Case 1

Question 1

Describe and identify the gross anatomy of the mediastinum

Answer 1

• Superior and inferior mediastinum. Inferior contains anterior, middle and posterior mediastinum.
• The boundaries of the entire mediastinum is the superior thoracic aperture (superiorly), the diaphragm (inferiorly), the sternum (anteriorly), the thoracic vertebrae (posteriorly) and the lungs and pleura (laterally).

Boundaries

• Superior mediastinum: superior thoracic aperture, sternum (specifically the manubrium), thoracic vertebrae, sternal angle/T4-5 vertebrae and the lungs and pleura.
• Inferior mediastinum: sternal angle/T4-5 vertebrae, sternum, thoracic vertebrae, diaphragm, lungs and pleura.
• Anterior mediastinum: sternum, anterior surface of the fibrous pericardium.
• Middle mediastinum: anterior and posterior surfaces of the fibrous pericardium.
• Posterior mediastinum: posterior surface of the fibrous pericardium, thoracic vertebrae

Contents

• Superior mediastinum: aortic arch and its branches, superior vena cava (SVC) and its tributaries, oesophagus and trachea, phrenic and vagus nerve, cardiac plexus and sympathetic chain, thymus and thoracic duct.
• Anterior mediastinum: thymus (and some fat and lymph nodes)
• Middle mediastinum: ascending aorta, superior vena cava, pulmonary trunk, heart and pericardium, trachea and bronchi, nerves (cardiac plexus, vagus and phrenic) and the tracheobronchial lymph nodes
• Posterior mediastinum: descending (thoracic) aorta, oesophagus, thoracic duct, azygos system of veins (azygos, hemiazygos, accessory hemiazygos), other blood vessels (bronchial, oesophageal and superior phrenic arteries) and sympathetic trunk.

Question 2

Describe and identify the major branches of the thoracic and abdominal aorta

Answer 2

Thoracic Aorta

• Ascending aorta - 2 branches: the right and left coronary arteries
• Aortic arch (ABCs) - 3 branches: brachiocephalic trunk (splits into right subclavian and right common carotid), left common carotid and left subclavian
• Descending (thoracic) aorta - many branches: posterior intercostal arteries, subcostal arteries (paired segmental), right and left bronchial arteries (paired lateral visceral), oesophageal branches (unpaired visceral), mediastinal branches (unpaired visceral), pericardial branches (unpaired visceral) and superior phrenic arteries.

Abdominal Aorta

• Comes from the aortic hiatus (at the level of T12)
• Inferior phrenic arteries: supply the inferior surface of the diaphragm
• **Coeliac trunk (unpaired visceral) - at the level of T12
• **Renal arteries (paired lateral visceral)
• **Superior mesenteric artery (unpaired visceral) - at the level of L1
• Gonadal arteries
• **Lumbar arteries (paired segmental parietal)
• **Inferior mesenteric artery (unpaired visceral) - at the level of L3
• Right and left iliac artery - to lower limbs

Question 3

Describe and identify the tributaries of the superior and inferior vena cava

Answer 3

Superior Vena Cava (SVC)

• Left internal jugular vein joins with left subclavian vein to form the left brachiocephalic vein.
• Right internal jugular vein joins with the right subclavian vein to form the right brachiocephalic vein.
• The right and left brachiocephalic veins join to from the superior vena cava, which drains into the right atrium.
• The azygos vein also drains into the SVC

Inferior Vena Cava (IVC)

• The right and left iliac veins join together to form the inferior vena cava (they themselves have come from veins that have drained the lower limbs and stretches of the pelvis).
• Lumber veins
• Right and left renal veins
• Hepatic veins
• There are a lot of other veins that don’t drain directly into the IVC because they will firstly go through the hepatic portal system to the liver to be metabolised.

Question 4

Describe how the composition of a blood vessel wall relates to its function

Answer 4

Arterioles

• Vascular smooth muscle cause vasoconstriction via the myogenic response
• Endothelial cells cause vasodilation via nitric oxide (NO) production.

Capillaries

• Involved in exchange of metabolites, gases and fluids.
• Are narrow, short and thin to give them a very large surface area for easier exchange.
• Due to the large surface area and them not having muscle in the walls, despite the small diameter, there is a very low resistance to flow.
• They are also arranged in parallel.
• Due to the microstructure of the capillaries and their large surface area, the speed at which the blood flows through the capillaries is very low - this means the surrounding cells have sufficient time to exchange metabolites and nutrients.

Veins and Venules

• The venous system acts as a huge reservoir for blood - at rest up to 70% of blood is stored in veins.
• In terms of their structure, they are thin-walled and very stretchy elastic vessels (high compliance).
• This means they can stretch without a significant increase in pressure.
• Veins can constrict via sympathetic nerve stimulation.
• This might be required if we need to shunt blood from the venous side to the arterial side of the circulation (e.g. during a haemorrhage), then sympathetic nerves can synapse onto the limited amount of smooth muscle present in veins and cause them to venoconstrict to squeeze the blood back to the heart to enter the arterial system.

Question 5

Describe the effects of local and external factors on arterioles and tissue fluid formation

Answer 5

• Arterioles are slightly contracted when resting - this is arteriolar tone.
• Changes in tone are controlled by vascular smooth muscle.

Local - Myogenic Response

• This triggers vasoconstriction in local arterioles.
• We normally have a normal resting vascular tone - the normal level of constriction in that arteriole.
• But if something happens such that our arterial BP increases (e.g. during exercise), that will cause the arteriole smooth muscle to stretch, which will cause the diameter to get bigger, meaning the resistance drops and the flow will increase.
• In certain tissues (e.g. kidneys) that need a steady supply of blood to not damage them, we need to locally decrease the arterial BP - this is the myogenic response.
• This increase in stretch in the arteriole will cause an intrinsic vasoconstriction in the smooth muscle of the arteriole to increase resistance and return blood flow to normal.

Local - Metabolites (most important)

• When a tissue is undergoing a lot of metabolic activity, it will produce a lot of metabolic by-products (e.g. H+, K+, CO2).
• These metabolites are vasodilators - therefore the more the tissue undergoes metabolic activity, the more vasodilator metabolites it produces and the stronger the vasodilation in the arteriole, thereby increasing blood flow.
• This mechanism allows the metabolic oxygen demand of a tissue to match the blood flow to that tissue (because if it is producing a lot of metabolites then it will need more oxygen).

External - Nerves (ANS)

• In particular, sympathetic nerves.
• Sympathetic nerves synapse onto arterial smooth muscle. When this happens, the arteriole will vasoconstrict.

External - Circulating Hormones

• An example of a circulating chemical that can cause vasodilation is nitric oxide (NO).
• NO is produced by endothelial cells lining blood vessels and it causes them to vasodilate.

Question 6

Describe the functional importance of the heart as a pump

Answer 6

To do

Question 7

Describe the resistance to flow, the changes in pressure and the distribution of blood volume in different parts of the circulation and the mechanisms that control this

Answer 7

Resistance to Flow

• Arterioles offer the main resistance to flow in the systemic circulation.
• The pressure drop across the arterioles is greater than that of any other class of vessel - they are resistance vessels.
• The combined effect of the resistance of all arterioles determines the total peripheral resistance (TPR).
• Resistance can be altered in arterioles to match local blood flow to local need.
• Vasodilation = increased diameter –> decreased resistance to flow –> increased blood flow
• Vasoconstriction = decreased diameter –> increased resistance to flow –> decreased blood flow

Changes in Pressure

• The pressure is highest in the aorta, it drops a little in the arteries but there is a major drop in the arterioles.
• It continues to decrease in the capillaries and venules and veins until it is just above 0 mmHg in the vena cava.
• The drop in pressure outside of the arterioles is due to friction.

Distribution of Blood Volume
- At rest, up to 70% of the body’s total blood volume is stored in veins.

Mechanisms

• The baroreceptors, myogenic response, metabolite production, sympathetic nerve stimulation and circulating hormones (NO) all help to control blood pressure.
• The kidneys control blood volume as well as pressure by varying the amount that is excreted.