Circulatory System

The circulatory system is a complex network of organs and vessels that is responsible for transporting nutrients, oxygen, carbon dioxide, hormones, and blood cells to and from the cells in the body. This system is vital for providing nourishment, fighting diseases, stabilizing temperature and pH, and maintaining homeostasis. The circulatory system consists of two main components: the cardiovascular system (heart, blood, and blood vessels) and the lymphatic system.

Blood

Blood is a specialized bodily fluid that delivers necessary substances to the body's cells and transports waste products away from those same cells. It is composed of plasma, red blood cells, white blood cells, and platelets.

Composition of Blood

• Plasma: The liquid component of blood, constituting about 55% of its volume. It is a yellowish fluid that carries blood cells, nutrients, hormones, and waste products.
• Red Blood Cells (RBCs or Erythrocytes): These are the most numerous cells in the blood. Their primary function is to transport oxygen from the lungs to the body's tissues and to carry carbon dioxide away from the tissues and back to the lungs.
  • Structure: RBCs are biconcave discs, which increases their surface area for oxygen exchange. In mammals, mature RBCs lack a nucleus and other organelles, which maximizes the space for hemoglobin, the protein that binds to oxygen.
  • Increase in Efficiency: The absence of a nucleus, mitochondria, and other organelles in mammalian RBCs allows for more space for hemoglobin, thus increasing their oxygen-carrying capacity. This also prevents the RBCs from consuming the oxygen they transport.
• White Blood Cells (WBCs or Leukocytes): These cells are a crucial part of the immune system. They help the body fight off infections and other diseases. There are several types of WBCs, each with a specific function.
• Platelets (Thrombocytes): These are small, irregular-shaped cell fragments that play a vital role in blood clotting. When a blood vessel is injured, platelets gather at the site and form a plug to stop the bleeding.

Blood Coagulation

Blood coagulation, or clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. This process is essential for preventing excessive bleeding when a blood vessel is damaged. The process involves a cascade of enzymatic reactions involving various clotting factors, ultimately leading to the formation of a fibrin mesh that traps platelets and red blood cells, forming a stable clot.

Tissue Fluid and Lymph

• Tissue Fluid (Interstitial Fluid): This is the fluid that surrounds the cells in the body. It is formed from blood plasma that leaks out of the capillaries. Tissue fluid helps to bring oxygen and nutrients to the cells and to remove waste products.
• Lymph: Some of the tissue fluid returns to the capillaries, while the rest enters the lymphatic vessels, at which point it is called lymph. Lymph is a clear fluid that contains white blood cells and circulates throughout the lymphatic system, playing a crucial role in the immune response.

The Heart

The heart is a muscular organ that pumps blood throughout the body. It is the central component of the circulatory system.

Structure of the Heart

The heart is divided into four chambers:

• Two Atria (singular: atrium): The upper chambers that receive blood returning to the heart.
• Two Ventricles: The lower chambers that pump blood out of the heart.

The right side of the heart (right atrium and right ventricle) deals with deoxygenated blood, while the left side (left atrium and left ventricle) handles oxygenated blood. Valves between the chambers and at the exit of the ventricles ensure that blood flows in only one direction.

Working of the Heart and Circulation

The heart pumps blood in a continuous cycle known as the cardiac cycle.

• Systole: The phase of the cardiac cycle when the heart muscle contracts and pumps blood from the chambers into the arteries.
• Diastole: The phase of the cardiac cycle when the heart muscle relaxes and allows the chambers to fill with blood.

Double Circulation: In mammals, the blood passes through the heart twice for each complete circuit of the body. This is called double circulation and consists of two parts:

1. Pulmonary Circulation: The right ventricle pumps deoxygenated blood to the lungs, where it gets oxygenated. The oxygenated blood then returns to the left atrium.
2. Systemic Circulation: The left ventricle pumps oxygenated blood to the rest of the body. The deoxygenated blood from the body then returns to the right atrium.

Main Blood Vessels

• Entering the Heart:
  • Vena Cava (Superior and Inferior): Bring deoxygenated blood from the body to the right atrium.
  • Pulmonary Veins: Bring oxygenated blood from the lungs to the left atrium.
• Leaving the Heart:
  • Pulmonary Artery: Carries deoxygenated blood from the right ventricle to the lungs.
  • Aorta: Carries oxygenated blood from the left ventricle to the rest of the body.
• To and from the Liver:
  • Hepatic Artery: Supplies oxygenated blood to the liver.
  • Hepatic Vein: Carries deoxygenated blood from the liver to the vena cava.
  • Hepatic Portal Vein: Carries nutrient-rich blood from the digestive system to the liver.
• To and from the Kidneys:
  • Renal Artery: Supplies oxygenated blood to the kidneys.
  • Renal Vein: Carries deoxygenated blood from the kidneys to the vena cava.

Pulse and Blood Pressure

• Pulse: The rhythmic expansion and recoil of arteries resulting from the contraction of the heart. It can be felt at various points on the body, such as the wrist and neck.
• Blood Pressure (BP): The pressure exerted by the circulating blood on the walls of the blood vessels. It is usually measured in millimeters of mercury (mm Hg) and is expressed as two numbers: systolic pressure over diastolic pressure.

Blood Vessels

Blood vessels are the channels through which blood circulates throughout the body. There are three main types of blood vessels:

• Arteries: These are thick-walled, muscular vessels that carry blood away from the heart. They are under high pressure and have elastic walls to withstand this pressure.
• Veins: These vessels carry blood towards the heart. They have thinner walls than arteries and are under lower pressure. Veins have valves to prevent the backflow of blood.
• Capillaries: These are the smallest blood vessels and form a network between the arteries and veins. They have very thin walls (one cell thick) to allow for the exchange of gases, nutrients, and waste products between the blood and the body's cells.

Lymphatic System

The lymphatic system is a network of tissues and organs that help rid the body of toxins, waste, and other unwanted materials. The primary function of the lymphatic system is to transport lymph, a fluid containing infection-fighting white blood cells, throughout the body.

Lymphatic Organs

• Spleen: The largest lymphatic organ. It filters blood, stores white blood cells, and recycles old red blood cells.
• Tonsils: These are masses of lymphatic tissue located at the back of the throat. They are part of the immune system and help to fight infections.

Blood Groups

Blood groups are determined by the presence or absence of specific antigens on the surface of red blood cells.

ABO Blood Group System

This system classifies blood into four main types:

• Type A: Has A antigens on the RBCs and anti-B antibodies in the plasma.
• Type B: Has B antigens on the RBCs and anti-A antibodies in the plasma.
• Type AB: Has both A and B antigens on the RBCs and no antibodies in the plasma.
• Type O: Has no antigens on the RBCs and both anti-A and anti-B antibodies in the plasma.

Rh Factor

The Rh factor is another antigen found on the surface of red blood cells. If a person has this antigen, they are Rh-positive (Rh+). If they do not, they are Rh-negative (Rh-). The Rh factor is important during pregnancy, as an Rh-negative mother carrying an Rh-positive fetus can develop antibodies that can harm the fetus.

Hepatic Portal System

The hepatic portal system is a unique part of the circulatory system. It is a series of veins that carry blood from the capillaries of the stomach, intestines, spleen, and pancreas to the capillaries in the liver. This allows the liver to process the nutrients absorbed from the digestive system before they are distributed to the rest of the body. It also allows the liver to detoxify any harmful substances that may have been absorbed.