Chapter 5

Chapter 5: The Circulatory System

5.1 Blood as a Transport Medium

Structure of Blood

Blood consists of two primary components:

  • Plasma: The liquid part of the blood.

  • Formed Elements: The non-liquid part, comprised of:

    • Erythrocytes (Red Blood Cells): Responsible for transporting oxygen.

    • Leucocytes (White Blood Cells): Play a crucial role in the immune response.

    • Thrombocytes (Platelets): Involved in blood clotting.

Plasma

Plasma is a straw-colored liquid composed of water, electrolytes, proteins, hormones, and waste products. Its primary function is to transport various components, including:

  • Cells (Erythrocytes, Leucocytes, Thrombocytes)

  • Nutrients (glucose, amino acids)

  • Wastes (urea, creatinine)

  • Hormones and Antibodies

Erythrocytes

Erythrocytes are the most abundant cells in blood, characterized by a biconcave shape that facilitates gas exchange and increases flexibility as they navigate through blood vessels. Key functions include:

  • Transporting oxygen from the lungs to body tissues.

  • Life span of approx. 120 days, lack of nucleus limits their longevity.

Leucocytes

Leucocytes are integral to the immune system, though they represent only about 1% of blood. They include various types, each with unique roles:

  • Neutrophils: Attack bacteria.

  • Monocytes: Differentiate into macrophages that engulf pathogens.

  • Lymphocytes: Key players in the immune response.

  • Basophils and Eosinophils: Involved in allergic reactions and combating parasites.

Thrombocytes

Thrombocytes or platelets are essential in blood clotting. They are small cell fragments that adhere to blood vessel injuries to form a matrix, initiating the coagulation process.

5.2 Moving Blood Through the Body

The Heart

The heart, located between the lungs in the mediastinum and surrounded by the pericardium, functions as the body’s pump. Its structure includes:

  • Cardiac Muscle: Composed of thick muscle layers.

  • Septum: Divides the heart into left and right sides.

  • Chambers: Four in total: two atria and two ventricles.

Heart Chambers

  • Right Atrium: Collects deoxygenated blood from the body.

  • Right Ventricle: Pumps blood to the lungs for oxygenation.

  • Left Atrium: Receives oxygenated blood from the lungs.

  • Left Ventricle: Pumps oxygenated blood into the body via the aorta.

Heart Valves

Valves prevent backflow of blood:

  • Atrioventricular Valves: Between atria and ventricles.

  • Semilunar Valves: At the exits of the ventricles.

Blood Vessels

Blood travels through arteries, capillaries, and veins:

  • Arteries: Carry blood away from the heart (e.g., aorta).

  • Capillaries: Sites for gas and nutrient exchange.

  • Veins: Bring blood back to the heart, featuring lower pressure and valves.

Blood Flow Regulation

Blood flow adjusts based on cellular activity via:

  • Altering heart output

  • Changing the diameter of blood vessels

Cardiac Cycle

The cardiac cycle consists of:

  • Systole: Heart muscles contract.

  • Diastole: Heart muscles relax and fill with blood. Both sides of the heart function simultaneously, ensuring effective circulation.

Circulation Pathways

  • Pulmonary Circulation: Deoxygenated blood travels to the lungs for gas exchange.

  • Systemic Circulation: Oxygenated blood is distributed to the body.

5.3 Blood Groups and Transfusions

Blood Groups

Blood types are determined by specific antigens on red blood cell surfaces:

  • ABO Grouping: Types include A, B, AB, and O, each defined by presence or absence of antigens.

  • Rh Grouping: Rh-positive individuals have Rh antigens; Rh-negative do not.

Transfusions

Transfusions are necessary for medical conditions and require matching of ABO and Rh types. Types of transfusions include:

  • Whole Blood and Red Cell Concentrates

  • Platelet Concentrates and Cryoprecipitate

  • Immunoglobulins for immune support and Autologous Transfusion for patients donating their blood ahead of time.

5.4 The Lymphatic System

Overview

The lymphatic system protects the body by returning fluid that escapes from the bloodstream and helps defend against infectious agents. It consists of:

  • Lymphatic Capillaries and Vessels: Transport lymph fluid away from tissues.

  • Lymph Nodes: Filter lymph and contain immune cells.

Lymphatic Vessels

Lymph is resolved and transported through lymph vessels:

  • One-way system: Fluid flows from tissue to veins.

  • Movement: Facilitated by surrounding muscles and valves.

Lymph Nodes

Lymph nodes are strategically located and play a pivotal role in the immune system:

  • Bean-shaped structures rich in lymphocytes.

  • Act as filters for lymph, capturing pathogens and debris before they re-enter circulation.