Human Circulatory and Lymphatic Systems: Structure and Function

Posted on Oct 5, 2025 in Biology

The Internal Environment

The circulatory system is responsible for transporting nutrients to and from the cells in the body. It is made up of the heart, blood vessels, and blood.

Interstitial Fluid Renewal

Interstitial fluid is a liquid that surrounds the body’s cells. It must be renewed constantly (by the flow of blood, which adds nutrients and collects waste products) because:

  • Cells consume nutrients from the interstitial fluid, and if it is not renewed, the nutrients would run out.
  • Cells also release their waste substances into it. If the interstitial fluid was not renewed, waste would accumulate in the body and become toxic.

1. The Heart

The heart is a muscular organ that pumps blood throughout the body. The wall has three layers:

  • Pericardium: A sac that surrounds and protects the heart, shielding the myocardium.
  • Myocardium: A thick layer of muscle tissue that allows the heart to contract.
  • Endocardium: A layer of tissue that lines the inside of the heart and the blood vessels.

Heart Chambers and Valves

The heart is divided into four chambers:

  • Atria: Receive blood returning from the body or lungs.
  • Ventricles: Pump blood out of the heart. Blood travels from the atria to the ventricles through valves.

The Tricuspid valve connects the right atrium and right ventricle.

The Bicuspid valve (or Mitral valve) connects the left atrium and left ventricle.

Blood Flow Pathways

Oxygenated blood travels from the lungs to the left atrium through the pulmonary veins, passes through the bicuspid valve into the left ventricle, and is pumped out through the aorta to the rest of the body.

Deoxygenated blood returns through the vena cava, enters the right atrium, passes through the tricuspid valve into the right ventricle, and is sent to the lungs via the pulmonary artery.

Semilunar valves in the aorta and pulmonary artery prevent blood from flowing back into the heart.

The Heartbeat Cycle

  1. Atrial Systole: The atria contract, pumping blood into the ventricles through the tricuspid and bicuspid valves.
  2. Ventricular Systole: The ventricles contract. Semilunar valves open, and blood exits through the pulmonary artery and aorta. The tricuspid and bicuspid valves close.
  3. Diastole: The heart relaxes. Blood from the vena cava and pulmonary veins is drawn into the corresponding atria. Semilunar valves close.

2. Blood Composition and Function

Blood is composed of plasma and cells. It transports O2, waste products, and nutrients throughout our body, defends the organism against infections, and regulates body temperature.

  • Red Blood Cells (Erythrocytes): Transport oxygen from the lungs to the body and carbon dioxide from the body back to the lungs.
  • White Blood Cells (Leukocytes): Play a key role in the immune response.
  • Platelets (Thrombocytes): Involved in blood clotting.
  • Plasma: The liquid component that carries cells, nutrients, hormones, and waste products.

3. Blood Vessels

Blood vessels are the conduits through which blood flows. They are classified into three main types:

  • Arteries: Carry blood away from the heart (typically oxygenated, except for the pulmonary artery).
  • Veins: Carry blood back to the heart (typically deoxygenated, except for the pulmonary veins).
  • Capillaries: Microscopic vessels where the exchange of gases, nutrients, and waste occurs between blood and tissues.

4. Blood Pressure Dynamics

Blood pressure is the force of blood as it moves through the vessels, driven by the heart’s contractions (systole) and relaxations (diastole). During systole, blood is pumped out of the heart into arteries; during diastole, the heart fills with blood. Valves in arteries prevent backflow, and their elasticity helps maintain continuous blood flow.

Arterioles control how much blood reaches organs by adjusting their diameter. Capillaries allow the exchange of substances with cells at low pressure. Sphincters regulate blood flow into capillaries. Blood pressure is higher during systole than during diastole, which is why both values are measured.

Venous Return

Blood returns to the heart through veins with the help of:

  • Suction during heart relaxation,
  • Surrounding muscle contractions, and
  • Valves that prevent backflow.

5. Blood Circuits (Systemic and Pulmonary)

The body has two main blood circuits:

  • Systemic Circuit: Carries oxygenated blood from the left ventricle to the body (except the lungs) through the aorta, arterioles, and capillaries. Deoxygenated blood returns to the right atrium via venules, veins, and the vena cava.
  • Pulmonary Circuit: Carries deoxygenated blood from the right ventricle to the lungs via the pulmonary arteries. In the lungs, it releases carbon dioxide and picks up oxygen. The oxygenated blood then returns to the left atrium.

Characteristics of the Circulatory System

The circulatory system is defined as:

  • Double: Blood passes through the heart twice per full cycle.
  • Complete: Oxygenated and deoxygenated blood never mix.
  • Closed: Blood always flows within vessels.

6. The Lymphatic System

The lymphatic system collects excess interstitial fluid (plasma that leaves capillaries) and returns it to the blood, helping prevent fluid buildup in tissues. It is also a crucial part of the immune system.

Components of the Lymphatic System

It consists of:

  • Thin lymphatic vessels: Closed at one end, they carry lymph (a transparent fluid made of interstitial plasma and lymphocytes, which fight infection).
  • Lymph nodes: Found in areas like the neck, armpits, and groin, which produce and release lymphocytes.

Differences from the Circulatory System

Unlike the circulatory system, the lymphatic system:

  • Carries lymph, not blood.
  • Has no pump like the heart.
  • Is open (unidirectional flow), meaning fluid flows in one direction only.

In short, the lymphatic system maintains fluid balance and helps defend the body against infections.

Functions of the Lymphatic System

  1. Collects, filters, and returns excess interstitial fluid to the bloodstream.
  2. Helps protect the body from infection by transporting lymphocytes.
  3. Absorbs and transports fats from the small intestine via lacteals in the villi.

Main Lymph Vessels and Nodes

  • The right lymphatic duct collects lymph from the head and right arm, draining into the right subclavian vein.
  • The thoracic duct begins at the chyle cistern (a lymph sac) and collects lymph from most of the body, draining into the left subclavian vein.
  • Lymph nodes: Found in key areas like the neck, armpits, and groin, serving as filtration centers.

Main Circulatory Disorders

Disorders of Blood Vessels

  • High blood pressure (Hypertension): Pressure in the arteries is too high.
  • Atherosclerosis: Fatty plaque builds up in arteries, narrowing them and reducing blood flow.
  • Aneurysm: A weak artery wall bulges; if it bursts, it can cause a stroke.

Disorders of the Heart

  • Myocardial infarction (Heart attack): Blocked coronary arteries reduce oxygen to the heart, killing heart cells.
  • Angina pectoris: Chest pain resulting from reduced oxygen supply to the heart; a warning sign of heart disease.

Disorders of the Blood

  • Anaemia: Low red blood cell or haemoglobin levels; causes fatigue, pale skin, and dizziness.
  • Leukaemia: Cancer of white blood cells; the cells cannot properly fight infections.
  • Thrombosis: A blood clot forms inside a vessel, potentially blocking blood flow.

Prevention Tips

Healthy habits like balanced eating, regular exercise, avoiding smoking, and managing stress can help prevent these disorders.