Architecture of the CVS

Question 1

What are the muscles of the heart?

Answer 1

• The heart wall consists of three layers:
• Endocardium - direct contact with blood, comprises of endothelial cells.
• Myocardium - thick, muscular, middle layer, responsible for the heart’s contractions.
• Epicardium (visceral layer of pericardium) - outermost layer of the heart.

Question 2

What is the heart encased in?

Answer 2

• In a double layer pericardial sac.
• Made up of two layers:
• Parietal Layer - anchored to the fibrous pericardium.
• Visceral layer (epicardium) – which is anchored to the myocardium.

Question 3

Where is the heart located?

Answer 3

The middle mediastinum - in between the lungs (between the two pleural cavities).

Question 4

Give me the characteristics of fibrous pericardium.

Answer 4

• Outermost layer of pericardium.
• Tough and protective.
• Ensures the heart remains in position in the mediastinum and restricts over-expansion.

Question 5

What is the space between the visceral and parietal pericardium called?

Answer 5

• The pericardial cavity.
• Contains serous fluid - typically around 15-50ml in a healthy individual.

Question 6

What is the purpose of serous fluid?

Answer 6

• Protection – Prevents overfilling over heart, by providing a layer of protection to external shocks.
• Lubrication – Due to the fluid, it allows the heart to slide over the pericardium without causing friction.
• Also reduces friction between the heart and the surrounding structures, facilitating smooth heart movements during contractions.
• Nutrition & Waste removal – Not it’s primary function. But can provide some nutrients to the cells of the serous membranes (mesothelium) and can also remove waste products.

Question 7

What type of epithelium is mesothelium?

Answer 7

• It is a simple squamous epithelium.
• Mesothelium secretes the serous fluid found between the parietal and visceral cavities.
• Mesothelium lines body cavities, like the pericardium, pleura (lungs), and peritoneum (abdominal cavity).

Question 8

How is the serous fluid that lines between the parietal and visceral pericardial cavity formed?

Answer 8

• It is formed by mesothelium which lines the parietal and visceral pericardial cavities.

Question 9

How is the serous fluid characterised?

Answer 9

• It is derived from blood plasma – but has different composition from blood due to selective permeability of serous membranes.
• A clear, pale yellow, watery fluid.

Question 10

What is pericarditis?

Answer 10

• It is the inflammation of pericardium.
• Could be caused by bacterial, viral, autoimmune, and other diseases.

Question 11

What is pericardial effusion?

Answer 11

• Build-up of excess fluids in the pericardial cavity.
• Pericarditis can cause a secretion of extra fluid leading to pericardial effusion.
• Injury can lead to bleeding in pericardial space.
• Cancer – tumours from other parts of the body might spread to the pericardium.

Question 12

What are the major (crucial) vessels returning blood to the heart?

Answer 12

• Superior & Inferior Vena Cava.
• Right and Left Pulmonary Veins

Question 13

What are the major (crucial) vessels pushing blood away from the heart?

Answer 13

• Pulmonary trunk.
• Then Right and Left Pulmonary arteries.
• Then Aorta.

Question 14

What are the walls of the arteries and arterioles made of?

Answer 14

• They are comprised of three layers:
• Tunica Intima – composed mainly of endothelium, simple squamous epithelium, minimal resistance to the blood flow.
• Tunica Media – muscular layer, helical arrangement of smooth muscle, quantity of elastin determines vessels stretchability.
• Tunica Adventitia (or Externa) – outermost layer primarily made of collagen and fibrous tissue. Provides strength to vessels.

Question 15

Compare and contrast arteries and arterioles.

Answer 15

• Arteries – Thick wall, have significant elastic fibres (allowing them to expand and recoil when blood is pumped into them), muscular (allowing them to constrict or dilate walls to regulate blood flow and pressure), large diameter of lumen, low resistance, and high pressure of blood.
• Arterioles – Smaller diameter of lumen, primary resistance vessels, play a major role in blood pressure regulation and determining blood flow to individual organs (by adjusting diameter of lumen constricting or dilating).

Question 16

Why are arterioles referred to as primary resistance vessels?

Answer 16

• Ability to change the size of the muscular walls easily.
• Able to control how much blood goes to different parts of the body.

Question 17

What do Aorta, artery, arterioles, veins, and vena cava contain in their walls?

Answer 17

• Endothelium.
• Elastic tissue.
• Smooth muscle.
• Fibrous tissue.

Question 18

What do venules contain in their walls?

Answer 18

• Endothelium.
• Smooth muscle.
• Fibrous tissue.

Question 19

Can you describe some key features of veins and venules?

Answer 19

• Veins – Capacitance vessels (At any given time, about 60% of the body’s total blood volume is contained within the veins], Unidirectional valves, thinner walls compared to arteries, less elastic tissue.
• Venules – smallest veins which receive blood from capillaries.
• They both return blood to the heart; except the hepatic portal vein which take it to the liver.
• They both have a low pressure of blood (<15mmHG)
• They have a large lumen.

Question 20

What are the three types of veins?

Answer 20

• Deep – Veins located deep within muscle tissues.
• Superficial – Veins located closed to the skins surface, often visible through the skin.
• Connecting/perforating veins – veins connecting deep veins to superficial veins. These perforate or pass through the muscle fascia (a band or sheet of connective tissue) which allow blood to flow between the deep and superficial venous systems.

Question 21

Can you describe the smooth muscle and elastic fibres in arteries?

Answer 21

• Smooth muscle – Provides the ability to constrict and dilate, regulating blood flow and pressure.
• Elastic – expand in response to high pressures and the recoil, maintaining blood pressure during diastole.

Question 22

What are the walls of the veins and venules made of?

Answer 22

• They are comprised of three layers:
• Tunica Intima – made of endothelium, less smooth due to presence of valves.
• Tunica Media – Contain less smooth muscle and elastic tissue than arteries, so thinner wall.
• Tunica Adventitia (or Externa) – comprised mainly of collagen, thickest layer in veins.

Question 23

How is the smooth muscle in the tunica media arranged in arteries?

Answer 23

• They are arranged in a helical pattern – allows for vasoconstriction and vasodilation, effectively controlling blood pressure and flow.

Question 24

What are pericytes?

Answer 24

• They are specialised cells that wrap around endothelial cells of capillaries.

Question 25

What are the functions of pericytes?

Answer 25

• Structural support – provide support to capillary walls – ensuring stability.
• Regulation – help regulate blood flow through capillaries by contracting or relaxing.
• Vessel formation – Can inhibit or promote angiogenesis. In certain tumour environments, may stabilise blood vessels by limiting angiogenesis or promote angiogenesis to promote vascular growth to supply tumour with nutrients.

Question 26

What is the structure of capillaries?

Answer 26

• Single tube of endothelial cells.
• Very close to cells for efficient diffusion.
• Leaky (fenestrated and sinusoidal) or tight (continuous) – can be more or less permeable.
• Connect arterioles to venules.
• Contain pericytes that support capillary walls.

Question 27

Where are pericytes located and how do they communicate with endothelial cells?

Answer 27

• Located on outer surface of capillaries and small venules.
• Communicate with endothelial cells via paracrine signals and gap junctions.

Question 28

What are capillary beds?

Answer 28

• They are a network of capillaries which supply tissues and organs.
• Provide a connection between arterioles and venules.

Question 29

What are the three types of capillaries?

Answer 29

• Continuous – Have a continuous endothelial lining.
• Fenestrated – Have intracellular pores, smaller pores than sinusoidal.
• Sinusoidal – Have intracellular and intercellular pores. Have larger gaps than fenestrated, allow larger cells and molecules to pass through.

Question 30

Where are the capillaries found in the body?

Answer 30

• Continuous – found in muscle, skin, and lungs.
• Fenestrated – found in kidneys, intestines, pancreas, endocrine glands.
• Sinusoidal – Found in liver, bone marrow, spleen, and lymph nodes.

Question 31

What do the peaks and troughs represent in a pressure graph in the circulatory system?

Answer 31

• In the pulse pressure region, where the blood is flowing through the arteries.
• The peak represents the systolic pressure.
• The trough represents the diastolic pressure.

Question 32

What mechanisms do veins have to push blood back to the heart?

Answer 32

• Skeletal muscles – muscular contractions squeeze veins, pushing blood back to heart.
• Changes in thoracic pressure during breathing – changes in thoracic pressure during breathing helps draw blood back towards the heart.
• Unidirectional valves

Question 33

How is blood flow regulated in capillaries?

Answer 33

• Precapillary sphincters found at the terminal arterioles control the flow of blood into individual capillaries, through a process called shunting blood.
• Sphincters constrict, if not much blood is needed – result in blood bypassing capillaries and flowing through thoroughfare channel, straight route from metarteriole to venule.)
• Sphincters relax if more blood is needed.

Question 34

Can you tell me the function of the artery walls during systole and diastole of the heart?

Answer 34

• In arteries:
• Elastic fibres expand during systole.
• Elastic fibres recoil between heart beats, to maintain continuous flow of blood.
• Smooth muscle primarily involved if stimulated by sympathetic nervous system – leading to vasoconstriction.
• Smooth muscle primarily involved if stimulated by parasympathetic nervous system – leading to vasodilation.
• Arterioles, control blood flow into capillary beds through vasoconstriction and vasodilation of smooth muscle.

Question 35

What function do capillaries play in blood volume?

Answer 35

• Capillaries – smallest blood vessels; facilitate exchange between blood and tissues.
• At the arterial end of the capillaries, due to the high hydrostatic pressure on the capillary walls, this pushes fluid out of capillaries into interstitial space. (hydrostatic pressure exceeds osmotic pressure causing efflux of fluid)
• At the venous end of the capillaries, osmotic pressure exceeds hydrostatic pressure, drawing fluid back into the capillaries.
• However, more fluid exits at arterial end than re-enters at venous end.
• Leading to net loss of fluid from capillaries to interstitial space.

Question 36

Tell me the diameters of the lumen of different vessels, and the thickness of walls.

Answer 36

• Aorta – 20-25mm, 2mm thick
• Arteries – 5-10mm, 1mm thick
• Arterioles – 10-50 um, 6um thick
• Capillary – 5-10 um, 0.5um thick
• Venule – 10-20 um, 1um thick
• Vein - 5-20 mm, 0.5mm thick
• Vena Cava – 25-30mm, 1.5mm thick

Question 37

What is the lymphatic system and what are their functions?

Answer 37

• A network of vessels, which have unidirectional valves, move lymph along lymph vessels (lymph propulsion) by contraction of smooth muscle in tunica media.
• Fluid balance – collect excess interstitial fluid (preventing oedema) and return to blood stream, ensuring fluid balance.
• Filtration and immunity – Carry lymph to nodes where it’s filtered. Any pathogens of foreign particles in lymph can be identified and evoke immune response.
• Pathway for immune cells – Provide a route for immune cells to travel the body, allowing them to reach sites of infection or injury.
• Dietary lipid absorption – Special lymphatic vessel in intestines called lacteals absorb dietary fats in the form of chylomicrons. Which are transported as part of lymph and delivered to bloodstream.

Question 38

What is the structure of the lymphatic’s vessels?

Answer 38

• Lymphatic primary vessel structure - same as arteries.

Question 39

What is the structure of lymph nodes?

Answer 39

• Comprised of 5 layers:
• Capsule – dense connective tissue covering with fibrous extensions (trabeculae – these extend inwards into the lymph node)
• Cortex – Contains follicles with germinal centres (B lymphocyte sites). Also houses dendritic cells.
• Paracortex – Beneath cortex, rich in T-lymphocytes.
• Medulla – innermost part; contains medullary cords (B cells, plasma cells, macrophages), and medullary sinuses (lymph channels)
• The hilum is also an indented region on the lymph node where lymph vessels exit the node.