32. properties of the circulation in the coronaries, skin, brain, splanchnic circulation and fetal circulation

Question 1

Circulation in the coronaries

Answer 1

• The coronary arteries arise from the aorta; they are separated into the left and the right coronary arteries.
• The right coronary arteries supply the right atrium and ventricle, it will continue down and supply also the posterior left ventricular wall and the posterior part of the interventricular septum. It is now known as the posterior descending coronary artery.
• The left coronary arteries divide into left anterior descending and left circumflex arteries. The descending one supplies the anterior septum and the anterior left ventricular wall. The circumflex supplies left atrium and ventricle.
• The conductive system is also supplied by the coronary arteries. Disease of the coronary arteries may therefore cause bardycardia, also stenosis of the left coronary arteries is extremely dangerous.

1. Beginning of systole: the tension of the left chamber of the heart is so high that the blood will be pressed out from the coronary vessels, then reversed blood flow will occur. Reverse flow will not occur in the right chamber, but it will have the same effect except from that.
2. Fast ejection phase: the high pressure in the aorta secures the flow in the coronary arteries.
3. Slow ejection phase: the pressure in the aorta drops, therefore the coronary perfusion slows down.
4. Diastole: more blood enter the coronary vessels (more than during systole), maximum coronary flow can be measured.

Question 2

Circulation in the brain

Answer 2

• It is very important that the venous and arterial blood flow to and from the brain is equal; if not the brain tissue could suffer from overpressure. The brain tissue is also very sensitive to hypoxia; therefore there are local reflex mechanisms that keep the blood flow constant despite altering peripheral circulatory situations.
• Cushing-effect: is when the intracranial pressure increases, then the peripheral blood pressure has to increase as well to keep the constant blood flow to the brain. This is most likely caused by hypoxia in the compressed vasomotor center.
• The constant blood flow should be 50ml blood/min/100g of tissue.
• Blood flow in the brain can be altered by the pCO2 and pH, low pH will increase blood flow. The facial nerve provides parasympathetic innervations of the brain vessels. The brain can tolerate a change in the mean blood pressure between 60 – 160mmHg, anything over will cause edema, and anything under will cause syncope (fainting, collapse).

Question 3

Circulation in the skin

Answer 3

• low metabolic demand
• crucial area of thermoregulation
• flow rate varies in a very large range
• vasoconstrictor tone is of major importance
• A-V anastomoses and arterioles regulate
• Vessel reflexes of the skin is of diagnostic importance

Question 4

Splanchnic Circulation

Answer 4

• Two serially attached capillary systems: portal circulation
• Myogenic tone is almost non existent
• The main regulator is the sympathetic tone (alpha receptor)
• Metabolic autoregulation is less developed
• In the liver, myogenic autoregulation occurs
• The splanchnic area (liver) serves as a reservoir, 15% of the circulating blood volume resides here in resting conditions

Question 5

Fetal circulation

Answer 5

• The fetus receives the oxygenated blood through theumbilical vein. This blood is saturated with oxygen by 85% _
• Most of the blood reaches the fetal liver, while a small part of it reaches the heart directly through the v. cava caudalis.
• Finally blood goes from the liver to the right ventricle. Left and right atria communicate with each other through the foramen ovale.
• In the fetus the left and right ventricles work as a parallely coupled system, as most of the blood pumped out from the right ventricle circumvents the resistant lung tissue.
• Blood goes through the ductus arteriosus into the aorta and is then added to the systemic circulation.
• This way the pressure in the a. pulmonalis isapproximately 5 mmHg higher that in the aorta. The left ventricle has only a 20 % larger volume than the right one.
• One third of the blood from the aorta goes to the cranial part of the body, while the rest is delivered to the caudal one. Half of this latter blood volume goes through the placenta.
• After delivery a sudden increase of the pulmonary circulation sets in. The pulmonary resistance decreases dramatically because of the appearance of surfactant factors after the onset of breathing.
• More and more blood goes through the lung.
• Pressure decreases in the right atrium as compared to the left one, which leads to the closure of the foramen ovale. Pressure decreases in both the arteria pulmonaris and the ductus arteriosus.
• The pressure drop elicits prostaglandin liberation, which closes the ductus arteriosus.
• Muscles of the left ventricle develop rapidly and finally the serially coupled pulmonary and systemic circulation evolves.