Human Circulatory System UPSC & Other Exams

Ragini Chopra (MSc, Biological Sciences, Science Faculty UPSC)

Human Circulatory System UPSC, Biology Notes UPSC

Human Circulatory System UPSC

The human circulatory system is a complex network responsible for transporting blood, oxygen, nutrients, and waste throughout the body. It can divided into two parts:

• Blood Circulatory System (Cardiovascular system)
• Lymphatic Circulatory System

Blood Circulatory System (Cardiovascular system)

It includes Pulmonary Circulation and Systemic Circulation.

Pulmonary Circulation: This involves the movement of blood between the heart and the lungs. Deoxygenated blood travels from the heart to the lungs to pick up oxygen, and then returns to the heart to be pumped out to the rest of the body. Oxygenated blood returns to the left side of the heart.

Systemic Circulation: This is the part of the circulatory system that delivers oxygenated blood from the heart to the rest of the body’s tissues and organs, and then returns deoxygenated blood back to right side of the heart.

The blood circulatory system can be divided into three main parts:

Human Heart:

• The heart is a muscular organ that acts as a pump, propelling blood through the blood vessels.
• It has four chambers: two atria (upper chambers) and two ventricles (lower chambers).
• Pumps oxygenated and deoxygenated blood to different parts of the body

Blood Vessels:

These are tubes that carry blood throughout the body. There are three main types:

• Arteries: Carry oxygenated blood away from the heart to the body’s tissues
• Veins: Return deoxygenated blood to the heart
• Capillaries: Tiny vessels where oxygen and nutrient exchange occurs

Blood:

• Composed of plasma, red blood cells, white blood cells, and platelets
• Carries oxygen, nutrients, hormones, and removes waste products
• Plays crucial roles in immune defense and healing

Lymphatic Circulatory System:

The lymphatic system is a subsystem of the circulatory system in the vertebrate body. This is a network of vessels, tissues, and organs that helps rid the body of toxins, waste, and other unwanted materials. It also plays a crucial role in the immune system by transporting lymph, a fluid containing white blood cells that fight infections.

The Pulmonary Circulation and Systemic Circulation are combined called Blood Circulation System (cardiovascular system).

Blood Circulatory System (Cardiovascular system)

Human Heart

The heart is a muscular organ, shaped roughly like a cone, located within the chest cavity (thoracic cavity) between the lungs, slightly tilted towards the left.

It is enclosed within a double-layered membrane called the pericardium, which provides protection and reduces friction.

Heart Chambers:

The heart is divided into four chambers:

• Two atria: Upper chambers that receive blood.
• Two ventricles: Lower chambers that pump blood out.

The heart is further divided into right and left halves by a muscular wall called the septum. This ensures that oxygenated and deoxygenated blood remain separate.

Heart Valves:

The heart has four Valve:

• Tricuspid valve: Located between the right atrium and right ventricle.
• Pulmonary valve: Located at the opening of the pulmonary artery.
• Mitral valve (bicuspid valve): Located between the left atrium and left ventricle.
• Aortic valve: Located at the opening of the aorta.  

These valves prevent the backflow of blood and ensure unidirectional blood flow.

Blood Flow:

• Deoxygenated blood (blood with high carbon dioxide content) returns from the body to the right atrium via the superior vena cava (from the upper body) and the inferior vena cava (from the lower body).
• From the right atrium, deoxygenated blood flows into the right ventricle through the Tricuspid valve.
• The right ventricle pumps deoxygenated blood to the lungs for oxygenation through the pulmonary valve and pulmonary artery.
• In the lungs, blood picks up oxygen and releases carbon dioxide.
• Oxygenated blood (blood with high oxygen content) returns from the lungs to the left atrium via the pulmonary veins.
• From the left atrium, oxygenated blood flows into the left ventricle through the Mitral valve (bicuspid valve).
• The left ventricle pumps oxygenated blood to the rest of the body through the aorta, the largest artery in the body.

Heart Function:

The heart’s primary function is to pump blood throughout the body. This is achieved through a rhythmic cycle of contraction (systole) and relaxation (diastole) of the heart chambers. Other functions are-

• Oxygen and Nutrient Delivery: Blood carries oxygen from the lungs and nutrients from the digestive system to all the cells and tissues of the body.
• Waste Removal: Blood also transports carbon dioxide (a waste product of cellular respiration) from the body’s cells to the lungs for exhalation. It also carries other waste products to the kidneys and liver for filtration and removal.
• Hormone Transport: Hormones, chemical messengers produced by various glands, are carried by blood to their target organs to regulate various bodily functions.
• Immune Response: White blood cells, a key component of the immune system, are transported by blood to fight infections throughout the body.
• Temperature Regulation: Blood helps to distribute heat throughout the body, maintaining a stable internal temperature.

Other Important Points:

• Ventricle walls are thicker than atrial walls.
• The left ventricle has the thickest muscular walls due to the high pressure required to pump blood throughout the body.
• The pulmonary artery is the only artery that carries deoxygenated blood.
• The pulmonary veins are the only veins that carry oxygenated blood.
• Blood circulation was discovered by William Harvey.
• Birds and mammals have a closed circulatory system. This means that their blood is always contained within vessels like arteries, veins, and capillaries.
• Insects have an open circulatory system. In this type of system, blood is not always confined to vessels and comes into direct contact with the organs.
• Blood circulation in humans is unidirectional, meaning it flows in a specific direction.
• Double circulation occurs in humans. This means that blood passes through the heart twice in one complete cycle: once to the lungs for oxygenation and once to deliver oxygen and nutrients to the body’s tissues. Separate circulation of oxygenated and deoxygenated blood ensures efficient oxygen delivery and waste removal.
• The average resting heart rate for an adult is around 72 beats per minute. Stroke volume, the amount of blood pumped out of the heart with each beat, is approximately 70 milliliters. Therefore, the heart pumps about 5 liters of blood per minute.
• Newborn babies have significantly higher heart rates, typically around 160 beats per minute.
• Tachycardia refers to a heart rate that is faster than normal.
• Bradycardia refers to a heart rate that is slower than normal.
• Electrocardiogram (ECG) is a test that records the electrical activity of the heart.
• Electrical impulses from the heart muscles cause to beat. This electrical signal begins in the sinoatrial node (S-Anode) located in the upper right chamber (the right atrium). The S-A node is a natural pacemaker. It is often called the “heart of the heart.”
• The pacemaker, located in the right atrium, initiates and regulates the heartbeat.
• The most common reasons to need a artificial pacemaker are bradycardia and heart block.
• Heart sounds are typically auscultated using a stethoscope.
• The average weight of the human heart is approximately 250-300 grams. (Men: Approximately 300 grams, Women: Approximately 250 grams)
• The study of the heart is known as cardiology.
• The blue whale has one of the slowest heart rates, around 25 beats per minute.
• The shrew (chhachhoondar) has one of the fastest heart rates, reaching up to 800 beats per minute.
• The first human heart transplant was performed on December 3, 1967, by Dr. Christiaan Barnard in South Africa.
• The first human heart transplant in India was performed on August 3, 1994, by Dr. P. Venugopal.
• Jarvik-7 was the first artificial heart, developed by Robert Jarvik in 1982. (Transplant by Dr. Willem Kolff to the patient was Barney Clark on December 2, 1982 in USA)
• The heart is considered the busiest organ in the human body due to its continuous pumping action.

Blood Vessels

Blood circulation in the human body occurs through three types of blood vessels: arteries, veins, and capillaries. Each type has unique characteristics and specific functions.

1. Arteries

– Carry oxygenated blood away from the heart to body tissues

– Characterized by thick, elastic walls that can withstand high blood pressure

– Blood flows rapidly and at high pressure

– No valves present in their walls

– The aorta is the largest artery in the body

– Color appears bright red due to oxyhemoglobin

Special Case of Pulmonary Artery

– Unique among arteries as it carries deoxygenated blood

– Transports blood from the right ventricle to the lungs for oxygenation

2. Veins

– Carry deoxygenated blood from body tissues back to the heart

– Characterized by thinner walls compared to arteries

– Blood flows at lower pressure

– Contain valves to prevent backflow of blood

– Color appears bluish-purple due to deoxyhemoglobin

Special Case of Pulmonary Vein

– Unique among veins as it carries oxygenated blood

– Transports blood from lungs to the left atrium of the heart

3. Capillaries

– Thinnest blood vessels in the circulatory system

– Connect arteries to veins

– Site of nutrient, oxygen, and waste exchange between blood and tissues

– Extremely narrow diameter

– Blood flows very slowly

– Thin, permeable walls allow for efficient exchange of substances

Blood Circulation Pathway:

Heart → Aorta → Arteries → Arterioles → Capillaries → Venules → Veins → Heart → Pulmonary Artery → Lungs → Pulmonary Veins → Heart → Aorta

Other Important Points:

– Arterial walls are thicker than venous walls

– Hemoglobin state determines blood color (oxy vs. deoxyhemoglobin)

– Capillaries are crucial for cellular metabolism and homeostasis

– Blood vessels form a closed circulatory system

– Different vessel types have specialized functions

– Structure is directly related to function

– Continuous blood flow is essential for life

Blood

Blood is a specialized fluid connective tissue that plays a crucial role in maintaining the body’s homeostasis.

The average blood volume in adults is approximately 5-6 liters in males (7% of body weight) and 4.5-5.5 liters in females.

In adults, Blood cells are primarily produced in the red bone marrow.

During embryonic development, the initial site of blood cell formation is the yolk sac. Later, the liver (mesoderm) becomes the major site of blood cell production. The spleen also contributes to blood cell formation during fetal development.

Finally, red bone marrow becomes the primary site of blood cell production in later fetal stages and continues throughout adult life.

It is a complex fluid composed of Plasma (Yellow liquid part) and corpuscles (solid part). These corpuscles are of three types: RBC, WBC, and platelets.

Plasma

The liquid component of blood, constituting approximately 55-60% of its total volume.

Plasma is primarily composed of water (90-92%) and contains various dissolved organic and inorganic substances:

Proteins:

• Albumin: Maintains osmotic pressure and transports various substances.
• Globulins: Include antibodies (immunoglobulins) that play a vital role in immunity.  
• Thrombin and Fibrinogen: Essential for blood clotting.
• Heparin: prevents blood clotting inside the body

Electrolytes: Maintain proper fluid and electrolyte balance.

Hormones: Transport hormones throughout the body.

Glucose: Provides energy for cells.

Waste products: Such as urea and creatinine, transported for excretion.

Antibodies: fight diseases

Serum = Plasma – fibrinogen protein

Blood Corpuscles (Blood Cells – RBCs, WBCs & Platelets):

These are the cellular components of blood, comprising approximately 40-45% of its volume. These corpuscles are of three types: RBC, WBC, and platelets.

Red Blood Cells (RBCs) or Erythrocytes: carry oxygen

• Biconcave, disc-shaped cells containing hemoglobin, a protein that binds and transports oxygen.
• RBC (hemoglobin) Responsible for oxygen transport from the lungs to tissues and carbon dioxide transport from tissues to the lungs. Deficiency of Hemoglobin leads to Anemia. Higher concentration in males compared to females.
• Anucleated (no nucleus) (exception – the RBCs of Camel and Lama)
• Count: 4.5-5.5 million cells per microliter (50-55 lakh/mm³) in males and 4-5 million cells per microliter in females.
• Production site: Red bone marrow
• Storage and Destruction site: Spleen (often called RBC graveyard and Blood Bank)
• Lifespan: Approximately 120 days.

White Blood Cells (WBCs) or Leukocytes: fight infections

• Crucial for the body’s immune defense.
• Count: 4,500-11,000 cells per microliter.
• RBC to WBC ratio: 600:1
• Lifespan: ~4 days
• Production site: Bone marrow and lymphoid tissues.
• Destruction site: Spleen
• Nucleated cells
• Found in both blood system and lymphatic system

Types of WBCs

Granular WBCs:

• Neutrophils (Maximum): Phagocytose bacteria.
• Acidophils or Eosinophils: Involved in allergic reactions and defense against parasites.
• Basophils: Release histamine and other inflammatory mediators.

Non-Granular WBCs:

• Lymphocytes: Involved in antibody production and cell-mediated immunity.
  • T cells are responsible for cell-mediated immunity, meaning they directly attack and destroy infected cells or abnormal cells (like cancer cells). T cells are a subset of lymphocytes.
  • Leukemia: Excessive WBC (T cells) production (Blood Cancer)
  • HIV AIDS damages T-cells, compromising immunity
• Monocytes (largest WBC): Phagocytose larger particles and debris.

Platelets or Thrombocytes: help in blood clotting

• Small, cell-like fragments involved in blood clotting.
• Count: 150,000-450,000 platelets per microliter.
• Produced in the bone marrow from megakaryocytes.
• Anucleated (no nucleus)
• Lifespan: 3-5 days
• Production site: Bone Marrow
• Destruction site: Spleen

Size: RBCs < Platelets < Some WBCs

• RBCs (Red Blood Cells): Smallest of the three.
• WBCs (White Blood Cells): Generally larger than RBCs, but vary significantly in size depending on the type of WBC.  
• Platelets: Smaller than most WBCs, but larger than RBCs.

Lifespan: RBCs > Platelets > Most WBCs

• RBCs: Longest lifespan, around 120 days.
• WBCs: Lifespan varies greatly depending on the type of WBC, ranging from a few hours to several years.
• Platelets: Relatively short lifespan, around 7-10 days.  

Number: RBCs > Platelets > WBCs

• RBCs: Most numerous, typically 4.5-5.5 million per microliter of blood.
• WBCs: Less numerous than RBCs, typically 4,500-11,000 per microliter of blood.  
• Platelets: More numerous than WBCs, typically 150,000-450,000 per microliter of blood.

Blood Pressure:

• Blood pressure is the force exerted by blood against the walls of blood vessels.
• It is typically measured in millimeters of mercury (mmHg).
• Normal blood pressure is generally considered to be 120/80 mmHg.
• Blood pressure is measured by an instrument called a sphygmomanometer.
• Systolic pressure: The higher number, representing the pressure in the arteries when the heart beats (contracts).
• Diastolic pressure: The lower number, representing the pressure in the arteries when the heart is resting between beats.
• The medical name for high blood pressure over a long period of time is hypertension. High blood pressure is when blood pressure is much higher than the normal range.
• The medical name for low blood pressure over a long period of time is hypotension. Low blood pressure is when blood pressure is much below than the normal range.

Blood pH:

The pH of blood is slightly alkaline, maintained within a narrow range of 7.35-7.45.

Bone Marrow Types

Red Bone Marrow

• Produces: Red Blood Cells, White Blood Cells, Platelets

Yellow Bone Marrow

• Produces: Fat cells, Cartilage, Bones
• Serves as energy reserve
• Can convert to red bone marrow during major blood loss

At birth: All bone marrow is red

With age: Gradual conversion to yellow bone marrow

Blood Coagulation (Blood Clotting)

Blood coagulation (clotting) is a complex, precisely regulated physiological mechanism that prevents excessive blood loss when blood vessels are damaged.

Key Stages of Blood Clotting

1. Initial Trigger

– Normally, blood does not clot inside blood vessels due to heparin protein

– When blood vessel is damaged, several processes are initiated

2. Platelet Activation

– Platelets (thrombocytes) come into contact with damaged tissue and air

– Platelets secrete thromboplastin

– Damaged Tissue + Platelets + Air → Thromboplastin

3. Enzyme Conversion

– Thromboplastin + Calcium ions convert prothrombin to thrombin

– Prothrombin + Thromboplastin + Ca++ → Thrombin

4. Fibrin Formation

– Thrombin catalyzes fibrinogen in plasma

– Converts fibrinogen into insoluble fibrin filaments

– Creates a net-like structure

– Fibrinogen + Thrombin → Fibrin

5. Clot Formation

– Fibrin is a net-like structure

– Blood cells (RBCs, WBCs, Platelets) get trapped in the Fibrin

– Fibrin + Blood Cells → Blood Clot

Proteins Involved

– Thrombocyte (Platelet)

– Thromboplastin

– Thrombin

– Fibrinogen

Significance

– Prevents excessive blood loss

– Initiates wound healing process

– Protects the body from potential infections

Note:

Disorders in this process can lead to:

– Excessive bleeding (hemophilia)

– Inappropriate clot formation (thrombosis)

Blood Groups

Discovery

– Karl Landsteiner first discovered human blood groups in 1900

– Identified that blood differs due to specific proteins called antigens on red blood cells

Antigens

– Composed of glycoproteins

– Also called agglutinogens

– Located on red blood cell plasma membrane

– Inherited from parents

Types of Antigens

1. Antigen-A (Blood Group A)

2. Antigen-B (Blood Group B)

Function

– Activate body’s defense system

– Stimulate antibody formation

Antibodies

– Also known as agglutinins

– Found in blood plasma or serum

– Synthesized from gamma globulin protein in white blood cells

Types of Antibodies

1. Anti-A (a)

2. Anti-B (b)

ABO Blood Group System

– Groups A, B, and O: Discovered by Karl Landsteiner in 1900

– Group AB: Discovered by Decastello and Sturli

Prevalence in Population

– Group A: 25%

– Group B: 35%

– Group AB: 10%

– Group O: 30%

Additional Information

– Blood groups are determined by inherited genetic factors

– Antigens and antibodies play crucial roles in immune response

– Understanding blood groups is essential in medical science

– Critical for safe blood transfusions

Genetic Inheritance

– Blood group inheritance follows Mendelian principles

– Parents’ blood groups determine potential offspring blood groups

Rare Blood Groups: PP and Bombay

PP (Pinal Phenotype) Blood Group

• Discovered by Kasturba Medical College, Mangaluru
• First reported case in India
• Extremely rare blood group
• Currently identified in only one person in the country

Bombay Blood Group (Hh Blood Type)

• Extremely rare blood group
• First discovered by Dr. Y.M. Bhende in 1952 in Mumbai
• Globally found in 0.0004% of population
• In India: 1 in 10,000 people
• Lacks H antigen
• Can donate to ABO blood groups
• Can receive blood only from same blood group
• Cannot receive blood from standard ABO groups
• Rarer than O negative blood group
• Considered “rarest of rare” blood type

Blood Transfusion and Blood Groups

Discovered by Karl Landsteiner, who identified the importance of antigens and antibodies in blood.

Incompatible blood transfusion can cause agglutination (clumping of blood cells). Agglutination can block blood vessels and potentially cause death. Hence, blood group matching is crucial during transfusion.

Note – Most commonly transfused component is Red Blood Cells (Erythrocytes). i.e., Antigens. Can not give to same antibody.

Inheritance of Blood Groups

Isohemagglutinin Gene (I Gene) is responsible for determining blood group antigens. It discovered by Karl Landsteiner in 1900.

It is a single gene with multiple alleles. Three primary allelic variations:

• IA (A antigen)
• IB (B antigen)
• IO (No antigen)

Dominant and Recessive Relationships

• IA and IB are co-dominant
• IO is recessive
• Determines blood group inheritance pattern

Possible Genotype Combinations

IAIAI (Homozygous A)

IAIO (Heterozygous A)

IBIB (Homozygous B)

IBIO (Heterozygous B)

IAIB (AB Blood Group)

IOIO (Homozygous O)

Genotype and Phenotype Correlation

1. Blood Group A

   – Genotypes: IAIAI or IAIO

   – Phenotype: Blood Group A

   – Dominant expression of IA gene

2. Blood Group B

   – Genotypes: IBIB or IBIO

   – Phenotype: Blood Group B

   – Dominant expression of IB gene

3. Blood Group AB

   – Genotype: IAIB

   – Phenotype: Blood Group AB

   – Codominant gene expression

4. Blood Group O

   – Genotype: IOIO

   – Phenotype: Blood Group O

   – Recessive gene expression

Inheritance Patterns: Possible Offspring Blood Groups Based on Parental Combinations.

Parents: A × B

A (IAIO) × B (IBIO)

Hence, Possible Offspring Blood Groups: A, B, AB, O

Rh Factor

Discovered by Karl Landsteiner and Wiener in 1940

Found in Rhesus monkey’s blood and human red blood cells

Rh Factor Classification

1. Rh Positive (Rh+)

   – Rh antigen present in red blood cells

   – Dominant trait

   – 85-90% of Indians are Rh+

2. Rh Negative (Rh-)

   – Rh antigen absent in red blood cells

   – Recessive trait

Inheritance Pattern

Rh+ is dominant over Rh-

  – Rh+ × Rh+ = Rh+

  – Rh+ × Rh- = Rh+

  – Rh- × Rh- = Rh-

Blood Transfusion Compatibility

  – Rh- can receive: Rh- only (i.e., Rh+ cannot be given to Rh-)

  – Rh+ can receive: Rh+ and Rh-

  – Universal Donor: O- (Rh-)

  – Universal Recipient: AB+ (No antibodies)

Erythroblastosis Fetalis

It happens during pregnancy

If mother is Rh-, and father is Rh+ then Rh+ fetal blood enters mother’s bloodstream. Mother develops anti-Rh antibodies. Can cause fetal death due to blood agglutination. Subsequent pregnancies at higher risk.

Treatment

Immunoglobulin (IgG) injection

Prevents antibody formation

Destroys Rh antigens in mother’s blood

Other Important Facts for One Day Exams

Heart Sounds

• Heart Sounds First Sound (Lubb) and Second Sound (Dub). Lubb sound is due to Closure of Atrioventricular Valves (Mitral Valve & Tricuspid Valve). Dub sound is due to Closure of Semilunar Valves (Aortic Valve & Pulmonary Valve).

Blood Terminology

• Hematology: Study of blood
• Hemopoiesis: Blood formation process

Blood Characteristics

• Red color due to ferrous ion in hemoglobin
• RBC increases at high altitudes
• Blood storage: 42 days at 4.4°C

Blood Donation

• Can donate 10% of blood at once
• Subsequent donation after 2 weeks
• Uses anticoagulants: Sodium citrate, sodium dextrate, EDTA

Blood Cell Cycle

• Each blood cell completes cycle in 60 seconds

Blood Disorders

• Excess RBCs: Polycythaemia
• Decreased RBCs: Anaemia
• Excess WBCs: Leukaemia
• Decreased WBCs: Leukopenia

Major Arteries (Pure Blood)

• Carotid: Head
• Subclavian: Arm
• Pulmonary: Lungs
• Aorta: Entire body
• Renal: Kidney
• Hepatic: Liver
• Gastric: Stomach
• Iliac: Leg
• Femoral: Leg

Major Veins (Impure Blood)

• Jugular: Head
• Subclavian: Arm
• Brachial: Arm
• Pulmonary: Lungs
• Vena Cava: Body to heart
• Renal: Kidney
• Hepatic: Liver
• Hepatic Portal: Intestines to liver
• Iliac: Leg
• Femoral: Leg

Lymphatic System and Immune System

Lymph

Lymph is a colorless, watery fluid that circulates throughout the lymphatic system. It’s essentially the fluid that drains from your cells and tissues and isn’t reabsorbed back into your blood capillaries.

It is derived from blood plasma as fluids pass through capillary walls at the arterial end. As the interstitial fluid begins to accumulate, it is picked up and removed by tiny lymphatic vessels and returned to the blood. As soon as the interstitial fluid enters the lymph capillaries, it is called lymph. Returning the fluid to the blood prevents edema and helps to maintain normal blood volume and pressure.

Composition of Lymph:

Lymph contains a variety of substances, including:

• White blood cells: These are crucial for fighting infection.  
• Proteins: Important for various bodily functions.  
• Fats: Absorbed from the digestive system.  
• Minerals: Essential for bodily processes.  
• Cellular debris: Including damaged cells and waste products.  
• Foreign substances: Such as bacteria, viruses, and cancer cells.

Functions of Lymph:

• Fluid balance: Lymph collects excess fluid from tissues and returns it to the bloodstream, helping to maintain proper fluid levels in the body.  
• Immunity: Lymph carries white blood cells (like lymphocytes) throughout the body, playing a vital role in the immune response. These cells help fight off infections and diseases.  
• Fat transport: Lymph transports fats absorbed from the digestive system to the bloodstream.  
• Waste removal: Lymph helps remove cellular debris and waste products from tissues.

The Lymphatic System

The lymphatic system is a complex network of vessels, nodes, and other lymphatic organs that work together to transport lymph throughout the body.  

• Lymph vessels: These are thin tubes that carry lymph fluid throughout the body.  
• Lymph nodes: These are small, bean-shaped structures located throughout the lymphatic system. They act as filters, trapping bacteria, viruses, and other harmful substances.  
• Other Lymphatic organs: The spleen, thymus, and tonsils are also part of the lymphatic system and play important roles in immune function.

Lymphatic Vessels

Lymphatic vessels, unlike blood vessels, only carry fluid away from the tissues. The smallest lymphatic vessels are the lymph capillaries, which begin in the tissue spaces as blind-ended sacs. Lymph capillaries are found in all regions of the body except the bone marrow, central nervous system, and tissues, such as the epidermis, that lack blood vessels. The wall of the lymph capillary is composed of endothelium in which the simple squamous cells overlap to form a simple one-way valve. This arrangement permits fluid to enter the capillary but prevents lymph from leaving the vessel.

The microscopic lymph capillaries merge to form lymphatic vessels. Small lymphatic vessels join to form larger tributaries, called lymphatic trunks, which drain large regions. Lymphatic trunks merge until the lymph enters the two lymphatic ducts. The right lymphatic duct drains lymph from the upper right quadrant of the body. The thoracic duct drains all the rest.

Like veins, the lymphatic tributaries have thin walls and have valves to prevent backflow of blood. There is no pump in the lymphatic system like the heart in the cardiovascular system. The pressure gradients to move lymph through the vessels come from the skeletal muscle action, respiratory movement, and contraction of smooth muscle in vessel walls.

Lymph Nodes

Lymph nodes are small bean-shaped structures that are usually less than 2.5 cm in length. They are widely distributed throughout the body along the lymphatic pathways where they filter the lymph before it is returned to the blood. Lymph nodes are not present in the central nervous system.

Other Lymphatic Organs

Lymphatic organs are characterized by clusters of lymphocytes. The lymphocytes originate in the red bone marrow with other types of blood cells and are carried in the blood from the bone marrow to the lymphatic organs. When the body is exposed to microorganisms and other foreign substances, the lymphocytes proliferate within the lymphatic organs and are sent in the blood to the site of the invasion. This is part of the immune response that attempts to destroy the invading agent.

Tonsils

Tonsils are clusters of lymphatic tissue just under the mucous membranes that line the nose, mouth, and throat (pharynx). Lymphocytes and macrophages in the tonsils provide protection against harmful substances and pathogens that may enter the body through the nose or mouth.

Spleen

The spleen is located in the upper left abdominal cavity, just beneath the diaphragm, and posterior to the stomach. It is similar to a lymph node in shape and structure but it is much larger. The spleen is the largest lymphatic organ in the body. The spleen consists of two types of tissue called white pulp and red pulp. The white pulp is lymphatic tissue consisting mainly of lymphocytes around arteries. The red pulp consists of venous sinuses filled with blood and cords of lymphatic cells, such as lymphocytes and macrophages. Blood enters the spleen through the splenic artery, moves through the sinuses where it is filtered, then leaves through the splenic vein.

The spleen filters blood in much the way that the lymph nodes filter lymph. Lymphocytes in the spleen react to pathogens in the blood and attempt to destroy them. Macrophages then engulf the resulting debris, the damaged cells, and the other large particles. The spleen, along with the liver, removes old and damaged erythrocytes from the circulating blood. Like other lymphatic tissue, it produces lymphocytes, especially in response to invading pathogens. The sinuses in the spleen are a reservoir for blood. In emergencies such as hemorrhage, smooth muscle in the vessel walls and in the capsule of the spleen contracts. This squeezes the blood out of the spleen into the general circulation.

Thymus

The thymus is a soft organ with two lobes that is located anterior to the ascending aorta and posterior to the sternum. It is relatively large in infants and children but after puberty it begins to decrease in size so that in older adults it is quite small.

The primary function of the thymus is the processing and maturation of special lymphocytes called T-lymphocytes or T-cells. While in the thymus, the lymphocytes do not respond to pathogens and foreign agents. After the lymphocytes have matured, they enter the blood and go to other lymphatic organs where they help provide defense against disease. The thymus also produces a hormone, thymosin, which stimulates the maturation of lymphocytes in other lymphatic organs.

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