Microcirculation

Question 1

Define blood flow rate (Q)

Answer 1

Volume of blood passing through a vessel per unit time.

Equation: Q = ΔP (pressure gradient) divided by R (resistance)

Question 2

What law is linked with the fluid circuit and state the equation of this law?

Answer 2

Darcy’s Law

Delta P = Q x R

Question 3

What happens to blood flow rate if the pressure gradient is increased?

Answer 3

Blood flow rate (Q) is also increased

Question 4

Define resistance (R) in terms of blood flow.

Answer 4

Hindrance to blood flow due to friction between moving fluid and stationary walls.

Question 5

What factors affect resistance (also give the equation)?

Answer 5

Blood viscosity (η)
Vessel radius (r) 
Vessel length (L)

R = 8Ln/pi.r^4

Question 6

How does resistance affect blood flow?

Answer 6

Increased vascular resistance would decrease blood flow (e.g. Vasoconstriction → Increased resistance and decreased blood flow)

Question 7

What effect does vasoconstriction and vasodilation have on radius (r), resistance (R) and blood flow (F) of blood vessels? Why is this important?

Answer 7

Vasoconstriction: Lower r, higher R and lower F

Vasodilation: Higher r, lower R and higher F

Arteriolar smooth muscle displays a state of partial constriction → described as vascular tone. Partial constriction enables alterations to blood flow through constriction and dilation (room to accommodate movements).

Question 8

What 2 functions are radii of arterioles adjusted independently to accomplish and what regulates each function?

Answer 8

1. Match blood flow to metabolic needs of specific tissues - Regulated by local (intrinsic) controls and independent of nervous or endocrine stimulation.
2. Regulation of systemic ABP: regulated by extrinsic controls (nervous influence and hormonal control).

Question 9

How is the first function in the answer above chemically and physically driven?

Answer 9

Chemically - Increase metabolites and oxygen usage leading to vasodilation of the arterioles → Active Hyperaemia.

Physically - Decrease blood temperature and increased stretch (distension) due to ^ BP leading to vasoconstriction of arterioles → Known as myogenic auto-regulation.

Question 10

What arterial responses are most relevant to skeletal muscle and small intestine arterioles respectively?

Answer 10

Skeletal muscle arterioles - Active hyperaemia

Small intestine arterioles - Myogenic vasoconstriction

Question 11

What is total peripheral resistance?

Answer 11

Sum of resistance of all arterioles present in systemic circulation.

Question 12

Explain how the brain is involved in helping regulate arterial blood pressure (ABP).

Answer 12

Regulated by cardiovascular control centre in the medulla oblongata. Vasoconstriction proceeds to increase BP, reducing blood flow to organs.

Question 13

Which 3 hormones can lead to vasoconstriction of arterioles in order to help regulate ABP?

Answer 13

Vasopressin (ADH)
Angiotensin II
Adrenaline/Noradrenaline

Question 14

What is the purpose of capillary exchange?

Answer 14

Delivery of metabolic substrates to the cells of the organism (this is the ultimate function of the CVS).

Question 15

Why is capillary density important?

Answer 15

Fick’s Law - Minimise diffusion distance, maximise SA and maximised diffusion time.

Increase in metabolic activity → Increase capillary density to supply available respiring cells.

Question 16

Give examples of tissues that have denser capillary networks.

Answer 16

Skeletal muscle
Myocardium
Brain
Lung

Question 17

List the 3 types of capillary structure and briefly describe each one.

Answer 17

Continuous - water-filled gap junctions between endothelial cells, enabling passage of electrocytes. The endothelial cells, enabling passage of electrolytes.

Fenestrated - pores within capillaries, enables relatively larger molecules to pass through capillaries into the tissue space (Larger gap junctions).

Discontinuous - relatively large holes in capillary

Question 18

What type of capillary structure does the blood brain barrier have?

Answer 18

Continuous

Question 19

Explain ‘Bulk Flow’

Answer 19

A volume of protein-free plasma filters out of the capillary, mixing with the interstitial fluid and is reabsorbed.

Involves oncotic ‘pulling’ force and hydrostatic ‘pushing’ force.

Question 20

What is oncotic pressure?

Answer 20

Osmotic force due to protein in capillary drawing water back in. Oncotic pressure is constant/uniform as protein plasma concentrations don’t change.

Question 21

What process occurs the pressure inside the capillary is greater than the interstitial fluid?

Answer 21

Ultrafiltration - fluid leaves capillary

other way around → Reabsorption

Question 22

How many litres of fluid are drained by the lymphatic system daily?

Answer 22

3 L

Question 23

What occurs if the rate of production of fluid (release of fluid into interstitial space) exceeds the rate of drainage?

Answer 23

Oedema

(Elephantiasis - type of oedema caused by parasitic blockage of lymph nodes leading to enlargement of affected lower limb)

Question 24

Generate an equation for flow across the whole circulation and rearrange to generate a BP equation.

Answer 24

Cardiac output = MAP (blood pressure)/TPR

MAP = Cardiac output x TPR