HBS204 - Week 3 - CV System Regulation

What is the primary variable that changes vessel resistance in the systemic circulation?

The radius of the arterioles

What is intrinsic (basal) tone?

Spontaneous, ongoing contraction of arteriolar smooth muscle that occurs independently of nerves or hormones

Which organs/tissues rely mainly on local control of blood flow, and which relies mainly on systemic control?

Heart, brain, skeletal muscle, lungs, and kidneys use local control; skin uses systemic control

Define active hyperemia.

An increase in blood flow that matches an increase in a tissue's metabolic activity, caused by local vasodilator buildup (e.g. low O2, high CO2/H+)

Define reactive hyperemia.

A surge in blood flow after a period of occlusion, caused by vasodilators that accumulated in the tissue during the blockage

A patient's finger has a rubber band removed after 2 minutes. Which type of hyperemia explains the redness, and why?

Reactive hyperemia — vasodilators built up in the tissue while blood flow was blocked, causing rebound vasodilation once flow resumed

What receptor does noradrenaline mainly act on in arterioles, and what effect does it have?

Alpha (α) receptors; causes vasoconstriction

Which tissues have beta-2 (β2) receptors on their arterioles, and what effect does adrenaline have there?

Heart, liver, and skeletal muscle; adrenaline causes vasodilation

During a fight-or-flight response, what happens to blood flow to the gut versus the skeletal muscles, and why?

Blood flow to the gut decreases (alpha-receptor vasoconstriction) while flow to skeletal muscle increases (beta-2 vasodilation), redirecting blood to where it's needed

What percentage of cardiac output do skeletal muscles receive at rest versus during exercise?

About 20% at rest, up to 85% during exercise

Define autoregulation of blood flow.

The ability of an organ to maintain a relatively constant blood flow despite changes in arterial pressure, without help from nerves or hormones

Compare the metabolic and myogenic theories of autoregulation.

Metabolic theory: high pressure washes out vasodilators, causing constriction back toward normal flow. Myogenic theory: stretching the vessel wall directly triggers smooth muscle contraction, independent of metabolism

Which three organs show the strongest autoregulation, and which shows almost none?

Brain, heart, and kidney show strong autoregulation; skin shows almost none

If arterial pressure drops below the autoregulatory range (below ~60-70 mmHg), what happens to the brain's blood flow?

The arterioles become fully dilated and can no longer compensate, so blood flow to the brain starts to fall along with pressure

Why are veins called capacitance vessels?

Because their thin, elastic-poor walls let them stretch and hold a large proportion (roughly 60-70%) of total blood volume

What is the approximate pressure gradient driving venous return, and why is it so small?

About 10-15 mmHg in peripheral veins down to ~0 mmHg at the right atrium, because most pressure has already been dissipated crossing the arterioles and capillaries

Name the three mechanisms that enhance venous return.

Venoconstriction (sympathetic activity), the skeletal muscle pump, and the respiratory pump

How does the skeletal muscle pump move blood toward the heart?

Contracting muscles squeeze the veins running through them, and one-way valves stop the blood flowing backward between contractions

During inspiration, what happens to pressure in the intra-abdominal veins versus the intra-thoracic veins, and why does this help venous return?

Intra-abdominal venous pressure increases (diaphragm descending) while intra-thoracic venous pressure decreases, increasing the pressure gradient that pushes blood toward the heart

Explain the Frank-Starling link between venous return and cardiac output.

Increased venous return fills the ventricle more (higher end-diastolic volume), which increases the force of contraction and therefore stroke volume and cardiac output

What are baroreceptors, where are they located, and what do they detect?

Stretch-sensitive receptors in the walls of the carotid arteries and aorta that detect changes in arterial blood pressure

Are baroreceptors normally silent or active at resting blood pressure?

Tonically active — they fire continuously even at normal blood pressure

List the baroreceptor reflex's stimulus, sensor, and integrating centre.

Stimulus: change in blood pressure. Sensor: baroreceptors (carotid and aortic). Integrating centre: medullary cardiovascular control centre

If blood pressure rises, what happens to sympathetic and parasympathetic output, and what is the net effect on heart rate, contraction force, and arteriolar resistance?

Sympathetic output decreases and parasympathetic output increases, causing decreased heart rate, decreased force of contraction, and arteriolar dilation (decreased resistance)

How quickly does the baroreceptor reflex correct a change in blood pressure?

Within about two heartbeats

What happens to baroreceptor firing rate if blood pressure stays elevated for several days?

The baroreceptors reset (adapt) to the new, higher pressure and reduce their firing rate at that pressure, so they no longer correct back to the original normal

Trace the sequence of events causing orthostatic hypotension when a person stands up.

Gravity pools blood in leg veins, decreasing venous return, which decreases cardiac output and therefore blood pressure

How does the baroreceptor reflex normally compensate for orthostatic hypotension?

Increased sympathetic activity raises heart rate, contraction force, and peripheral resistance, restoring mean arterial pressure within about two heartbeats

Why might standing up too quickly cause dizziness or fainting even in a healthy person?

The baroreceptor reflex may not have enough time to compensate for the sudden pressure drop, temporarily reducing oxygen delivery to the brain

What are the two Starling forces that control fluid movement across a capillary wall?

Hydrostatic pressure (pushes fluid out) and colloid osmotic pressure (pulls fluid in)

What is the formula for Net Filtration Pressure, and what does a positive versus negative result mean?

NFP = HP - OP; a positive value indicates net filtration (fluid leaves the capillary), a negative value indicates net reabsorption (fluid enters the capillary)

Why does hydrostatic pressure drop along the length of a capillary, while osmotic pressure stays roughly constant?

Hydrostatic pressure drops due to friction/resistance as blood moves away from the heart, while osmotic pressure stays constant because the plasma proteins causing it are too large to leave the capillary

At the arterial end of a capillary, hydrostatic pressure is 35 mmHg and osmotic pressure is 25 mmHg. What is the NFP and what does it mean?

NFP = +10 mmHg, indicating net filtration (fluid is pushed out of the capillary)

What happens to fluid that is filtered out of capillaries but not reabsorbed?

It is picked up by the lymphatic system and returned to the bloodstream

Name the three types of capillaries and give one location for each.

Continuous (skin, muscle, brain), fenestrated (kidney, small intestine, endocrine organs), sinusoid (liver, spleen, bone marrow)

Why do continuous capillaries in the brain form the blood-brain barrier?

Their tight junctions are complete and extend around the entire perimeter of the endothelial cells, preventing substances from passing between cells

Which capillary exchange process depends on lipid solubility, and which substances use it most easily?

Diffusion; lipid-soluble substances like oxygen, carbon dioxide, and fatty acids diffuse directly through the endothelial cell membrane

Why do large water-soluble substances like proteins generally stay inside continuous capillaries?

They are too large to pass through the tight junctions between endothelial cells or the cell membrane itself

List three causes of edema related to disrupted capillary filtration/reabsorption balance.

Increased capillary hydrostatic pressure (e.g. heart failure), decreased plasma protein concentration (e.g. liver disease or malnutrition), and increased interstitial protein concentration (e.g. inflammation)