Cardiovascular Physiology - Blood Vessels and Fluid Compartments

Question 1

Explain the function and features of capillaries

Answer 1

Specialised transfer blood vessels that allow exchange of substances and heat with the surrounding interstitial fluid.

They have the following features:

Thin walls to facilitate diffusion (Fick’s law of diffusion)

A large number of small capillaries provide >6000m2 of endothelium for diffusion throughout the body, yet only contain around 5% of blood volume.

Very slow blood flow (<1cm/s flow rate)

Question 2

Describe sinusoidal capillaries

Answer 2

Flat, irregular capillaries with larger gaps between the cells, and a thinner basement membrane.

This facilitates greater exchange of molecules as big as proteins between the blood and interstitium.

Seen in the liver

Question 3

Describe the blood-brain barrier

Answer 3

A plexus of continuous capillaries with tight junctions to maintain membrane integrity. It prevents the passage of hydrophilic molecules.

This structure is reinforced by astrocyte foot processes

There are extra transport mechanisms, such as GLUT-1 glucose transporters to allow efficient transfer of water-soluble molecules into the brain.

Question 4

Describe the glomerulus

Answer 4

A highly specialised network of fenestrated capillaries designed to facilitate rapid filtration of small molecules and fluid into the renal tubule.

Glomerular capillaries are 5,000x more permeable than the BBB, with pores 80nm wide.

Three layers contribute to flomerular filtration

Capillary endothelium
Bowman’s capsule endothelium (Formed from podocytes which prevent molecules larger than 70,000 Da from filtering through)
Basement membrane

Question 5

How is bloodflow controlled through capillary beds?

Answer 5

Pre-capillary sphincters can contract and relax to divert blood through or away from capillary besd.

These undergo rhythmic contraction every 5 seconds, to generate pulsatile flow int he capillary bed and to allow rapid response to substances such as inflammtory mediators, environmental temperature and medications.

Question 6

What is the formula for net filtration in the bowman’s capsule?

Answer 6

This is the balance between the hydrostatic pressure of the plasma vs the hydrostatic pressure in the bowman’s capsule, countered by the effect of plasma oncotic pressure.

Plasma hydrostatic pressure (Pc) 48mmHg (Due to the efferent arteriole)

Interstitial hydrostatic pressure (Pi) 10mmHg

Plasma oncotic pressure (πi) 25mmHg

Net filtration pressure (NFP)

NFP = Pc - Pi - πi
NFP = 48 - 10 -25
NFP = 15mmHg

Question 7

Define the terms Tonicity and Osmole

Answer 7

Tonicity is the relative difference in osmolality between two compartments across a semipermeable membrane.

An Osmole is the amount of solute that exerts an osmotic pressure of one atmosphere when diluted in 22.4L of solution at 0°C

Osmolarity is the number of osmoles per litre of solution

Omolality is the number of osmoles per kilogram of solution

Normal osmolality is 280-300mOsm/kg, and is estimated by 2(Na+K)+Urea+Glucose

This distinction between osmolality and osmolarity is important because of the presence of fats and proteins in the blood.

Question 8

How is osmolality measured and calculated?

Answer 8

Osmolality is measured by testing the depression of freezing point caused by the solute

Normal osmolality is 280-300mOsm/kg, and and estimation can be calculated through:

2(Na+K)+Urea+Glucose

Question 9

What is osmotic pressure and how is it calculated?

CONFIRM universal gas constant units

Answer 9

It is the pressure needed to prevent movement of solvent molecules by osmosis across a semipermeable membrane.

Normal osmotic pressure in the blood is just over 7 atmospheres.

Calculated by the Van’t Hoff equation: p = RTC

p = osmotic pressure in pascals
R = universal gas constant of 8.32 J/mol/K
T = absolute temperature in Kelvin
C = Osmolality in mOsm/kg

Question 10

What is an osmolar gap?

Answer 10

The difference between the calculated and measured osmolality due to the presence of osmotically active molecules not included in the physiological osmolality equation:

Examples include:
Mannitol
Ethanol
Glycine

Question 11

What are osmoreceptors?

Answer 11

Specialised cells in the anterior hypothalamus that monitor and respond to chanjges in plasma osmolarity, regulating thirst and ADH secretion.

Question 12

What is the difference between oncotic and osmotic pressure?

NEEDS CLARIFICATION

Answer 12

Osmotic pressure is the hydrostatic pressure needed to prevent the solvent moving down a concentration gradient across a semipermeable membrane

Oncotic pressure is the poteion of osmotic pressure generated only by proteins, again measured as the hydrostatic pressure needed to oppose the resultant fluid shift.

Question 13

How does fluid move between intra- and extra-cellular fluid?

Answer 13

The osmolality of the intracellular environment is tightly controlled, therefore any variations in tonicity are brought about by a change in the extracellular osmolality.

This is primarily determined by sodium ion concentration

This is of clinical significance when managing hyponatraemia, as rapid changes in sodium levels can induce swelling of delicate tissues such as the brain and lead to pontine myelinolysis

Question 14

How is water distributed throughout the body?

IMAGES

Answer 14

In men, 60% of body weight is water, whereas women have higher body fat, so 50-55% of total body weight is water.

Therefore in a 70kg man, this is 42 litres, distributed between three main compartments:

67% intracellular fluid (28L)

33% Extracellular (14L)
Of these 14L, 67% is interstitial (10L), including the lymphatic system - it is similar to plasma but without the protein)
Trans-cellular fluid: CSF, bile, sweat, eye humour, GI secretions

33% of Extracellular fluid is intravascular (4L), 55% of this is plasma (the other 45% being blood cells), of which 90% is water, and 7% is protein, and 3% solutes.