L62: Venous Return

Question 1

Properties of veins

Answer 1

1. Large diameter —> low resistance
2. Thin wall —> easy to stretch
3. Low pressure —> valves to prevent backflow
4. Valves
5. Volume reservoir (75%)
6. Very high compliance (thin wall, smooth muscle —> easy to stretch, sympathetic nerve —> allow compliance to be changed)

Question 2

Venous pressure, Central venous pressure, Mean circulatory filling pressure

Answer 2

Venous pressure: Pressure in peripheral veins (10-15 mmHg)

Central venous pressure: Pressure in vena cava (~ R atrial pressure: 0-5 mmHg)

Mean circulatory filling pressure: Pressure in circulation when flow = 0
—> Pressure equalised throughout circulatory / blood volume re-distributed around system (死水一潭)

Question 3

Venous return and Vascular function curve

Answer 3

Vascular function curve: Venous return vs R atrial pressure

• Venous return decreases with R atrial pressure
  —> due to ↓ P gradient from vena cava to R atrium
• Venous return ∝ (Venous pressure - R atrial pressure)
  —> Resistance has no factor due to low resistance / consistent throughout
• X-intersect: Mean circulatory filling pressure (when venous return = 0) (no flow)
  —> veins contains most blood —> greatly affect MCFP
• RAP below zero: vascular function curve levels off
  —> ↑ P gradient counteracted by collapse of vein in -ve R atrial pressure

Question 4

Factors affecting venous return (6 factors)

Answer 4

1. Sympathetic activity (venomotor tone)
   - ↑ sympathetic activity —> ↓ venous compliance —> ↑ venous pressure —> ↑ venous return
   - MCFP = volume/compliance —> ↓ venous compliance —> ↑ MCFP
   - Overall: whole curve parallel shift upwards
2. Blood volume
   - ↑ blood volume —> ↑ venous pressure —> ↑ venous return
   - ↑ blood volume —> ↑ MCFP
   - Overall: whole curve parallel shift upwards
3. Peripheral resistance
   - ↑ TPR —> ↓ less blood flow from artery to vein —> ↑ arterial pressure + ↓ venous pressure —> ↓ venous return
   - ↑ TPR only changes distribution of blood —> flows stop —> blood re-distribute again to equalise pressure —> no change in MCFP (only depends on compliance + blood volume)
   - Overall: slope of curve ↓, X-intersect no change
4. Skeletal muscle pump
   - muscle contraction —> ↑ Pout —> compression of vessels (veins are most affected (thin walled)) —> ↑ venous pressure —> ↑ venous return
   - only rhythmic contractions ↑ venous return (allow refilling of blood)
   - maintained contraction —> ↑ vascular resistance —> ↓ flow from artery —> ↓ venous return
5. Breathing
   - Inspiration —> ↓ Thoracic P + ↑ Abdominal P —> ↑ P gradient from abdominal vena cava to thoracic vena cava —> ↑ venous return
   - deeper the inspiration, greater the effect, more venous return
6. Gravity
   - contributes to P(in) of vessels
   * **- Transmural pressure = Pin - Pout = (hydrostatic P + gravitation P) - tissue P
   - horizontal: little effect
   - upright: ↑ lower limbs pressure —> ↑ lower limb transmural pressure —> distend lower limb vessel —> blood accumulate in lower limb —> ↓ venous return —> ↓ cardiac output —> ↓ BP —> Baroreflex —> ↑TPR (early) + ↑HR + ↑force (late) —> restore mean BP
   - TPR higher, venous return lower, cardiac output lower —> ↑ diastolic P + ↓ systolic P —> ↓ pulse pressure

Question 5

Cardiac function curve (CFC) + Vascular function curve (VFC)

Answer 5

Cardiac function curve: CO vs RAP
Vascular function curve: VR vs RAP
—> Since heart is a closed system —> venous return = cardiac output
—> heart must operate at intersection point with CFC x VFC

1. Blood volume
   - ↑ blood volume —> ↑ venous return —> VFC shift parallel upwards
   - ↑ blood volume —> ↑ filling of heart (heterometric ↑ force) —> shift along CFC
   Overall: new intersection point: ↑CO + ↑RAP
2. Contractility
   - ↑ contractility —> CFC shift upwards (hemeometric ↑ force) —> ↑ CO
   - ↑ contractility —> ↑ SV —> ↓ residual volume (NOT diastolic volume) —> ↓ RAP
   - ↓ RAP —> ↑ P gradient —> ↑ venous return (shift along VFC)
   - no change in VFC
   Overall: new intersection point: ↑CO + ↓RAP
3. TPR
   - ↑ TPR —> ↓ venous pressure —> ↓ venous return (VFC slope decrease)
   - ↑ TPR —> ↑ arterial pressure —> ↓ P gradient (ventricular - artery) —> ↓ SV (***but does not ↑ end diastolic volume because ↓ venous return) —> CFC slope decrease
   Overall: new intersection point (shift vertically downwards): CO ↓ + no change in RAP