Double Circulation in Humans

Circulatory System

• The circulatory system consists of two separate circuits: the pulmonary circuit and the systemic circuit.
• The pulmonary circuit is responsible for transporting oxygenated blood from the lungs to the heart.
• The systemic circuit is responsible for delivering oxygenated blood from the heart to the body and returning deoxygenated blood back to the heart.

Blood Components

• Blood is a connective tissue composed of several components, including red blood cells, plasma, platelets, and white blood cells.
• Red blood cells (erythrocytes) are the densest component, settling at the bottom of a centrifuge.
• Plasma is the least dense component, rising to the top of a centrifuge.
• Platelets and white blood cells form a thin layer in the middle.

Red Blood Cells

• Red blood cells have no nucleus when mature and are biconcave disks, increasing their surface area for gas exchange by 20-30%.
• They are thinner at the center than the edges.
• Antibodies (immunoglobulins) help guard against foreign microorganisms.

White Blood Cells

• White blood cells (leukocytes) are formed from pluripotent stem cells within bone marrow.
• They protect the body from harmful bacteria, viruses, and other foreign invaders.
• Unlike red blood cells, white blood cells have a nucleus.

Types of White Blood Cells

• Monocytes become enlarged to form macrophages, which clean up bacteria, dead cells, and other debris in the bloodstream.
• Lymphocytes produce antibodies that attack invading microorganisms and toxins.

Platelets

• Platelets (thrombocytes) have no nucleus.
• They are membrane-bound fragments of cells that form when larger cells in the bone marrow break apart.
• Platelets play a key role in blood clotting, which prevents excessive blood loss after an injury.

Blood Clotting

• When a blood vessel is broken, it releases chemicals that attract platelets.
• The platelets then rupture and release chemicals that interact with other nearby chemicals in the plasma to produce enzyme thromboplastin.
• A series of enzyme reactions produces fibrin, which forms a fibrous mesh over the site of injury, preventing the loss of blood cells and eventually forming a clot.

Blood Vessels

• A complex network of tubes that branch and re-branch to distribute blood and its contents to body cells.
• Arteries have thicker walls than veins and are more elastic.
• Arteries carry oxygenated blood away from the heart (towards cells), while veins carry deoxygenated blood toward the heart (away from cells).
• Capillaries are the smallest form of blood vessels, with walls that are just one cell thick.

Purpose of Circulation

• The circulatory system plays a crucial role in delivering nutrients, oxygen, and hormones to cells and removing waste products.
• A diffusion membrane is necessary for efficient exchange of oxygen, waste, and nutrients.
• There are two types of transport: passive transport, which requires no energy, and active transport, which requires energy to force molecules from a low to high concentration.

Evolution of Circulatory System

• The need for a circulatory system is an evolutionary adaptation to increasing body size.
• Larger organisms need a network to transport materials throughout the organism because some cells do not have direct contact with the external environment.