Blood vessel properties - capillaries

Question 1

Capillaries: sites of exchange

Answer 1

Exchange occurs across capillary walls by diffusion no carrier- mediated transport systems (except for the blood-brain barrier)

Question 2

How is diffusion increased?

Answer 2

• Minimal distance
• Single layer of endothelial cells
• Thin wall (1 μm) & small
  diameter (7 μm)
• Proximity to cells
• Maximal surface area
• High numbers (10-40 billion) = 600m2
• Maximal Bme
• Velocity is slow due to extensive branching

Question 3

Rate vs velocity of blood flow

Answer 3

• Flow rate = volume of blood per unit of 4me

- Flow velocity = distance of blood travelled per unit of time

Question 4

Rate vs velocity of blood flow 2

Answer 4

Flow rate is stable at all levels of vascular tree - CVS is a closed loop so the volume of blood, i.e. the flow rate, must equal the CO (~5L/min at rest)
Flow velocity varies it is inversely proportional to the total cross sectional area of all vessels

Question 5

vascular tree

Answer 5

• Flow rate is the same throughout the vascular tree

- Flow velocity varies & is inversely related to total cross-sectional area

Question 6

Permeability

Answer 6

• Endothelial cells fit together like a jigsaw to form the capillary wall
• The majority of capillaries have tightly joined endothelial cells with narrow water-filled spaces between them
• This allows for passage of small water-soluble substances

Question 7

Tightness of endothelial cell fit varies between organs

Answer 7

• Cerebral: tight junctions so no cleas are present>blood- brain barrier
• Skeletal muscle & lung (&most others):water-filled clefts (4nm) > small, water soluble substances pass (ions, glucose, amino acids) but large water-soluble substances cannot (proteins via vesicular transport)
• Kidneys & intestines: water-filled clefts + fenestrations (20-100nm) > ‘leaky’ capillaries
• Liver: water-filled clefts (10-1000nm) + fenestrations > very large inter- & intra-cellular spaces, known as sinusoids

Question 8

Control of blood flow through capillaries

Answer 8

• No smooth muscle
• Pre-capillary sphincters are smooth muscle cells that spiral capillaries: myogenic tone (not innervated) & sensitive to local metabolic factors
• If metabolic activity increases, sphincters relax > increase flow - helps control how much blood flow goes through capillary
• If metabolic activity decreases, sphincters contract > flow is bypassed

Question 9

Blood flow through an organ is regulated by:

Answer 9

1. Number of open capillaries

2. Degree of arteriolar resistance

Question 10

Interstitial fluid: the space between blood & Tissue

Answer 10

• Interstitial fluid (80%) + plasma (20%) = extracellular fluid (ECF)
• Movement between blood & cells is not direct
• Movement in or out of cells can be passive (down cg) or active (against)
• Movement in or out of capillary endothelial cells is mostly passive similar composition between arterial blood & interstitial fluid, except proteins

Question 11

Exchange between blood & organs can occur via:

Answer 11

1. Diffusion: Diffusion allows for individual solute exchange
   between blood & cells
   This is passive, down a concentration gradient
2. Bulk flow (maintains plasma volume) regulates distribution of ECF between plasma & interstitial fluid, i.e. maintains plasma volume

Question 12

bulk flow

Answer 12

• Does not play a major role in individual solute exchange
• Hydrostatic & oncotic pressures influence bulk flow via ultrafiltration (fluid comes out) & reabsorption (fluid goes in)
• Capillary blood pressure & plasma proteins - Interstitial fluid & proteins
• Capillaries are the only vessels that have ‘pores’ to allow fluid to pass through

Question 13

In the context of bulk flow, what would happen in response to a haemorrhage?

Answer 13

net inward pressure is increased so fluid moves from the interstitial and into plasma (incre. reabsorption and reduces filtration)