Special Circulation

Question 1

Describe the distribution of cardiac output

Answer 1

About 80% at rest goes to the GI tract, kidneys, skeletal muscle, heart, brain. With most going to kidneys and GI tract

There is a range of blood flow for each organ that represents the vasodilator reserve for that organ

Question 2

What are the phasic changes in coronary blood flow during heart contraction?

Answer 2

Most of the blood flow to the myocardium occurs during diastole due to muscle compression on the coronary vessels during systole

Question 3

How can increased heart rate reduce coronary perfusion with diseased coronary vessls?

Answer 3

Length of diastole is shortened at higher heart rates, normally coronary arteries dilate to compensate

If they are diseased then their vasodilator reserve is limited, leading to myocardial ischemia and anginal pain with high heart rate

Question 4

How is coronary blood flow primarily regulated?

Answer 4

Changes in tissue metabolism

Production of adenosine and NO, both vasodilators

Question 5

Describe the unique sympathetic control of coronary blood flow

Answer 5

Unlike other vasculature, activation of sympathetic nerves causes only transient vasoconstriction followed by vasodilation

Metabolite production overrides sympathetic stimulation within seconds

Question 6

What is functional sympatholysis?

Answer 6

Term used to describe the functional relationship between sympathetic and metabolic control of coronary blood flow

Question 7

How can insufficient coronary blood flow be mitigated in coronary artery disease?

Answer 7

Collateral vessels increases myocardial blood supply, helping to supply blood flow to ischemic regions

Question 8

What are the four major arteries supplying the brain?

Answer 8

Two internal carotid

Two vertebral

Question 9

Why does increased intracranial pressure cause lower cerebral perfusion?

Answer 9

Collapses veins

Decreases effective CPP and reduces blood flow

Question 10

How is cerebral blood flow regulated?

Answer 10

Highly related to metabolism of tissue

Increased blood flow in response to excess CO2 or H+

Large decrease in arterial PO2 will increase cerebral blood flow

Question 11

Describe the autoregulation of cerebral blood flow

Answer 11

Brain has excellent autoregulation between 60 and 130 mmHg

With chronic hypertension, autoregulation curve shifts to protect the brain at higher pressures

Question 12

What is the role of the sympathetic nervous system in controlling cerebral blood flow?

Answer 12

There is innervation of the larger cerebral vessels

However it has relatively little influence because autoregulation will override its stimulation

Question 13

Describe the microvascular organization in skeletal muscle

Answer 13

Highly organized

When not contracting, only about 25% of the capillaries are perfused

In contrast, during contraction and active hyperemia, all of the capillaries may be perfused

Question 14

Describe muscle blood flow during contraction

Answer 14

With rhythmic contraction (e.g. locomotion) the mean blood flow increases, however flow decreases during contraction and increases during relaxation

A sustained contraction decreases mean blood flow, followed by a post-contraction hyperemic response

These effects are due to compression of the vessels

Question 15

How is skeletal muscle blood flow regulated?

Answer 15

Local regulation during activity (metabolic mechanisms)

Sympathetic generated vascular tone during resting

Question 16

What is the splanchnic circulation?

Answer 16

Blood flow to the GI tract, spleen, pancreas and liver

Question 17

How is the splanchnic circulation arranged and what is its significance?

Answer 17

The liver circulation is in series with the GI, splenic, and pancreatic circulation

This means that changes in blood flow in these vascular beds will have a significant effect on hepatic flow

Question 18

What are the main two sources of hepatic blood flow?

Answer 18

Hepatic artery - 75% of oxygen source

Portal vein - 75% of total blood flow

Question 19

What is the effect of gut activity and metabolic factors on GI blood flow?

Answer 19

During active absorption of nutrients, blood flow to the GI tract is increased

Metabolic control

Question 20

What is the effect of sympathetic stimulation on GI blood flow and why is this significant?

Answer 20

Causes intense vasoconstriction and greatly decreased blood flow

Allows blood to be shunted to more vital organs during exercise and shock

The splanchnic circulation functions as a venous reservoir

Question 21

Describe the shunt system in fetal circulation

Answer 21

Intracardiac: foramen ovale

Extracardiac: Ductus arteriosus and ductus venosus

Designed so that the most highly oxygenated blood is delivered to the myocardium and brain

**Study the slides for specific path of blood shunting

Question 22

Where does the exchange between the fetal circulation and maternal circulation take place?

Answer 22

The chorionic villi (fetus) and intervillous space (maternal) at the placenta

The maternal and fetal blood does not mix

Question 23

What is the combined cardiac output or combined ventricular output?

Answer 23

Sum of the outputs of the right and left ventricles

Refers to the cardiac output of the fetus because the right and left hearts pump in series rather than parallel

Right output is slightly larger than left

Question 24

Why is the fetal pulmonary circuit high resistance?

Answer 24

Collapsed lungs and hypoxic vasoconstriction

Results in low blood flow to the lungs in the fetus (although some is retained for development)

Question 25

What is the arterial and venous pressure in a fetus?

Answer 25

Low, but steadily increases as development occurs

Low-resistance in systemic circuit

Question 26

What is the function of the ductus venosus?

Answer 26

Bypass liver circulation

This shunting is more prominent in early pregnancy

Question 27

What is the function of the ductus arteriosus?

Answer 27

Bypass pulmonary circulation

Contains substaintial smooth muscle in its vessel wall, and its patency is due to active relaxation caused by circulating substances (progtaglandin E2)

Distribution between pulmonary circuit and systemic circuit depends heavily on the impedance of the pulmonary vasculature

Question 28

What results from closure of the placental circulation?

Answer 28

Represents a major parallel path in the systemic circulation, accounting for the low vascular resistance of the fetal systemic circulation

Total peripheral resistance increases at birth, when this circulation disappears

Question 29

What results from the opening of the pulmonary circulation?

Answer 29

Expansion of the lungs due to increase in PO2 from breathing significantly decreases pulmonary vascular resistance

Question 30

When does the ductus venosus close?

Answer 30

Functionally closed within minutes of virth due to cessation of blood flow

Obliterated in 1-2 weeks

Question 31

How does the foramen ovale close?

Answer 31

Left atrial pressure increases due to increased return from the lungs

Right atrial pressure falls due to decreased venous return from removal of the placental circulation

The pressure gradient reversal causes the valve of the foramen ovale to close, and gradually a permanent seal forms

Question 32

How does the ductus arteriosus close?

Answer 32

Two phases:

Functional closure within 24 hours after birth via smooth muscle constriction

Anatomical occlusion over the next few days or weeks

Question 33

What is patent ductus arteriosus?

Answer 33

Failure of the ducts arteriosus to close, resulting in a persistant shunt that causes flow from the aorta to the pulmonary circulation

This can lead to left-sided heart failure due to volume overload in the left atrium and ventricle

Question 34

What is Eisenmenger syndrome?

Answer 34

PDA with pulmonary vascular disease

Reversal of the shunt causing blood flow from the pulmonary artery to the descending aorta

Results in cyanosis of the lower extremities