SDL Microcirculation

microcirculation definition

End part of circulation system transporting nutrients to tissues and removing waste

capillary wall structure

Single-layer thin endothelium, highly permeable, 20–30 µm thick

number of capillaries in body

~10 billion capillaries

total capillary surface area

500–700 m²

distance of capillary from any cell

20–30 µm

functions of microcirculation

Transport nutrients, remove waste, exchange gases, maintain homeostasis

modulation of microcirculation

Nervous system, body fluids, local factors

length of all capillaries

90–110 thousand km (~1.5 times Earth’s perimeter)

size of capillaries

~1/20th thickness of human hair

capillary exchange definition

Exchange of substances between blood and tissues in capillaries

three mechanisms of capillary exchange

Diffusion, transcytosis, bulk flow

diffusion in capillaries

Movement of small molecules (O₂, CO₂, glucose) down concentration gradients

transcytosis definition

Transport of large, lipid-insoluble molecules across endothelial cells via vesicles

bulk flow definition

Movement of solutes with water across capillary wall governed by Starling forces

starling forces list

Capillary hydrostatic pressure, interstitial hydrostatic pressure, capillary oncotic pressure, interstitial oncotic pressure

capillary hydrostatic pressure definition

Pressure of blood inside capillaries pushing fluid out

interstitial hydrostatic pressure definition

Pressure of fluid outside capillaries pushing fluid in

capillary oncotic pressure definition

Osmotic pull by plasma proteins (esp. albumin) pulling fluid in

interstitial oncotic pressure definition

Osmotic pull by proteins in interstitial fluid pulling fluid out

net filtration pressure formula

(Pc + πi) – (πc + Pi)

positive net filtration pressure

Favors filtration (fluid leaves capillary)

negative net filtration pressure

Favors reabsorption (fluid enters capillary)

arteriolar end of capillary

High Pc → favors filtration

venular end of capillary

High πc → favors reabsorption

effect of decreased plasma proteins

↓ πc → oedema

effect of increased blood pressure

↑ Pc → oedema

blood flow definition

Movement of blood through circulatory system

blood pressure definition

Force exerted by blood on vessel walls due to heart contraction

resistance definition

Opposition to blood flow determined by vessel length, radius, viscosity

main site of resistance

Arterioles (vasodilation/vasoconstriction)

systemic vascular resistance definition

Resistance provided by peripheral circulation

pulmonary vascular resistance definition

Resistance provided by lung vasculature

relationship between resistance and flow

↑ resistance → ↓ flow unless pressure increases

relationship between resistance and pressure

↑ resistance → ↑ pressure if flow maintained

pulmonary circulation definition

Heart → lungs → heart, oxygenates blood

systemic circulation definition

Heart → body tissues → heart, delivers O₂ and nutrients

arteries role

Carry blood away from heart

veins role

Return blood to heart

arterioles role

Divide arteries into smaller vessels, regulate resistance

capillaries role

Exchange gases, nutrients, waste between blood and tissues

venules role

Collect blood from capillaries into veins

blood distribution modulation

Heart rate changes, vasodilation, vasoconstriction

lymphatic system role

Returns excess interstitial fluid and proteins to circulation

fluid balance equation

Filtration = Reabsorption + Lymph Flow

lymphoedema definition

Accumulation of fluid in tissues due to impaired lymphatic drainage

causes of lymphoedema

Blocked lymphatics (tumor, infection, surgery), congenital malformation

mechanism of lymphoedema

Failure of lymph flow → imbalance between filtration and reabsorption → fluid retention

clinical features of lymphoedema

Swelling, heaviness, non-pitting oedema

types of capillaries

Continuous, fenestrated, sinusoidal/discontinuous

continuous capillaries

No pores, exchange via diffusion/transcytosis, found in muscle, skin, CNS

fenestrated capillaries

Have pores, allow rapid exchange, found in kidneys, endocrine glands, intestines

sinusoidal capillaries

Large gaps, allow passage of cells and proteins, found in liver, spleen, bone marrow

movement of water across capillary endothelium

Regulated by Starling forces (Pc, Pi, πc, πi)

starling hypothesis

Fluid movement across capillaries depends on balance of hydrostatic and oncotic pressures

arteriolar end pressures

High Pc → favors filtration

venular end pressures

High πc → favors reabsorption

oncotic pressure definition

Osmotic pressure exerted by plasma proteins

oncotic pressure exam MCQ answer

Osmotic pull by proteins

major site of exchange of gases and nutrients

Capillaries

exchange in continuous capillaries

Through active channels, not pores

clinical cause of oedema

Loss of plasma proteins (malnutrition, nephrotic syndrome) or ↑ blood pressure

importance of albumin

Major plasma protein maintaining capillary oncotic pressure

effect of vasodilation

↓ resistance → ↑ blood flow

effect of vasoconstriction

↑ resistance → ↓ blood flow

danger of high blood flow

Can damage vessels and tissues

danger of low blood flow

Causes tissue hypoxia

summary of microcirculation function

Transport nutrients, remove waste, maintain tissue homeostasis

capillary exchange processes

Diffusion, transcytosis, bulk flow

starling forces role

Determine direction of fluid movement across capillary wall

lymphatic system importance

Prevents oedema by returning excess fluid to circulation

blood flow distribution factors

Resistance, pressure, flow

clinical relevance of microcirculation

Disorders cause oedema, lymphoedema, tissue hypoxia