Blood Vessels and Lymphatic System
Arteries
An Artery is a blood vessel that carries blood away from the heart to different parts of the body. Its wall is composed of three distinct layers: an outer layer of connective tissue and elastic fibres, a middle layer of thick smooth muscle tissue and elastic fibres, and an inner Endothelium. This thick, muscular, and elastic construction enables arteries to withstand the high pressure of blood ejected from the ventricles during systole.
Outer Layer: Composed of connective tissue and elastic fibres that provide structural support and allow the artery to expand under pressure
Middle Layer: Contains thick circular smooth muscle and elastic fibres — contraction and relaxation of this layer controls the vessel diameter
Inner Layer: The Endothelium is a single layer of flat cells forming the smooth inner lining that minimises resistance to blood flow
The contraction of circular smooth muscles in arteries and arterioles is controlled by the nervous and endocrine systems. When stimulated, the muscle contracts, narrowing the arterioles — a process called Vasoconstriction, which reduces blood flow. When the muscles relax, the arterioles widen through Vasodilation, allowing more blood to pass through. Arterioles divide repeatedly to form a dense network of Capillary vessels.
Vasoconstriction: Reduces blood flow to a tissue by narrowing the arteriole lumen, controlled by nerve signals and hormones
Vasodilation: Increases blood flow by relaxing smooth muscle in the arteriole wall, widening the lumen
Arteriole Branching: Repeated division of arterioles ultimately produces the capillary network supplying body tissues
Atherosclerosis is a condition involving coexisting atheroma and arteriosclerosis. Atheroma is the deposition of a hard yellow plaque of lipoid material (primarily cholesterol) in the innermost layer of arteries. Arteriosclerosis is a degenerative arterial change associated with advancing age, characterised by thickening of the middle layer of arteries. Together, these cause narrowing and hardening of arteries, increasing the risk of Thrombus formation. A Thrombus formed in the brain or heart can be fatal, and atherosclerosis is a major contributor to heart attacks.
Atheroma: Deposition of yellow cholesterol-rich plaque in the inner arterial layer, caused by high blood cholesterol levels
Arteriosclerosis: Age-related thickening of the middle muscular layer, reducing arterial elasticity
Consequence: Narrowed and hardened arteries restrict blood flow and promote clot formation, which is dangerous in the coronary and cerebral arteries
Capillaries
Capillary vessels are the smallest blood vessels, with walls that are only one cell thick — the Endothelium. Although blood appears confined within the capillary walls, they are permeable, allowing water and dissolved substances to pass in and out. This exchange delivers oxygen and nutrients to tissues while removing carbon dioxide and waste products. The capillary network is so dense that no living cell is far from a supply of oxygen and food; in the liver, every cell is in direct contact with a Capillary.
Single-Cell Wall: The wall consists of a single layer of endothelial cells, making it extremely thin and ideal for diffusion
Permeability: Small molecules (water, gases, nutrients, wastes) pass freely through, while large proteins and blood cells remain inside
Density: The network is extensive enough to reach virtually every cell in the body
The exchange of materials between blood and body tissues through Capillary walls occurs in three distinct ways:
Active Transport and Diffusion: Materials move through the endothelial cells lining the capillary wall into the interstitial fluid, and then to body cells, and vice versa
Intercellular Spaces: Materials pass through the gaps between endothelial cells of the capillary wall to and from the extracellular fluid
Endocytosis and Exocytosis: Materials are taken up by endocytosis from the capillary lumen and passed to the other side by exocytosis — the reverse also occurs for some materials entering from the interstitial fluid
The diameter of Capillary vessels can be altered by nervous stimulation, which tends to close them, and by chemicals such as histamine, which dilates them. This change in diameter is brought about by a change in the shape of the endothelial cells forming the wall. Precapillary Sphincter rings of smooth muscle also regulate the amount of blood flowing into capillaries, precisely controlling blood supply to each tissue. The pressure within capillaries causes continuous leakage of fluid from blood plasma into the surrounding tissue spaces, forming Interstitial Fluid that consists primarily of water with dissolved nutrients, hormones, gases, wastes, and small proteins. Large proteins, red blood cells, and platelets cannot cross the capillary wall, but some white blood cells can squeeze through. Interstitial Fluid is the medium through which all material exchange between blood and nearby cells occurs. Capillary vessels join to form venules, which then join to form Vein vessels.
Precapillary Sphincters: Rings of smooth muscle at the entrance of capillaries that control blood flow into individual capillary beds
Nervous Regulation: Nerve signals can constrict capillaries by changing endothelial cell shape, reducing flow to specific areas
Chemical Regulation: Histamine released during inflammation dilates capillaries, increasing blood flow and causing swelling
Veins
A Vein is a blood vessel that transports blood from body tissues towards the heart. Its wall has the same three layers as an artery — outer connective tissue, middle muscular layer, and inner Endothelium — but the middle layer is relatively thin and only slightly muscular, with few elastic fibres. This makes vein walls much thinner and less elastic than artery walls. The lumen (bore) of a Vein is larger than that of an artery.
Three Layers: Present but the middle muscular layer is thin and poorly elastic compared to arteries
Larger Bore: The wider lumen of veins accommodates blood flowing at lower pressure
Thin Walls: The reduced muscular and elastic tissue reflects the lower pressure of blood within veins
Semilunar Valve structures are present in Vein vessels and prevent the backflow of blood as it moves towards the heart. The pressure of surrounding skeletal muscles, when they contract, tends to squeeze the veins and assist the return of blood towards the heart. Muscular activity including breathing movements help maintain normal flow of blood in the body.
Semilunar Valves: Pocket-like valves that open in the direction of blood flow and close to prevent backflow
Muscle Pump: Contraction of surrounding skeletal muscles compresses veins, pushing blood towards the heart
Respiratory Pump: Breathing movements create pressure changes that assist venous return
Vein vessels join to form progressively larger veins, ultimately forming the Venae Cavae — the inferior vena cava (from the lower body) and the superior vena cava (from the upper body). These two great veins empty blood into the right atrium of the heart. The oxygenated blood from the lungs is returned to the left atrium by the pulmonary veins.
Inferior Vena Cava: Collects deoxygenated blood from the lower body and delivers it to the right atrium
Superior Vena Cava: Collects deoxygenated blood from the upper body (head, neck, arms) and delivers it to the right atrium
Pulmonary Veins: The only veins carrying oxygenated blood — four pulmonary veins return blood from the lungs to the left atrium
Comparing Blood Vessels
Artery, Vein, and Capillary vessels differ in structure and function. The following comparison covers the key structural and functional differences used to distinguish these three types of blood vessels.
Direction of Blood Flow
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Arteries: Transport blood away from the heart to body parts through capillaries
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Veins: Collect blood from body tissues through capillaries and transport it towards the heart
•
Capillaries: Link arteries with veins
Type of Blood Carried
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Arteries: All carry oxygenated blood except pulmonary arteries
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Veins: All carry deoxygenated blood except pulmonary veins
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Capillaries: Mixed — exchanging oxygenated and deoxygenated blood
Valves
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Arteries: No valves (except at the base of the aorta and pulmonary trunk)
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Veins: Semilunar Valve structures present to prevent backflow
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Capillaries: No valves
Blood Pressure
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Arteries: High blood pressure
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Veins: Low blood pressure
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Capillaries: Falling pressure throughout
Pulse Detection
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Arteries: Pulse can be detected — wave of pressure from heartbeat
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Veins: No pulse
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Capillaries: No pulse
Rate of Blood Flow
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Arteries: Rapid — 400 to 500 mm per second in the aorta, decreasing in smaller arteries and arterioles
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Veins: Flow rate increases from smaller to larger veins
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Capillaries: Slowest — less than 1 mm per second
Bore and Wall Thickness
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Arteries: Smaller bore with thick walls
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Veins: Larger bore with thin walls
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Capillaries: Larger bore; wall is only one cell in thickness
Muscle and Elastic Fibres
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Arteries: Thick muscle layer and elastic fibres — elasticity helps smooth the pulsating blood flow
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Veins: Thin muscle layer with fewer elastic fibres — less elastic than arteries
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Capillaries: No muscle layer and no elastic fibres
Exchange of Materials
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Arteries: No exchange of materials
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Veins: No exchange of materials
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Capillaries: Responsible for all exchange of materials between blood and tissues
Blood Pressure and Rate of Flow
Blood pressure is the measure of force with which blood pushes against the walls of blood vessels. This pressure is generated by the contraction of ventricles (ventricular systole) and is highest in the aorta, gradually reducing as blood moves through arteries. The elastic walls of Artery vessels stretch with each pulse of blood, producing the feel of a pulse. During diastole (relaxation phase), the heart does not exert pressure on arterial blood, and the pressure falls.
$$P_{pulse} = P_{systolic} - P_{diastolic}$$
Pulse pressure is the difference between the maximum pressure during ventricular contraction and the minimum pressure during ventricular relaxation. It reflects the force of each heartbeat.
$P_{systolic}$=Maximum arterial pressure during ventricular contraction (normal: 120 mm Hg)(mm Hg)
$P_{diastolic}$=Minimum arterial pressure during ventricular relaxation (normal: 75–85 mm Hg)(mm Hg)
$P_{pulse} > 50 \text{ mm Hg}$
→May indicate arterial stiffness, often associated with atherosclerosis in older individuals
Systolic Pressure: The peak pressure reached during systole when ventricles contract — normally 120 mm Hg
Diastolic Pressure: The lowest pressure during diastole when ventricles relax — normally 75 to 85 mm Hg
Pulse: The wave of pressure felt in arteries due to the heartbeat — detectable in arteries but absent in veins and capillaries
Blood pressure declines progressively along the circulatory circuit from the aorta through arteries, arterioles, capillaries, venules, and veins. The difference between systolic and diastolic pressure diminishes until it disappears in capillaries and veins. Blood moves from regions of higher pressure to regions of lower pressure. The rate of blood flow is lowest in Capillary vessels and increases again in venules and veins. This pattern results from changes in the total cross-sectional area of the vessel system.
Pressure Gradient: Blood always flows from higher pressure (arteries) to lower pressure (veins) — this gradient drives circulation
Velocity Changes: Flow rate is fastest in the aorta, slowest in capillaries (due to enormous total cross-sectional area), and increases again in veins
Cross-Sectional Area: The combined cross-sectional area of all capillaries is far greater than that of the aorta, explaining the slow capillary flow despite continuous cardiac output
Hypertension is a condition of persistently high blood pressure. Prolonged high blood pressure damages the lining of blood vessels and weakens the heart muscles, which become thickened from continuous strain. The pumping efficiency of the heart declines, and blood may be retained in the heart and lungs, leading to a potentially fatal condition called congestive heart failure. High blood pressure can also cause Artery vessels in the brain to burst, resulting in brain haemorrhage.
Vessel Damage: Chronic high pressure injures the endothelial lining, promoting atherosclerosis and weakening vessel walls
Heart Strain: The left ventricle thickens (hypertrophy) trying to pump against high resistance, eventually weakening
Brain Haemorrhage Risk: Hardened, less elastic arteries in the brain are more likely to rupture under sustained high pressure
Thrombus Formation and Related Conditions
A Thrombus is a solid mass or plug of blood constituents (a clot) that forms within a blood vessel. It may partially or completely block the vessel where it forms. If dislodged and carried elsewhere in the circulatory system, it is called an Embolus. The formation of a thrombus is called Thrombosis, and thromboembolism (when an embolus lodges and blocks a vessel) is a leading cause of death.
Thrombus: A stationary blood clot formed within a vessel — can partially or completely obstruct blood flow
Embolus: A dislodged thrombus that travels through the bloodstream until it lodges in a narrower vessel
Thromboembolism: The condition caused when an embolus blocks a blood vessel, cutting off blood supply to downstream tissue
Causes of Thrombus Formation
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Irritation or infection of the inner lining of blood vessels
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Reduced rate of blood flow due to prolonged inactivity
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Lung conditions such as pneumonia, tuberculosis, and emphysema
Blockage of a blood vessel in the heart by an Embolus or a locally formed Thrombus causes damage to a portion of the heart muscles — a condition known as a heart attack, or technically Myocardial Infarction. It involves necrosis (tissue death) of part of the heart muscle due to disrupted blood supply. If the normal flow of blood is blocked in a blood vessel supplying the brain, it causes death of surrounding neural tissue due to lack of oxygen — a Stroke (or cerebral infarction). Symptoms vary depending on which part of the brain is damaged. Haemorrhage refers to the discharge of blood from blood vessels — brain haemorrhage results from bursting of an Artery supplying the brain, often due to hardened arterial walls and high blood pressure.
Myocardial Infarction: Necrosis of heart muscle tissue caused by blocked coronary blood supply, often accompanied by dangerous arrhythmias (especially ventricular fibrillation)
Stroke (Cerebral Infarction): Death of brain tissue (neural necrosis) due to blocked blood supply in a cerebral artery — symptoms depend on the affected brain region
Haemorrhage: Discharge of blood from blood vessels — brain haemorrhage results from bursting of an artery supplying the brain, often due to hardened walls and high blood pressure
Prevention of Vascular Disorders
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Reduce cholesterol intake in food and maintain normal body weight
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Control blood pressure through regular walking and exercise
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Do not smoke — smoking damages blood vessel lining and accelerates atherosclerosis
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Engage in regular physical exercise
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Avoid chronic stress and tension
Lymphatic System
The lymphatic system is responsible for transporting and returning materials from the body tissues to the blood. It comprises Lymph capillaries, lymph vessels, lymphoid masses, Lymph Node structures, and lymph — the fluid that flows through the system. About three litres more fluid leaves the blood Capillary vessels each day than is reabsorbed by them. The lymphatic system returns this excess Interstitial Fluid along with its dissolved proteins and other substances back to the bloodstream.
Components: The system consists of lymph capillaries, lymph vessels, lymph nodes, lymphoid organs (spleen, thymus, tonsils), and the lymph fluid itself
Fluid Balance: Returns approximately 3 litres of excess interstitial fluid per day that blood capillaries cannot reabsorb
Connection to Blood: Lymph vessels empty into veins, making lymph a fluid in transit between interstitial fluid and the blood
Lymph capillaries end blindly in body tissues, where pressure from accumulated Interstitial Fluid forces the fluid into the lymph capillaries. Once this fluid enters the lymph capillaries, it is called lymph. The intercellular spaces in lymph vessel walls are larger than those of blood Capillary vessels, allowing larger molecules from the interstitial fluid to enter. Lymph capillaries join to form larger lymph vessels, ultimately forming the Thoracic Lymph Duct, which opens into the subclavian vein. The flow of lymph is always towards the thoracic duct.
Blind-Ended Capillaries: Lymph capillaries are closed at one end, unlike blood capillaries which form continuous loops
Pressure-Driven Entry: Accumulated interstitial fluid pressure forces fluid into lymph capillaries
Larger Intercellular Spaces: Allow larger molecules (proteins, fat globules) to enter lymph capillaries that cannot fit through blood capillary walls
In the intestine, branches of lymph capillaries within villi are called Lacteal structures. The flow of lymph is maintained by the activity of skeletal muscles, movement of viscera, breathing movements, and valves that prevent backflow of lymph. Along the lymphatic pathway, Lymph Node structures are present — masses of connective tissue containing Lymphocyte cells. Several afferent lymph vessels enter each lymph node, which is drained by a single efferent lymph vessel. Lymph Node structures are present in the neck region, axilla, and groin.
Lacteals: Lymph capillaries within intestinal villi that absorb fat globules — after a fatty meal, these globules may constitute 1% of the lymph
Flow Maintenance: Skeletal muscle contraction, visceral movement, breathing, and valves all work together to push lymph towards the thoracic duct
Lymph Node Structure: Afferent vessels carry lymph in; efferent vessel carries filtered lymph out — lymphocytes and macrophages within the node filter and destroy pathogens
Locations of Lymph Nodes
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Cervical lymph nodes — neck region
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Axillary lymph nodes — armpits
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Inguinal lymph nodes — groin
Lymphoid Organs and Masses
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Spleen — filters blood, destroying foreign particles and aged red blood cells
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Thymus — produces and matures lymphocytes
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Tonsils and adenoids — lymphoid tissue in the throat that traps pathogens
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Lymphoid masses in walls of digestive tract (mucosa and submucosa)
The lymphatic system performs four critical functions that support circulation and immunity.
Fluid Return: Returns approximately 3 litres of excess Interstitial Fluid and dissolved proteins back to the blood each day, maintaining fluid balance
Fat Absorption: Lacteal structures in intestinal villi absorb large fat globules released by intestinal cells after fat digestion — these may make up about 1% of lymph after a fatty meal
Defence Against Pathogens: Lymph Node structures contain Lymphocyte and Macrophage cells that destroy bacteria and viruses — the painful swelling of lymph nodes during infection (e.g., mumps) results from accumulation of dead pathogens and immune cells
Blood Filtration: The spleen filters blood, exposing it to Macrophage and Lymphocyte cells that destroy foreign particles and aged red blood cells
Application Strategy
Worked Examples
Problem Statement
A person's blood pressure is measured as 140/90 mm Hg. Calculate the pulse pressure and determine whether this reading is within the normal range.
hypertension
Strategic Toolkit
$$P_{pulse} = P_{systolic} - P_{diastolic}$$
Step 1
Identify systolic and diastolic pressures
The first number (140 mm Hg) is the systolic pressure measured during ventricular contraction. The second number (90 mm Hg) is the diastolic pressure measured during ventricular relaxation.
$$P_{systolic} = 140 \text{ mm Hg}, \quad P_{diastolic} = 90 \text{ mm Hg}$$
Step 2
Calculate pulse pressure
Pulse pressure is the difference between the systolic and diastolic values, found by simple subtraction.
$$P_{pulse} = 140 - 90 = 50 \text{ mm Hg}$$
Step 3
Evaluate the reading against normal values
Normal systolic pressure is 120 mm Hg and normal diastolic is 75 to 85 mm Hg. Both values in this reading exceed the normal range. The pulse pressure of 50 mm Hg is at the upper end of the normal range (typically 30 to 50 mm Hg). This reading indicates elevated blood pressure, consistent with hypertension.
Problem Statement
Blood flows at approximately 450 mm/s in the aorta and less than 1 mm/s in the capillaries. Explain why the flow is slowest in capillaries despite the aorta having the highest blood pressure.
capillaryartery
Strategic Toolkit
$$v \propto \frac{1}{A_{total}}$$
Step 1
Compare total cross-sectional areas
Each individual capillary is very narrow, but there are billions of capillaries. Their combined total cross-sectional area is approximately 1000 times greater than that of the single aorta.
$$A_{capillaries} \approx 1000 \times A_{aorta}$$
Step 2
Apply the inverse relationship between velocity and area
The same volume of blood must pass through each section of the circulatory system per unit time. Since velocity is inversely proportional to cross-sectional area, the enormous combined area of capillaries results in very slow flow.
$$v \propto \frac{1}{A} \implies v_{capillary} \approx \frac{450}{1000} < 1 \text{ mm/s}$$
Step 3
Explain the functional significance
The slow capillary flow is essential because it provides sufficient time for the exchange of oxygen, carbon dioxide, nutrients, and waste products between blood and surrounding tissue cells. If blood rushed through capillaries as fast as in the aorta, effective exchange would not occur.
Problem Statement
A blood vessel has a wall one cell thick, carries blood at a rate of less than 1 mm/s, has no valves, and is the site of material exchange between blood and tissues. Identify the vessel type and explain two ways in which it differs from an artery.
capillaryarteryvein
Step 1
Identify the vessel type
The wall being one cell thick and the vessel being the site of material exchange are definitive features of capillaries. The slow flow rate and absence of valves further confirm this identification.
Step 2
Compare with artery — wall structure
Unlike capillaries which have only an endothelial cell layer, arteries have three thick layers: outer connective tissue, a thick middle muscular layer with elastic fibres, and an inner endothelium. Arteries also have a much thicker wall relative to their bore.
Step 3
Compare with artery — pressure and exchange
Arteries carry blood at high pressure (approximately 120 mm Hg systolic) and do not participate in material exchange. Capillaries have falling pressure and are the sole site of material exchange between blood and body tissues.
Memorize formulas
Normal Blood Pressure
Normal systolic pressure is 120 mm Hg (during ventricular systole) and normal diastolic pressure is 75 to 85 mm Hg (during ventricular diastole).
Formula
$$P_{normal} = 120/75\text{--}85 \text{ mm Hg}$$
Pulse Pressure
The difference between systolic and diastolic pressure. Normal range is 30 to 50 mm Hg.
Formula
$$P_{pulse} = P_{systolic} - P_{diastolic}$$
Blood Flow Velocity Inverse to Area
Blood flow velocity is inversely proportional to total cross-sectional area. Aorta: 400 to 500 mm/s; Capillaries: less than 1 mm/s.
Formula
$$v \propto \frac{1}{A_{total}}$$
Daily Lymph Return
Approximately 3 litres of excess interstitial fluid per day is returned to the blood by the lymphatic system.
Formula
$$V_{excess} \approx 3 \text{ L/day}$$
Review
1
The innermost layer of an artery wall, composed of a single layer of endothelial cells, is called:
Ans
Tunica intima
Review
Tap to reveal
Recall the three arterial wall layers
Arteries have three concentric coats: tunica intima (inner), tunica media (middle), and tunica adventitia (outer).
$$\text{Artery wall} = \text{intima} \rightarrow \text{media} \rightarrow \text{adventitia}$$
Match the description to the correct layer
A single layer of endothelial cells defines the tunica intima — it is in direct contact with flowing blood.
$$\text{Tunica intima} = \text{endothelium}$$
2
Which of the following is a characteristic feature of veins that distinguishes them from arteries?
Ans
Presence of semilunar valves that prevent backflow
Review
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Compare vein and artery features
Arteries: thick walls, narrow lumen, no internal valves, high pressure. Veins: thin walls, wide lumen, semilunar valves present, low pressure.
$$\text{Vein feature} = \text{semilunar valves}$$
3
Capillary walls are composed of:
Ans
A single layer of endothelial cells only
Review
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Identify capillary wall composition
Unlike arteries and veins, capillaries lack tunica media (smooth muscle) and tunica adventitia (connective tissue). Only the endothelial lining remains.
$$\text{Capillary wall} = \text{endothelium} + \text{basement membrane only}$$
Relate structure to function
The thin single-cell wall minimizes the diffusion distance, enabling efficient exchange of oxygen, carbon dioxide, glucose, and other solutes.
$$\text{Thin wall} \rightarrow \text{short diffusion distance} \rightarrow \text{rapid exchange}$$
4
Which statement correctly describes a structural difference between arteries and veins?
Ans
Arteries carry blood under significantly higher pressure than veins
Review
Tap to reveal
Compare arterial and venous pressure
Blood leaves the heart via arteries at high pressure (~120 mm Hg systolic) and returns via veins at low pressure (~5–15 mm Hg). This pressure difference explains the structural differences.
$$P_{artery} \gg P_{vein}$$
Relate pressure to wall structure
High-pressure arteries need thick, elastic walls to withstand and smooth out pressure surges. Low-pressure veins need thin walls and valves to prevent backflow.
$$\text{High pressure} \rightarrow \text{thick walls (artery)}; \quad \text{Low pressure} \rightarrow \text{thin walls + valves (vein)}$$
5
When body tissues become more metabolically active, precapillary sphincters respond by:
Ans
Relaxing to increase blood flow into the capillary bed
Review
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Understand precapillary sphincter function
Precapillary sphincters regulate perfusion of individual capillary beds. Relaxation opens the capillary; constriction reduces or stops flow through it.
$$\text{Active tissue} \rightarrow \text{sphincter relaxes} \rightarrow \text{increased capillary flow}$$
Apply to metabolic demand
Increased metabolic activity produces vasodilator metabolites (CO₂, H⁺, lactic acid) that signal sphincters to relax, directing blood where it is needed most.
$$\uparrow \text{Metabolism} \rightarrow \uparrow \text{vasodilators} \rightarrow \text{sphincter relaxation}$$
6
A person's blood pressure is recorded as 150/95 mm Hg. What is their pulse pressure?
Ans
55 mm Hg
Review
Tap to reveal
Apply the pulse pressure formula
Pulse pressure is calculated as systolic pressure minus diastolic pressure.
$$P_{pulse} = P_{systolic} - P_{diastolic}$$
Substitute the recorded values
Using the given reading of 150/95 mm Hg, subtract diastolic from systolic.
$$P_{pulse} = 150 - 95 = 55 \text{ mm Hg}$$
Assess the result
A pulse pressure of 55 mm Hg is above the normal range of 35–45 mm Hg, suggesting increased arterial stiffness — a finding consistent with the elevated blood pressure reading.
$$55 \text{ mm Hg} > 45 \text{ mm Hg (normal upper limit)}$$
7
Blood flows most slowly through which of the following vessels?
Ans
Capillaries, because their total cross-sectional area is the greatest
Review
Tap to reveal
State the flow-velocity relationship
Blood velocity is inversely proportional to the total cross-sectional area at any circulatory level.
$$v \propto \frac{1}{A_{total}}$$
Compare cross-sectional areas
Aorta: ~5 cm². Arterioles: ~40 cm². Capillaries: ~2500 cm². Veins: ~20 cm². Capillaries have by far the greatest total cross-sectional area.
$$A_{capillaries} \gg A_{aorta} \rightarrow v_{capillaries} \ll v_{aorta}$$
8
Vasoconstriction is the process by which:
Ans
Smooth muscle in the vessel wall contracts to narrow the lumen
Review
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Define vasoconstriction
The prefix 'vaso-' refers to blood vessels. 'Constriction' means narrowing. Together, vasoconstriction means the active narrowing of blood vessels by smooth muscle contraction.
$$\text{Vasoconstriction} = \text{smooth muscle contraction} \rightarrow \text{decreased lumen diameter}$$
Distinguish from related terms
Vasodilation (widening) is the opposite. Angiogenesis (new vessel growth) and valvular opening are unrelated processes.
$$\text{Constriction} \neq \text{Dilation}; \quad \text{Constriction} \neq \text{Angiogenesis}$$
9
The progressive accumulation of fatty deposits (atheroma) in the inner lining of arterial walls, leading to narrowed and stiffened arteries, is called:
Ans
Atherosclerosis
Review
Tap to reveal
Identify the defining feature
The question describes fatty deposit (atheroma) buildup in the inner arterial wall — this is the hallmark of atherosclerosis.
$$\text{Atheroma in arterial intima} = \text{Atherosclerosis}$$
Eliminate distractors
Thrombosis involves clots (not fatty deposits), haemorrhage involves bleeding (not deposit accumulation), and embolism involves traveling debris (not progressive wall buildup.
$$\text{Atherosclerosis} \neq \text{thrombosis, haemorrhage, embolism}$$
10
A blood clot that breaks free from its site of formation and travels through the bloodstream until it lodges in a narrower vessel is called:
Ans
Embolus
Review
Tap to reveal
Distinguish thrombus from embolus
A thrombus remains fixed at its site of formation. When a thrombus or fragment detaches and travels, it is reclassified as an embolus.
$$\text{Thrombus} \xrightarrow{\text{detaches}} \text{Embolus}$$
Identify the key feature of an embolus
The defining characteristic is movement — an embolus is a mobile obstruction that travels through the bloodstream until it lodges.
$$\text{Embolus} = \text{traveling clot/debris}$$
11
Blockage of a coronary artery by an atheroma or thrombus, resulting in the death of cardiac muscle tissue due to oxygen deprivation, is termed:
Ans
Myocardial infarction
Review
Tap to reveal
Identify the organ and vessel involved
The coronary arteries supply the heart muscle (myocardium). Blockage here affects cardiac tissue specifically.
$$\text{Coronary artery blockage} \rightarrow \text{myocardial damage}$$
Determine the outcome
Complete blockage causes tissue death (infarction) due to sustained oxygen deprivation. This is myocardial infarction.
$$\text{Tissue death} = \text{infarction}; \quad \text{Heart muscle} = \text{myocardium}$$
12
A stroke involves disruption of blood supply to the brain. Which of the following mechanisms could directly cause a stroke?
Ans
An embolus lodging in a cerebral artery
Review
Tap to reveal
Identify what causes a stroke
A stroke results from interrupted blood supply to brain tissue, leading to neuronal death. Common causes include embolus, thrombus, or haemorrhage in cerebral vessels.
$$\text{Stroke} = \text{disrupted cerebral blood flow}$$
Evaluate each mechanism
Only an embolus lodging in a cerebral artery directly obstructs brain blood supply. The other options either improve flow or affect unrelated systems.
$$\text{Embolus in cerebral artery} \rightarrow \text{obstruction} \rightarrow \text{stroke}$$
13
Lymph is formed when:
Ans
Blood plasma filters out of capillaries into interstitial tissue and is collected by lymphatic capillaries
Review
Tap to reveal
Trace the origin of lymph
At the arterial end of capillaries, hydrostatic pressure exceeds osmotic pressure, pushing plasma out into interstitial spaces. This excess fluid becomes interstitial fluid, then lymph when collected by lymphatic vessels.
$$\text{Plasma} \rightarrow \text{capillary filtration} \rightarrow \text{interstitial fluid} \rightarrow \text{lymph}$$
Note the volume
About 3 liters of excess interstitial fluid are collected as lymph each day and returned to the circulation via the thoracic duct and right lymphatic duct.
$$V_{excess} \approx 3 \text{ L/day}$$
14
Which cells within lymph nodes are primarily responsible for engulfing and digesting pathogens and cellular debris?
Ans
Macrophages
Review
Tap to reveal
Identify the cell types in lymph nodes
Lymph nodes contain two main functional cell types: lymphocytes (for adaptive immunity) and macrophages (for phagocytosis of pathogens and debris).
$$\text{Lymph node cells} = \text{lymphocytes} + \text{macrophages}$$
Match the function to the correct cell
Engulfing and digesting pathogens is phagocytosis, the defining function of macrophages. Lymphocytes attack pathogens through antibody-mediated or cell-mediated mechanisms, not by ingestion.
$$\text{Phagocytosis} \rightarrow \text{macrophages}$$
15
Lacteals are specialized lymphatic capillaries located in the intestinal villi. Their primary function is to absorb:
Ans
Dietary fats in the form of chyle
Review
Tap to reveal
Identify the location and structure of lacteals
Each intestinal villus contains a central lacteal (lymphatic capillary) surrounded by a network of blood capillaries. The two systems absorb different nutrients.
$$\text{Villus} = \text{central lacteal} + \text{blood capillary network}$$
Match absorbed substances to the correct vessel
Fats are too large and hydrophobic to enter blood capillaries directly, so they enter lacteals as chylomicrons and travel via the lymphatic system. Amino acids and glucose are small and water-soluble, so they enter blood capillaries directly.
$$\text{Lacteal} \rightarrow \text{fats (chyle)}; \quad \text{Blood capillary} \rightarrow \text{amino acids, glucose}$$
Question 1 of 15
0:00
1
The innermost layer of an artery wall, composed of a single layer of endothelial cells, is called: