Cardiovascular and Lymphatic System: Blood Vessels, Circulation, and Immunity

Blood Vessels and Circulation

Structure and Function of Blood Vessels

The cardiovascular system consists of various types of blood vessels, each with specialized structures and functions that contribute to the regulation of blood flow and systemic circulation.

• Tunica intima, tunica media, tunica externa: The three layers of blood vessel walls. The tunica media is most critical in regulating systemic blood pressure due to its smooth muscle content.

• Vasodilation and vasoconstriction: Vasodilation is the increase in vessel diameter due to smooth muscle relaxation, while vasoconstriction is the decrease due to contraction.

• Arteries vs. veins: Arteries carry blood away from the heart; veins return blood to the heart. Capillaries are the site of gas and nutrient exchange.

• Capillary beds: The cross-sectional area of capillary beds is much greater than that of the aorta, allowing efficient exchange.

Example: The aorta has a small cross-sectional area but high pressure, while capillaries have a large area and low pressure, facilitating exchange.

Blood Flow and Pressure Regulation

Blood flow is regulated by vessel diameter, blood volume, and resistance. The body uses several mechanisms to maintain homeostasis.

• Peripheral resistance: Resistance to blood flow in the vessels, primarily determined by vessel diameter.

• Pulse pressure: The difference between systolic and diastolic pressure; it decreases as blood moves through the capillaries.

• Venous return: Valves in veins and the skeletal muscle pump help return blood to the heart.

• Blood volume distribution: Most blood is found in the systemic veins at any given time.

Equation:

$\text{Blood Pressure} = \text{Cardiac Output} \times \text{Peripheral Resistance}$

Capillary Exchange and Lymphatic System

Capillaries are the primary site for exchange of gases, nutrients, and wastes between blood and tissues. The lymphatic system assists in returning excess interstitial fluid to the bloodstream and in immune defense.

• Filtration and reabsorption: Hydrostatic pressure pushes fluid out of capillaries; osmotic pressure draws it back in.

• Lymphatic vessels: Collect excess interstitial fluid and return it to the venous system.

• Lymph nodes: Filter lymph and house immune cells.

Example: Lymph capillaries are highly permeable due to overlapping endothelial cells that act as one-way valves.

Immunity and the Lymphatic System

Innate and Adaptive Immunity

The immune system protects the body from pathogens through innate (nonspecific) and adaptive (specific) mechanisms.

• Innate immunity: First line of defense, including skin, mucous membranes, and phagocytic cells.

• Adaptive immunity: Involves lymphocytes (T cells and B cells) and the production of antibodies specific to antigens.

• Antigen-presenting cells (APCs): Such as macrophages and dendritic cells, present antigens to T cells to initiate adaptive responses.

Example: Vaccination induces adaptive immunity by exposing the immune system to a harmless form of an antigen.

Cells and Organs of the Immune System

Several cell types and organs are involved in immune responses.

• Lymphocytes: B cells (produce antibodies), T cells (cell-mediated immunity), and Natural Killer (NK) cells.

• Primary lymphoid organs: Bone marrow (B cell maturation) and thymus (T cell maturation).

• Secondary lymphoid organs: Lymph nodes, spleen, tonsils.

Table: Major Lymphoid Organs and Functions

Organ	Function
Bone marrow	Production and maturation of B cells
Thymus	Maturation of T cells
Lymph nodes	Filter lymph, site of immune activation
Spleen	Filters blood, removes old RBCs, immune surveillance
Tonsils	Trap pathogens entering through mouth/nose

Immune Responses and Disorders

The immune system can malfunction, leading to hypersensitivity, autoimmunity, or immunodeficiency.

• Hypersensitivity: Exaggerated immune response (e.g., allergies).

• Autoimmunity: Immune system attacks self-tissues.

• Immunodeficiency: Inadequate immune response (e.g., HIV/AIDS).

Example: Type I hypersensitivity involves IgE antibodies and mast cell degranulation, leading to allergic reactions.

Respiratory System Overview

Structure and Function

The respiratory system is responsible for gas exchange, supplying oxygen to the blood and removing carbon dioxide.

• Upper respiratory tract: Nasal cavity, pharynx, larynx.

• Lower respiratory tract: Trachea, bronchi, bronchioles, alveoli.

• Alveoli: Site of gas exchange; surrounded by capillaries.

Example: The alveolar membrane consists of alveolar epithelium, capillary endothelium, and their fused basement membranes.

Mechanics of Breathing

Breathing involves changes in thoracic volume and pressure to move air in and out of the lungs.

• Inspiration: Diaphragm contracts, thoracic cavity expands, pressure decreases, air flows in.

• Expiration: Diaphragm relaxes, thoracic cavity decreases, pressure increases, air flows out.

• Transmural (transpulmonary) pressure: The difference between alveolar and intrapleural pressure; keeps lungs inflated.

Equation:

$\text{Transpulmonary Pressure} = \text{Alveolar Pressure} - \text{Intrapleural Pressure}$

Gas Exchange and Transport

Oxygen and carbon dioxide are exchanged across the respiratory membrane by diffusion and transported in the blood.

• Oxygen transport: Mostly bound to hemoglobin in red blood cells.

• Carbon dioxide transport: Dissolved in plasma, bound to hemoglobin, or as bicarbonate ions.

Example: The majority of oxygen is carried as oxyhemoglobin; CO2 is primarily transported as bicarbonate ($\text{HCO}_3^-$).

Additional info:

• Some questions reference specific hormones (e.g., ADH, aldosterone, ANP) involved in blood volume regulation.

• Immunological memory is established by memory B and T cells after exposure to antigens.

• Respiratory system also plays a role in pH regulation via CO2 removal.