Sonoma State University
Department of Biology - Hanes
Animal Physiology
Application
Discussion
Reflection
General Plans for Circulation
Many animals, like earthworms, move blood by peristaltically contracting blood vessels. Peristalsis is a contraction preceded by relaxation so that the internal fluid moves along in one direction as in the earthworm p 474. The vessels in more sophisticated animals may have valves, which are flaps of endothelium shaped like a hammock with one edge attached to the vessel wall. Two of these flaps on opposing walls catch the blood and prevent its return. Mammalian hearts start out in just this manner (as contracting tubes), but then begin to fold into the four chambered heart.
Most invertebrates have open circulatory systemsas the clam or crayfish p 474. In this systems hearts pump blood into arteries which distribute blood to different parts of the body. Then the vessels open up to the interstitial space and the blood becomes lymph. The blood settles in the lower parts of the animal where it is picked up by an open vein & sucked up to the heart again. Open systems have lower pressures and less of an ability to distribute blood to needy areas.
Cephalopods, large annelids and vertebrates have closed circulatory systems. In these systems blood is always contained in blood vessels. (Fig 12-2 p 474) Fluid filtered from blood becomes interstitial fluid and it contains very little protein. Pressures are higher in these systems, circulation is faster, blood volume less and control of blood flow more precise. Since fluid is lost to the interstitial system, there must be a means of getting fluid back into the circulatory system. What are the advantages of either the closed or open circulatory systems?
Blood flows from the sinus venosus into the right atrium (no valve). Then with the contraction of the atrium, into the right ventricle through the right atrio-ventricular valve (tricuspid); upon ventricular contraction through the right semi-lunar (pulmonary) valve into the pulmonary arteries. Blood returns to the left atrium where there is no valve and upon its contraction through the left atrio-ventricular valve (bicuspid) to the left ventricle; thence through the left semi-lunar (mitral or aortic) valve to the aorta.
Electrical Activity of the Heart
We have already discussed the action potentials of cardiac muscle with its Na+ gates and Ca++ gates and Ca++ controlled K+ gates.Cardiac muscle is composed of two cell types. One has lots of glycogen, very little contractile protein, and is specialized to conduct action potentials (AP) from cell to cell rapidly. The other cell type is contractile muscle and conducts AP rather slowly. Cardiac muscle cells are interconnected by gap junctions (in this tissue called intercalated disks), thus when one cell produces an AP, it is passed to all connecting cells. The frog pacemaker activity occurs via a K+ gate that reduces permeability during diastole. The resulting depolarization can set off Na+gates and a new cycle occurs. The fastest cycling cells will thus control the rate of the heart beat. The fastest cycling cells are near the sinus venosus and this group of Purkinje cells is called the sino-atrial node (SA node). Atrial tissue conducts at 0.8 M/s and the wave goes from right to left. The electrical disturbance that can be picked up on the skin and caused by this traveling wave is called the P wave of the electrocardiogram (ECG). Valves are made of connective tissue and do not transfer AP's. The only muscular connection between the atria and ventricles is the atrial-ventricular node (AV node), again composed of Purkinje fibers. These cells conduct slowly (about .05 M/s) and account for the delay between atrial and ventricular contractions. Two bundles of Purkinje cells run down the septum of the heart and one branches to each of the ventricular outer walls. These are called bundles of His and they conduct very rapidly (4-5 M/s) so that all of the ventricular cells will contract nearly together. The first place that you can see the ventricular contraction is at the apex and the contraction wave goes up the heart. The electrical activity of the ventricular AP wave at its turns produces the QRS waves of the ECG. Ventricular muscle cells conduct at about 0.5 M/s. Repolarization of the ventricular mass produces the T wave of the ECG.
Frank-Starling "Law of the Heart" p 483
The Law of the Heart states that the normal heart will maintain a blood pressure of 0 at the sinus venosus no matter how much blood is returned to the heart. In other words, the heart adjusts its pumping rate to the rate of blood return. It adjusts to the amount of returning blood in the following ways:
The amount of blood a heart pumps per minute is called the minute volume or cardiac output. The amount pumped per beat is the stroke volume. The amount of blood that returns to the heart is called the venous return. Review Fig 12-12 p 485.
Comparative Circulatory Patterns of Vertebrates
Carefully observe the differences between fish circulation, amphibian circulation and mammal circulation. pp 489, 491,492, and I will draw mammal.
Note the modifications of a fetal heart p 495.
Note the charts of blood velocity on p 490 and blood pressure p 497.
Note the structure of arteries, capillaries and veins p. 500, 507
Rete Mirabile -and counter-current exchanges p 505
The rete mirabile are lines of arterioles surrounded by venules in close contact. These may be found in the blood supplies to mammalian testes, tuna muscles, seal flippers, moose feet, duck feet, etc... The purpose here is to produce a counter-current flow that allows heat transfer between arterioles and venules so that outer appendages remain cool, while the interior body conserves its heat. A rete may also be found before the gas gland of fish to maintain high O2 levels in the gland.
Starling's "Law of the Capillary" - p 509
The Law of the Capillary states that in a capillary bed there is an outflow of fluid from the arteriolar end of capillaries. The fluid circulates around cells and returns to the capillaries near their venule ends.
There are a number of forces acting to force fluid through the endothelial cell of the capillary. These are:
|
Outward Flow |
Inward Flow |
|
Blood pressure (hydrostatic) |
Plasma Protein Osmotic Pressure |
|
Gravity pres. (if below heart) |
Gravity Pres. (if above heart) |
|
Interstitial Protein Osmotic Pres. |
Tissue Elasticity |
|
Outward Flow |
||
|
|
At Arteriolar end of capillary |
At Venule end of capillary |
|
BP |
35 torr |
15 torr |
|
PPOP |
-25 |
-26 |
|
IPOP |
1 |
1 |
|
TE |
-3 |
-3 |
|
|
9 torr outward |
-13 torr outward |
Of course, small amounts of protein do leak through the epithelium or are given off by parenchymal cells. This protein could not get back to the blood stream if it were not for lymphatic drainage. Whenever fluid or pressure build up outside the circulatory system, blind-ended lymph vessels have a valve that flaps inward and allows fluid entrance and even materials as large as cells. Valves prevent what is trapped within the lymph vessels to return to interstitial fluid.
From your recently acquired knowledge, what would happen if blood pressure rises in the capillaries? If venous pressure rises and backs fluid up in the capillaries? If the lymph channels are blocked by filarial worms? If diet or liver damage prevents synthesis of much blood protein as in Kwashiorkor? Right! ... edema.
Nervous and hormonal
Epinephrine from the adrenal gland or from sympathetic nerves can constrict sphincter muscles around arterioles or in some cases the arterioles relax (arterioles in skeletal muscle) from epinephrine stimulation.
Local control
Just before an arteriole splits into many capillaries, one last smooth muscle cell surrounds it. This is the precapillary sphincter. It is on the outside of the blood vessel along with the other tissue cells and it is sensitive to low O2, acid, high CO2, and high K+. All of these tend to make the sphincter relax and allow blood flow through the capillary bed. There opposites cause contraction and cut off capillary blood flow. Most of the time capillary beds are shut off; only about one in six is open. They may open briefly several times a minute. Capillaries are responding to the needs of the cells they supply because one of these cells is the precapillary sphincter.
What new insight does this give you about Starling's Law of the Capillary?
CNS Control of Blood Flow
The CNS monitors blood pressure in the large arteries by stretch receptors in the aorta and carotids. What would happen if these were to stiffen? Small bodies along the aorta and at the branch of the carotid arteries have large blood flows and nerves that detect pO2, pH, and pCO2. This information is carried to the pons-medulla area. There are also cells lining the floor of the medulla that are sensitive to pCO2 in the cerebral-spinal fluid flowing by. The CNS can control arterial and arteriolar contraction, venous contraction and thus blood storage, and heart rate.
Kidney Control of Blood Flow
The other organ with major control mechanisms for blood pressure is the kidney. Blood flow is somehow monitored by the juxtaglomerular apparatus. It can cause the endothelium of the afferent arterioles to release the hormone renin (not rennin the milk digesting enzyme). Renin cleaves a piece off angiotensinogen (a normal plasma protein synthesized by the liver) to convert it to Angiotensin I. Angiotensin I is converted by an enzyme in the lung to angiotensin II. Angiotensin II is one of the most potent stimulator of vascular smooth muscle known, thus raising blood pressure. Angiotensin II also causes the release of aldosterone, an adrenal cortex hormone that increases Na+ retention and thus builds blood volume.
Cardiovascular Responses to Exercise
Exercise causes an immediate increase in venous return. Muscles contracting push venous blood toward the heart.
Sympathetic nervous stimulation is increased causing: decreased blood flow to skin and gut, increased blood flow to skeletal muscle, contraction of spleen and large storage veins like the splanchnic vein, faster heart rate.
The results of this are that during exercise stroke volume, pH, pCO2, and pO2 of arterial blood remain nearly the same. Minute volume may increase by about 5X maximum. At this level however, stroke volume would have to increase. Limits to exercise levels are the ability to pump blood, not the ability to take in O2.
Cardiovascular Response to NO
Endothelial cells can also control blood flow. When endothelial cells take up more Ca++ as when stretched and Ca++ gates are opened, the synthesis of Nitric Oxide (NO) is promoted from a conversion of arginine to citrulline. NO passes on to local smooth muscle where it causes three effects. The effects are all due to a stimulation of guanylate cyclase activity converting GTP to cGMP.
Cardiovascular Responses to Diving
Diving mammals and birds have a reflex apnea caused by sensors in the nose and pharynx. Some of the responses are as follows: p 521
Blood Cotting p.522
Application
1. Explain the contraction cycle of the heart in relation to its
ECG.
2. Explain Starlings Law of the Heart. The capillary.
3. How is blood flow controlled at the capillary level? at the
arteriolar level?
4. What does the heart monitor to control blood flow? Can you think
of instances when the information is inadequate to sustain life?
Anemia drastic blood loss.
5. What is the importance of the lymph system?
6. How does circulation change during exercise? diving?
What is high blood pressure?
What is cardiac insufficiency?
Could you map out all of the factors that are involved in blood flow?