BLOCK 2: BP and Hemodynamics

Lecture 2.1, 2.2, 2.3

Capillaries

0.1m/sec blood flow. 1mm long. 8um diameter

Capillary density is proportional to…

tissue’s metabolic activity

capillary bed

network of capillaries where exchange of materials with the tissue cells can take place

Tissue Cell

1.3 cell diameters from capillary

Routes for Capillary Exchange

1. Intercellular Clefts

2. Endothelial Cell Membranes

3. Fenestrations

Intercellular Clefts/Pores

water and most small substances can diffuse through pores/clefts

Endothelial Cell Layer

some small molecules and gases can diffuse/ be transported across the endothelial cell layer

Fenestrations

large molecules can pass easily through fenestrations

Continuous Capillary

in lungs, skeletal muscle, & connective tissue

Fenestrated Capillary

(“little window”) 70-100 nm. in kidneys, endocrine glands, small intestine

Sinusoidal Capillary

in liver, spleen, bone marrow & anterior pituitary gland

Diffusion

movement of molecules/ions from high → low conc until equilibrium is reached. PASSIVE.

Bulk Flow

movement of fluid from regions of high → low pressure. PASSIVE

Transcytosis

substances enter pinocyotic vesicles, move to endothelial cells via endocytosis, and exit on opposite side via exocytosis. ACTIVE

Push Fluid OUT of Capillary

1. Blood Hydrostatic Pressure

2. Interstitial Fluid Osmotic Pressure

Blood Hydrostatic Pressure

pushes fluid out through capillary pores

Interstitial Fluid Osmotic Pressure

pulls fluid out via osmosis. IFOP smaller compared to BHP

Pull Fluid INTO Capillary

1. Blood Colloid Osmotic Pressure

2. Interstitial Fluid Hydrostatic Pressure

Blood Colloid Osmotic Pressure

result of differences in protein concentration between plasma and ISF; tends to pull water

from ISF and into capillaries

Interstitial Fluid Hydrostatic Pressure

due to the pressure exerted by interstitial fluid; is normally very small

NFP EQUATION

NFP = (BHP + IFOP) - (BCOP + IFHP)

Net Filtration Pressure

net movement of fluid is driven by the difference between the inward and outward pressures. NFP = Filtration (Outward) - Reabsorption (Inward)

Filtration

results in a constant flow of fluid that washes over the tissue cells at the arterial end of the capillary, carrying nutrients and oxygen with it

Reabsorption

results in a return of fluid to the capillary at the venous end, thereby depositing wastes into the venous system.

Fluid in Capillaries

3L/day

Hemodynamics

the study of fluid flow in the vascular system

Resistance to fluid flow

caused by friction between the molecules in the fluid and the walls of the tub. Frictional resistance always reduces flow.

Driving Pressure

pressure difference required for blood flow.

What happens if BPs are equal?

No blood flow

Blood Pressure

pulsatile in sync with heartbeat. Peaks during systole & falls during diastole.

Mean Arterial Blood Pressure Map

MAP = DBP + 1/3 (SBP – DBP)

Arterial Blood Pressure

reflects the elasticity and the amount of fluid forced into arteries closest to the heart

Elastic Aorta

stretches with ventricular contractions

recoil with ventricular relaxation

Arterial Compliance

volume change for any change in pressure is relatively larg

Arterial Compliance Equation

C = dV / dP

Pulse

pressure wave created by high compliance & elasticity) allow flow to be propelled throughout cardiac cycle, owing to alternating expansion and recoil of arteries after each contraction of the left ventricle

Pulse Pressure

reflects stroke volume if C is constant

PP Equation

PP = SBP - DBP

Pulmonary Circulation

Very compliant. Pressures are lower than systemic circuit

Mean Pressure Pulmonary Artery

~14 mmHg at rest. (SBP = 24 mmHg; DBP = 9 mmHg)

Mean Left Atrial Pressure

~5 mmHg

Pressure Gradient

9mmHg (14-5 = 9)

Vascular Resistance

Blood flow is proportional to the driving pressure, and inversely proportional to the “resistance” to flow: Sum of all forces that retard flow. Flow = driving pressure / Resistance

Resistance Calc

R = (viscosity) (vessel length) / radius^4

Cardiac Output Formulas

1. CO = SV x HR

2. CO = pressure gradient/R

Cardiovascular Center

located in medulla oblongata and helps regulate HR, SV, and blood vessel diameter

INPUT to CV CENTER

Higher Brain Systems: limbic system, cerebral cortex, hypothalamus

Proprioceptors: position (joint movements)

Baroreceptors: blood pressure

Chemoreceptors: blood acidity

increased frequency of sensory nerve impulses

OUTPUT to CV Center

Heart: decrease (vagus nerve) or increased rate & contractibility (cardiac accelerator nerves)

Cardioacceleratory Center

collection of sympathetic neurons that increase HR and contractility

Cardioinhibitory Center

collection of parasympathetic neurons that decrease heart rate

Vasomotor Center

collection of “vasomotor neurons” that regulate blood vessel diameter via sympathetic nerves that synapse on arteriolar smooth muscle and cause vasoconstriction.

Chemoreceptors

located in ascending aorta (signal w cranial nerve X) & carotid sinus (cranial nerve IX)

activate the sympathetic division, leading to increased HR, SV, and vasoconstriction. Responds strongly to hypoxia/elevated CO2

High Pressure Baroreceptors

located in carotid sinus (cranial nerve IX) & Aortic Arch (cranial nerve X)

Low Pressure Baroreceptors

ocated in the walls of the right atrium & vena cava (cranial nerve X)

Baroreceptors response to low BP

Low BP → baroceptors in aortic arch stretch less → decreased nerve impulses to CV center → increased sympathetic stimulation → increased HR, BP, SV, vasocontriction

Barorecptors response to high BP

High BP → arteries stretched → baroreceptors increase firing rate → cardioinhibitory center stimulate & vasomotor center inhibited → reduce HR & BP