07: Capillaries and Lymphatics

Which tissues have more capillaries

Those with a higher metabolic rate

T/F: blood flow through the capillaries is pretty constant

False

Pattern of flow through the capillaries

Vasomotion: intermittent, back-and-forth flow due to arteriolar vasodilation and vasoconstriction

Types of capillaries

• Continuous

• Fenestrated

• Sinusoidal

What determines what type of capillary is present in a tissue

What needs to be exchanged

How do molecules primarily move across capillary walls

Diffusion according to their concentration gradient

Factors that increase diffusion rate of molecules

• Lipophilic character

• Increased concentration gradient

• Increased diffusion surface area

• Lower molecular weight

• Decreased diffusion distance

Most common type of capillary

Continuous

Features of continuous capillaries

Tight cellular junctions with small intracellular pores

Molecules that can travel through continuous capillaries

• Lipid sol diffuses through

• Water sol has to fit through the pores

• Proteins can be moved via transcytosis

Why does exercise training increase the number of capillaries in skeletal muscle

To increase the rate of nutrient supply and waste removal to match metabolic demand

Locations with modified continuous capillaries that have pores blocked by tight junctions

Blood brain barrier

Molecular transport mechanisms across the blood brain barrier

• Passive diffusion

• Facilitated diffusion (carrier proteins)

• Transcytosis

Types of drugs kept out by the blood brain barrier

Hydrophilic drugs

Areas in the brain that have more permeable capillaries

Areas that need to sense or release cytokines and hormones

Features of fenestrated capillaries

Pores and additional fenestra

Molecules that can travel through fenestrated capillaries

• Lipid sol diffuses through

• Water sol rapidly diffuses through pores and fenestra

• Proteins still need transcytosis

Common locations of fenestrated capillaries

• Glomerulus

• Intestinal villi

• LNs

Features of sinusoidal/discontinuous capillaries

LARGE holes

Molecules that can travel through sinusoidal/discontinuous capillaries

Basically everything can pass through, even proteins and whole cells

Common locations of sinusoidal/discontinuous capillaries

• Liver

• Spleen

• Bone marrow

Starling forces that are the primary control of fluid movement

• Capillary hydrostatic pressure

• Capillary osmotic/colloid/oncotic pressure

Result of imbalanced starling forces

Edema

Where is the capillary hydrostatic pressure between

Capillary and tissue (ECF)

How does capillary hydrostatic pressure change along the length of the capillary

It is high at the arteriolar end and lower at the venous end due to fluid volume lost as net filtration

How does interstitial hydrostatic pressure change along the length of the capillary

It stays constant

Structures that keep the interstitial hydrostatic pressure relatively constant

Lymphatic vessels that drain filtered fluid

Which end of the capillary has the most filtration

Arteriolar end

Force that opposes the capillary hydrostatic pressure

Capillary oncotic pressure

Protein that provides most of the capillary oncotic pressure

Albumin

Where is the capillary oncotic pressure between

The plasma and the subglycocalyx space

Additional protective function provided by the glycocalyx

Keeps clotting proteins away from the endothelial surface

Structures that pick up filtered fluid

Lymphatics

Morphology of lymphatic vessels

Endothelial cells with lots of gaps

Lymphatic structures that prevent backflow

Valves

When do lymphatics have a “pump”

• Larger lymphatics have smooth muscle

• Lymphatics that are between muscle groups experience increased fluid movement due to the muscle pump action

How is lymphatic fluid returned to circulation

• Dumped into the vena cava

• Reabsorbed at the LNs

Result of impaired lymphatic drainage

Edema

Result of R sided heart failure

Ascites and pleural effusion

Result of L sided heart failure

Pulmonary edema