in the sino-atrial node (SAN) which is in the walls of the right atrium
• the SAN is like a pacemaker, it sets the rhythm of the heartbeat by sending out regular waves of electrical activity to the atrial walls
• this causes the right and left atria to contract at the same time
• a band of non-conducting collagen tissue prevents the waves of electrical activity from being passed directly from the atria to the ventricles
• instead these waves of electrical activity are transferred from the SAN to the atrioventricular node (AVN)
• the AVN is responsible for passing the waves of electrical activity onto the bundle of His, but there’s a slight delay before the AVN reacts to make sure the ventricles contract after the atria have emptied
• the bundle of His is a group of muscle fibres responsible for conducting the waves of electrical activity to the finer muscle fibres in the right and left ventricle walls called the purkyne tissue
• the purkyne tissue carries the waves of electrical activity into the muscular walls of the right and left ventricles causing them to contract simultaneously from the apex up

Question 13

Describe what cardiac output is and how it is regulated

Answer 13

• cardiac output - the amount of blood ejected from the heart each minute
• cardiac output = stroke volume x heart rate
• it is regulated by both the autonomic nervous system and by hormones and other chemicals
• nervous control is initiated in the cardiovascular centre in the medulla of the brain
  - -> when the body is involved in exercise, signals are received there from moving limbs
  - -> at the same time chemoreceptors and baroreceptors monitor the levels of chemicals in the blood and blood pressure respectively and this information is also sent to the medulla
  - -> in responses to all these signals the SAN which is innervated by the sympathetic nervous system is stimulated to increase the heart rate by initiating and increasing the number of action potentials
  - -> the myocardial muscle cells are also stimulated to contract more effectively so raising stroke volume
• hormonal regulation of cardiac output involves the adrenal glands, exercise stimulates them to increase their production of adrenaline, this acts on cardiac muscle to raise both heart rate and stroke volume

Question 14

Describe the blood supply to the heart

Answer 14

• coronary arteries branch off from the aorta soon after it leaves the left ventricle thus receiving oxygen saturated blood
• these arteries divide to supply all parts of the heart
• many anastomoses are present in the coronary circulation
• venous return is through small veins that empty into the coronary sinus

Question 15

Describe the structure and function of arteries

Answer 15

• arteries are large vessels that transport oxygenated blood from the heart to the rest of the body, walls are made up of 3 layers
• tunica interna - surrounds the lumen of the artery and is composed of endothelium covering a basement membrane and a layer of elastic tissue
• tunica media - usually the thickest and is composed of smooth muscle interspersed by elastic fibres
• tunica externa - consists of a though layer of fibrous tissue
• the presence of muscle and elastic fibres give the arteries elasticity and contractility
  - -> elasticity- when the ventricles of the heart contract they eject blood into the large arteries, the elasticity of these allows them to expand to accommodate the extra blood
  - as the ventricles relax the elastic recoil of the arteries forces the blood onwards
  - -> contractility - property originates in the tunica media, it receives its nerve supply from sympathetic branches of the autonomic nervous system
  - when stimulated, the walls contract narrowing the lumen (vasoconstriction), when the stimulation ceases the muscle wall relaxes and the lumen increases (vasodilation)
  - contractility is important both in maintaining homeostasis and in instances of arterial haemorrhage

Question 16

Describe the structure and function of arterioles

Answer 16

• arterioles are small arteries that connect the arterial network to the capillary network
• have a similar 3 layer structure to arteries
• arterioles closest to the capillary network have very thin walls consisting of tunica interna enclosed in a few muscle cells
• arterioles with tunica media play a vital role in vasodilation and vasoconstriction due to the amount of muscle present, they therefore control the amount of blood arriving to the capillary bed

Question 17

Describe the structure and function of veins

Answer 17

• made up of the same 3 layers as arteries however the layers of tunica interna and tunica media are very thin, whereas the tunica externa is much thicker than arteries
• larger lumen than arteries
• veins are still able to expand to accommodate the quantity of blood passing through
• in addition many veins contain valves to prevent backflow of blood which is under much less pressure to move forward than in arteries
• venous sinuses - specialised veins in brain and heart which walls only consist of tunica interna

Question 18

Describe the structure and function of venules

Answer 18

• the walls of the venous end are composed of all 3 layers whereas in the capillary end the tunica media is absent
• small veins connecting to the capillary network

Question 19

Describe the structure and function of capillaries

Answer 19

• supply almost every cell in the body
• composed of a single layer of tunica interna
  - -> this thin wall allows capillaries to carry out their primary function which is to allow the diffusion of gases, nutrients and wastes between the blood and surrounding tissues

Question 20

What is blood pressure and what are some of the factors that can affect it?

Answer 20

• it is the pressure exerted on the interior walls of blood vessels
• blood volume -> the total volume of fluid present in the circulatory system affects the pressure it exerts on the walls of the blood vessels, if there is a decrease in blood volume the pressure exerted will decrease
• viscosity of the blood -> blood consists of specialised cells suspended in plasma, the number of cells and proteins present in the blood affects the viscosity and the ease with which it can be moved along the blood vessels, to move thickened blood the heart has to increase the strength of its contractions resulting in an increased pressure exerted within blood vessels
• elasticity of the vessel walls -> if the structure of blood vessel walls it may become more difficult for them to stretch to accommodate extra blood when the ventricles contract, this increases the pressure in the blood and hence blood pressure (fat plaques)
• length of blood vessels -> the longer the vessel the lower the blood pressure
• diameter of the blood vessels -> pressure in blood vessels increases as their diameter decreases
• sympathetic nervous system -> the vasomotor centre in the medulla is stimulated by receptors that detect pressure and blood gas levels in the arterial system, this centre then stimulates arterioles to vasoconstrict or vasodilate which affects blood pressure
• hormonal regulation -> adrenaline and noradrenaline from the adrenal gland influence cardiac output and control vessel diameter, angiotensin 2 induces vasoconstriction and water balance by controlling the amount of aldosterone released

Question 21

What are the three main circulatory routes through which blood can be directed?

Answer 21

• systemic circulation
  - -> when blood leaves the heart through the left ventricle it is directed to all tissues and organs except the pulmonary arteries
  - -> the aorta is the largest artery involved and from this artery smaller arteries branch off to supply body organs
  - -> once the blood has been through the capillary networks of the body tissues and organs it is returned to the right atrium via the vena cava
• hepatic-portal system
  - -> blood passes through the gastrointestinal organs and then through the liver before returning to the heart
  - -> this allows nutrients and toxins taken into the body through the gastrointestinal system to be stored or removed before entering the general circulation
• pulmonary circulation
  - -> deoxygenated blood leaving the heart from the right ventricle goes via the pulmonary arteries to the lungs
  - -> here the blood exchanges oxygen and carbon dioxide with the air within the lungs
  - -> oxygenated blood then returns to the left atrium of the heart before entering the systemic circulation rich in oxygen

Question 22

What is plasma and what does it contain?

Answer 22

• plasma makes up approx 55% of blood volume
• it is composed of 90% water
• plasma proteins
• albumin
• globulins
• fibrinogen
• nutrients (amino acids, glucose, fatty acids and glycerol)
• wastes (urea, creatinine and bilirubin)
• oxygen
• carbon dioxide
• electrolytes
• hormones
• enzymes

Question 23

Describe the different blood cells

Answer 23

• the remaining 45% of blood is formed by blood cells
• erythrocytes (RBC)
  - -> make up 99% of blood cells and are responsible for transporting oxygen from the lungs to all parts of the body
  - -> biconcave disks - their design allows them to squeeze through capillaries
  - -> cell membrane contains antigens which are responsible for cell recognition (basis of blood groups)
  - ABO system based on the presence of antigens A and B
  - Rhesus system based on the presence or absence of the Rhesus factor
  - -> contain haemoglobin
  - -> with a lifespan of only 120 days new RBC’s must be produced constantly
• leucocytes (WBC’s)
  - -> largest of the blood cells and contain nuclei
  - -> granulocytes are WBC’s that contain granules
  - neutrophils - protect the body from foreign cells that invade tissues and remove debris, any host cells that are damaged at the site of an infection release chemotaxins which attract neutrophils, they surround the invader and release lysosomes
  - basophils - contain heparin and histamine which enable other cells to reach their sites of action
  - eosinophils - neutralise histamine and contain lysosomes that act against parasites and are found in large numbers around tissues close to the eternal environment
  - -> agranulocytes are WBC’s that contain no granules
  - Lymphocytes - responsible for identifying foreign or abnormal cells (antigens), on recognising an antigen they reproduce rapidly releasing an antibody which attacks it to render it harmless
  - monocytes - develop into macrophages and have an active role in immunity and inflammatory response
• thrombocytes (platelets)

Question 24

Describe the control of blood pressure in the renin-angiotensin system

Answer 24

• renin is a chemical which is secreted by the kidney and circulates in the blood
• it leads to the formation of angiotensin one, as it circulates through the lung angiotensin one is converted into angiotensin two by angiotensin converting hormone
• angiotensin two exerts a powerful constrictor effect on the arterioles, which can raise blood pressure
• angiotensin 2 also acts on the adrenal gland to release aldosterone which acts on the kidney and can cause retention of sodium
• this system is also a defence mechanism to maintain blood pressure in situations such as haemorrhage or extreme salt depletion

Question 25

Describe the changes that occur during pregnancy

Answer 25

• the heart is enlarged by chamber dilation and a degree of myocardial hypertrophy in early pregnancy leading to a 10-15% increase in ventricular wall muscle
  - -> the increasing blood volume also results in increased diastolic filling and progressive distension of the heart chambers
• the engaging uterus raises the diaphragm and the heart is correspondingly shifted upwards and to the left
• dramatic systemic and pulmonary vasodilation of arteries to increase blood flow
• increased permeability of capillaries
• vasodilation and impended venous return in lower extremities
• haemodilution of blood
• increased capacity for clot formation
• blood volume increases by 30-50%
• increase in cardiac output - ensures blood flow to the brain and coronary arteries is maintained while distribution to other organs is modified as pregnancy advances
  - -> increase in cardiac output is due to an increase in stroke volume and heart rate
• cardiac output is affected when a pregnant woman lies supine, he gravid uterus impedes venous return from the inferior vena cava resulting in a decrease in cardiac output (supine hypotension) - dizziness, nausea, light-headedness
• little change to systolic blood pressure, diastolic is lower in first two trimesters and returns to normal by term
• the effects of progesterone and relaxin on the smooth muscles of the vein walls and the increased weight of the growing uterus all contribute to the increased risk of valvular incompetence (varicose veins)
• plasma volume increases by 50% over the course of the pregnancy followed by a relatively small increase in red blood cell volume (haemodilution)
• RBC’s became more spherical with increased diameter due to the fall in plasma colloid pressur encouraging more water to cross the cell membrane