peripheral circulation & respiration 1

What is the primary "job" of the heart?

To deliver blood to the systemic and pulmonary circulations at a rate and pressure sufficient to meet the needs of the entire animal

What is the primary job of blood vessels serving individual tissues?

To adjust their relative resistance to flow so capillary beds receive sufficient blood to meet that tissue's needs, and to return blood to the heart at a rate/pressure sufficient for the whole animal

What is the primary job of the lungs?

To move air in and out of the alveoli at a rate that allows gas exchange with blood circulation appropriate to the animal's metabolic needs

What are the two basic functions of both loops (systemic and pulmonary) of the vascular system?

Distribution (transport of blood to/from organs, nutrients, hormones, immune cells) and Exchange (heat and gases to/from environment and tissues)

Distribution (vascular function)

Transportation of blood to and away from organs; includes delivering nutrients, vitamins, oxygen, water, and electrolytes, removing metabolic byproducts, conducting hormones, and transporting immune cells

Exchange (vascular function)

Movement of heat and gases to/from the environment, and gases/metabolites to/from tissues

What are key structural/functional characteristics of the vascular system?

Structurally complex, senses and responds to environmental changes, produces local mediators, and metabolizes circulating vasoactive substances and hormones

Vascular Smooth Muscle (VSM) - major function

To maintain vascular tone (a state of contraction) which determines vascular resistance and thus the ability of blood pressure to accomplish flow

Where is the majority of vascular resistance to flow contributed?

At the level of the arterioles

Striated Muscle

Muscle type with visible banding pattern; includes skeletal and cardiac muscle

Non-striated ("smooth") Muscle

Muscle lacking visible striations; found in vascular, bronchial, and gastric (hollow organ) walls

How do Vascular Smooth Muscle (VSM) cells differ from striated muscle cells?

VSM cells are smaller, not a syncytium (single nucleus), elongated and thin, have less-developed sarcoplasmic reticulum, no T-tubules (have caveolae instead), and are linked by gap junctions

Caveolae

Small invaginations of the VSM cell surface membrane that function like T-tubules, coupling membrane potential changes to the sarcoplasmic reticulum

Gap Junction (VSM)

Provides electrical link between adjacent vascular smooth muscle cells, similar to cardiac muscle

Dense Bodies (VSM)

Structures analogous to the Z-disk in skeletal muscle; anchor thin filaments, provide mechanical connection between cells, and contribute to elastic recoil

Intermediate Filaments (VSM)

Supporting proteins that link dense bodies together; do not contribute to the contractile process

Intermediate Junction (VSM)

Provides a mechanical link between two adjacent smooth muscle cells

Arterioles (capillary bed)

Vessels surrounded by smooth muscle that regulates vascular tone and resistance to flow, controlling entry into capillary beds

Capillaries (capillary bed)

Vessels where exchange of material with surrounding tissue takes place

Venules (capillary bed)

Post-capillary vessels that collect blood flowing out of the capillary bed

Lymphatics (capillary bed)

Vessels that collect interstitial/extravascular fluid and return it to venous circulation

Why are capillaries good exchange vessels?

Very thin walls (one cell thick), walls have holes/pores, and there are millions of them causing blood to flow very slowly

Why is blood flow slowest at the capillary level?

Because capillaries represent the highest total cross-sectional area in the circulation

The Lymphatic System

A second system of vessels, anatomically distinct from blood vessels, that returns fluid and plasma protein leaked from capillaries back to the circulating pool

How does lymph enter circulation?

Lymphatic vessels originate in tissues and collect fluid filtered but not reabsorbed by capillaries; lymph is filtered at a lymph node before being transported into a vein

Primary function of the respiratory system

To act as a gas exchanger between blood and atmosphere (O2 in, CO2 out)

Secondary functions of the respiratory system

Aiding acid-base balance (CO2/HCO3-), defending against inhaled particles, filtering blood clots (thrombi) from systemic circulation, and regulating humoral substances (e.g., Angiotensin I → II)

Major components of the respiratory system

The lungs, the respiratory muscles, and the CNS respiratory control centers

Trachea

Airway structure kept open by horseshoe-shaped cartilage rings connected by smooth muscle; warms and humidifies inspired air

Bronchi

Airway branches that gradually contain less cartilage than the trachea

Bronchioles

Airway branches where cartilage disappears and smooth muscle increases

Six components of respiratory function

Ventilation, Distribution, Diffusion, Perfusion, Transport, and Exchange

Ventilation

Movement of air in and out of the lungs

Distribution (respiratory)

Distribution of ventilated air among the branches of the airway

Diffusion (respiratory)

Movement of gases across the alveolus-capillary barrier

Perfusion (respiratory)

Blood flow to the lung provided by the circulatory system

Transport (respiratory)

Movement of gases in the blood

Exchange (respiratory)

Exchange of gases in peripheral tissues

What drives fluid (air or blood) to flow spontaneously?

A pressure gradient — fluid flows from a region of high pressure toward a region of low pressure

Pleura

Membranes that lie between the lungs and the wall of the thoracic cavity

Parietal Pleura

Pleural layer that lines the walls of the thoracic cavity

Visceral Pleura

Pleural layer that covers the surface of the lungs

Intrapleural Space (pleural cavity)

The fluid-filled space between the parietal and visceral pleura

Why is intrapleural pressure negative?

Because of the opposing elastic recoils of the thorax (outward) and lungs (inward), creating a sub-atmospheric pressure in the intrapleural space

What holds the lungs open at constant thoracic volume?

The non-expandability of the fluid in the intrapleural space

What dictates airflow in the lungs?

Changes in intrapleural pressure, governed by changes in thoracic cavity volume

How does inspiration occur (mechanically)?

The chest wall moves outward, increasingly negative intrapleural pressure overcomes lung inward recoil, lungs expand, and alveolar pressure drops below atmospheric pressure (P(alveolar) < P(barometric))

Major muscles of inspiration

Diaphragm and external intercostal muscles; contract with every inspiration, and relaxation causes passive expiration

Accessory muscles of inspiration

Sternocleidomastoid and scalenus muscles; contract only during forceful inspiration

Muscles of active expiration

Internal intercostal and abdominal muscles; contract only during active (forced) expiration

How is inspiration controlled neurologically?

Inspiratory neurons in the medulla (DRG) fire, activating motor neurons via the phrenic nerve (diaphragm) and intercostal nerve (external intercostals)

How does expiration occur (during quiet breathing)?

Inspiratory neurons cease firing, motor neurons are no longer activated, inspiratory muscles relax, and lungs return to original volume by elastic recoil

Ohm's Law analogy for fluid flow

F = P/R (Flow = Pressure/Resistance), analogous to I = V/R for electrical current

What determines flow between two points in a vessel?

The resistance in the vessel between an upstream and downstream point, given a particular pressure gradient

Arterial Pressure (hydrostatic)

Pa - PB (arterial pressure minus atmospheric/barometric pressure)

Tissue Pressure (hydrostatic)

PT - PB (tissue pressure minus atmospheric/barometric pressure)

Transmural Arterial Pressure

Pa - PT (arterial pressure minus tissue pressure)

Transmural Venous Pressure

Pv - PT (venous pressure minus tissue pressure)

Systemic Driving Pressure

Pa - Pv (arterial pressure minus venous pressure)

Why are pressures in the body measured relative to a reference level?

Because blood pressure is always measured as a pressure difference/gradient between two points; atmospheric pressure (760 mmHg) is often used as the "zero" reference

Laminar Flow

Streamlined, silent flow where fluid moves in parallel layers (lamina); occurs normally in smaller branched portions of circulation and airway

Turbulent Flow

"Noisy" flow that occurs beyond a critical velocity, causing increased resistance; occurs normally in ventricles, aorta, and main pulmonary artery

Factors that induce turbulent flow

Increased velocity of flow (constricted vessels/narrowed valves), decreased blood viscosity (anemia), and alterations in vessel walls

Effect of turbulent flow on pressure requirements

A greater pressure gradient is required to maintain a given rate of flow under turbulent conditions

Where does laminar flow normally occur in blood vessels?

Normal arteries, arterioles, venules, and veins

Where does turbulent flow normally occur in blood vessels?

Ventricles, aorta, main pulmonary artery, and stenosed (abnormally narrowed) arteries

"Single-file" flow

Flow pattern occurring in capillaries where erythrocytes pass through in a single line

Poiseuille's Equation

F = πΔPr⁴ / 8ηL; describes factors (vessel radius, fluid viscosity) affecting flow rate under a pressure gradient

Poiseuille's Equation variables

F = flow velocity, ΔP = pressure gradient (driving pressure), r = radius of tube, L = length of tube, η = viscosity of fluid

Simplified Poiseuille relationship (constant length & viscosity)

Flow ≅ ΔPressure × Radius⁴

Why are small changes in vessel radius significant for blood flow?

Because flow is proportional to radius to the 4th power, so small radius changes cause relatively large changes in blood flow when pressure is constant

Three factors influencing the radius of an artery or arteriole

The pressure gradient distending it outward (blood pressure), the external pressure compressing it (vascular smooth muscle contraction), and the circumferential tension in the wall (elastic structural factors)