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
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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.
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.
=Maximum arterial pressure during ventricular contraction (normal: 120 mm Hg)(mm Hg)
=Minimum arterial pressure during ventricular relaxation (normal: 75–85 mm Hg)(mm Hg)
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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