Chapter 20: Blood Vessels and Circulation

*Things to know about the tunica interna*

1. A protein carbohydrate layer, the basal lamina, is between the endothelium and the basement membrane, which helps anchor the membrane to the endothelium. 2. The endothelium acts as a selectively permeable barrier to materials entering or leaving the bloodstream; it secretes chemicals that stimulate dilation or constriction of the blood vessels; and it repels blood cells and platelets so that these can flow freely without sticking to the vessel wall. *If the endothelium is damaged, however; platelets may adhere to it and cause a blood clot.

What is the second mechanism of movement through the capillary wall?

2. Transcytosis: Endothelial cells pick up material on one side of the plasma membrane by pinocytosis or receptor-mediated cytosis, transport the material in vesicles across the cell and discharge the material on the other side by exocytosis. This movement is important for the movement of fatty acids, albumin, and some hormones such as insulin. Can be in either direction: from the blood to the tissue fluid or from the tissue fluid to the blood.

What is the third mechanism of movement through the capillary wall?

3. Filtration and reabsorption. Hydrostatic pressure takes the fluid out of the blood, (filtration), as fluid leaves the capillaries, while osmotic pressure brings the fluid into the blood (reabsorption), as fluid enters the capillaries.

What is angina pectoris?

A partial obstruction of coronary blood flow, which can cause chest pain. It results from temporary and reversible ischemia, which is a deficiency of blood flow to the cardiac muscle.

What is the portal system?

A system where blood flows through two consecutive capillary networks or beds before returning to the heart. They occur in: 1. the kidneys; 2. connect the hypothalamus to the anterior pituitary gland; 3. and connect the intestines to the liver. For example, because all blood must go through the liver, portal systems connect the capillary beds in the intestines to the capillary beds in the liver.

What are veins?

Afferent vessels that carry blood towards the heart. They start to merge as the blood returns to the heart. They are looser than arteries, which have a more fixed shape.

What are anastomoses?

An anastomosis is a point of convergence between two blood vessels other than capillaries.

What is an arteriovenous anastomosis?

An arteriovenous anastomosis is a shunt, where blood flows from an artery directly into a vein, bypassing the capillary bed. They are seen in extremities and are important in fingers, palms, toes, and ears, where they reduce heat loss in cold weather by allowing warm blood to bypass these exposed surfaces (bypassing capillary beds). The shunt also protects major internal organs, which buys a person's time to get to a warmer state during cold temperatures.

What are arteries?

Efferent vessels that carry blood away from the heart. They start to split into smaller branches as the blood leaves the heart.

What are arterial anastomoses?

Two arteries merge when they deliver blood to the same place. They also provide alternative routes of blood supply to a tissue. Found in the coronary circulation and around joints where limb movement may obstruct an artery's pathway.

What is capillary exchange?

It refers to the two-way movement exchange between blood and surrounding tissue through the walls of the capillaries. The most important blood in the body is in the capillaries, even though only 5% of the blood in the body is in them.

What are capillaries? (detailed)

Materials are exchanged between the blood and the tissue fluids through capillaries. They consist of endothelium and a basement membrane (with its basal lamina). Red blood cells are bigger than capillaries, so they need to straighten out to be able to squeeze through the smallest capillaries. *Nearly every cell in the body is close to a capillary*

What are capillaries?

Microscopic, thin walled vessels that connect the smallest arteries to the smallest veins.

What are chemicals that leave the capillary blood to enter the tissues?

Oxygen, electrolytes, glucose, amino acids,, lipids, other organic nutrients, antibodies, and hormones.

What are metarterioles?

Short vessels that connect arterioles to capillaries. In here, muscle cells are spaced a short distance apart, each forming a precapillary sphincter that encircles the entrance to one capillary. Gas exchange occurs in the capillaries, as they have no layers.

Hydrostatic and osmotic pressures are

opposing forces. This is how protein-free fluids move through capillaries. The shift in fluid movement direction (from arterial to venous end) is a result of change in the difference between hydrostatic and osmotic pressure.

What is myocardial infarction (heart attack)?

The long term obstruction of coronary circulation, which causes cardiac cell death in the affected area. When heart muscle cells die, they can lead to a heart attack. This obstruction is usually due to a fat deposit or a blood clot, leading to no blood flow to the heart for a period of time, which then causes tissue death, eventually leading to a heart attack.

*Circulatory routes*

The most common route of blood flow is heart-arteries-capillaries-veins-heart. Blood usually passes through only one network of capillaries from the time it leaves until it returns, but exceptions include, such as portal systems and anastomoses.

What is hydrostatic pressure?

The physical force exerted by a compressed fluid against a surface, (including the pressure on the fluid itself), such as a capillary wall. Blood pressure is one example.

What is osmotic pressure?

The pressure due to the movement of water from a low concentration of non penetrating solutes (and a high H2O) to a high concentration of non penetrating solutes (and a low H2O). *The pressure is driven by the amount of solute, not by the amount water. *

What are arterioles?

The smallest arteries, which control how much blood an organ or tissue receives.

What are venules?

The smallest veins

What is atherosclerosis?

The stiffening of the arteries, lessening their recoil, which causes stress to the arteries, increasing the chance of an aneurysm. If this aneurysm occurs in a weak point in an artery or in the heart wall, it can cause a rupture.

What is the most external tunic?

Tunica externa: outermost layer. It consists of loose connective tissue that often merges with neighboring vessels/organs. It also anchors the vessel to adjacent tissues and provides a passage for small nerves, lymphatic vessels, and smaller blood vessels that supply the tissue of larger vessels. This layer also helps nourish the vessel's walls.

What is the most inner tunic?

Tunica interna: lines inside the vessels and is exposed to blood. It consists of a simple squamous epithelium called the endothelium. The endothelium sits on top of a basement membrane, which also lies on a loose connective tissue (typically areolar tissue). From deepest to superficial: endothelium-basement membrane-connective tissue. This connective tissue anchors the tunica interna to the tunica media.

What are chemicals that enter the capillary blood from the tissues?

CO2, ammonia, and other metabolic waste that needs to be removed.

What are capillary beds?

Capillaries are organized into weblike networks called capillary beds. They are supplied from a single metarteriole (de aqui salen todas las capillaries). The metarteriole connects the bed to the arteriole and as the blood goes from arteriole to venule, the metarteriole becomes a thoroughfare channel, connecting the capillary bed to the venule. Capillaries can either empty into the distal end of the thoroughfare channel or directly into the venule. Precapillary sphincters control which beds are well perfused with blood.

What are the fenestrated capillaries?

Capillaries that have endothelial cells with many filtration pores, called fenestrations. They allow for rapid passage of small molecules, but still block most proteins and large molecules. The basal lamina helps stop the movement of large molecules through the fenestrations. These capillaries are important in organs that engage in rapid absorption or filtration such as in the kidneys, endocrine glands, and in the small intestine.

What is the middle tunic?

Tunica media: the middle and usually, the thickest layer. It consists of smooth muscle, collagen, and in some cases, elastic tissue. The relative amounts of smooth muscle and elastic tissue varies greatly from one vessel to the next, so the middle layer provides a basis for classifying vessels. The smooth muscle in this layer causes vasomotion (changes in a blood vessel's diameter), thus it determines whether the vessel will enlarge or constrict. The tunica media also strengthens the vessel and prevents it from rupturing due to BP.

What are walls of arteries and veins made of?

Tunics

In terms of pressure, what happens at the venous side? )

*Two kinds of hydrostatic pressure:* 1. Blood pressure 2. Interstitial fluid The positive BP inside the capillary and the negative interstitial pressure produce a net outward pressure of: 10 mmHg (due to BP) and 3 mmHg that's been suck out by interstitial fluid = 13 mmHg. *Osmotic pressure (opposing hydrostatic):* The net oncotic pressure at this side is also 20 mmHg in. These opposing forces create a net reabsorption rate of 7 mmHg in (20 COP-13 due to hydrostatic). *More fluid leaves than what enters*

What are venous sinuses?

*They have no equal in arteries* They are veins with especially thin walls, large lumens (capillaries) and no smooth muscle. They do not exhibit vasomotion. *The majority of vasomotion occurs in arteries anyway, but some veins may exhibit it too*

In terms of pressure, what happens at the arterial side?

*Two kinds of hydrostatic pressure:* 1. Blood pressure: (the major one, where the heart compresses the blood, creating BP). *Seen in the arterial end.* 2. Interstitial fluid (ECF): helps draw blood out of the capillary (sucks it out). At the arterial end, the positive hydrostatic pressure within the capillary pushes the fluid out, favoring filtration, and the negative interstitial pressure draws blood out as well, so they both create a net outward force of 30 mmHg (by BP) + 3 mmHg = 33 mmHg. The blood is then directed towards the venule. *Osmotic pressure (opposing hydrostatic):* 1. Colloid osmotic pressure opposes the hydrostatic pressure by drawing fluid (blood) into the capillary by osmosis. The blood has a COP of 28 mmHg, while interstitial fluid (ECF) has a COP of 8 mmHg (so fluid moves from a low solute concentration, ECF, to a high solute concentration (capillary). The difference between the blood COP and the interstitial fluid COP is the oncotic pressure, which is 28 in - 8 out = 20 in. *COP is more concentrated in the blood due to the plasma proteins, such as albumin, found in it. COP then is created because proteins want to leave the capillary but as they cannot, water leaves instead through osmosis.* However, net COP (20 in) is the same at both ends of the capillary. These opposing forces create a net filtration rate of 13 mmHg out (33 due to hydrostatic- 20 COP).

What are the two kinds of receptors?

1. Baroreceptors 2. Chemoreceptors

Why are capillaries able to do reabsorb 85% of the fluid?

1. Capillaries branch along their length, which means that there are more of them at the venous end which reabsorb than at the arterial end. 2. They also have nearly twice the diameter at the venous end than at the arterial end, so there is more capillary surface area available to reabsorb fluid.

Three types of arteries:

1. Conducting (elastic or large) arteries 2. Distributing (muscular) arteries 3. Small arteries

What are the types of capillaries?

1. Continuous 2. Fenestrated 3. Sinusoid

What is the first mechanism of movement through the capillary wall?

1. Diffusion: The most important mechanism for exchange. It is characterized by the movement from high to low concentrations. Lipid soluble substances that can go through the membrane are steroid hormones, O2, CO2, which diffuse easily. Lipid insoluble substances, such as glucose and electrolytes, must either pass through membrane channels, pores (fenestrations) or intercellular clefts. Large molecules are held back.

Give three reasons why flow peaks in coronary arteries when the heart relaxes:

1. During diastole, the myocardium does not squeeze the coronary arteries, thus it allow for blood to come into them. 2. When the ventricles relax, the aortic valve closes and its cusps don't cover the opening of the coronary arteries, allowing blood to flow into them. 3. During relaxation, blood flow in the aorta falls back towards the heart, filling the cusps of the closed aortic valve, with some blood flowing into the coronary arteries (like filling a bucket (valve) that has holes in it that lead to the coronary arteries).

Process of the coronary circulation

1. It begins as the blood leaves the aorta. Here, arterial supply begins. The aorta branches off into the right and left coronary arteries, which lead the blood back to the heart. The openings of both arteries lie deep in the pockets formed by two of the aortic valve cusps. 2. Blood flow increases in the coronary arteries during ventricular relaxation (unlike other arteries). While the heart is resting, the valves are locked, allowing blood to go to the coronary vessels during diastole (heart relaxation). *In other organs, blood flow usually peaks when the heart contracts and ejects blood into arteries, while diminishing when the heart relaxes, filling the ventricles. The opposite is true for coronary arteries: Flow peaks when the heart relaxes.*

*Things to know about capillary activity:*

1. It varies from moment to moment. For example, in a resting tissue, most precapillary sphincters are constricted, which makes for a low BP, thus reabsorption predominates. In a metabolically active tissue, on the other hand, its capillary flow and BP increases, thus filtration predominates along the entire length of the capillary.

What are the types of venules?

1. Post capillary venules 2. Muscular venules

Which circumstances affect capillary exchange?

1. The kidney has high BP capillary networks called glomeruli, which are dedicated to filtration only. 2. Alveolar capillaries in the lungs have low BP capillary networks that are almost entirely dedicated to absorption, so fluid does not fill the air spaces. *Thus, not all capillaries have to have both features*

*Things to know about filtration and reabsorption in a capillary*

1. The only pressure that changes significantly from the arterial end to the venous end is the capillary BP, which is the change that leads from filtration to reabsorption. 2. Even though more fluid leaves than what is reabsorbed, the capillaries are able to absorb about 85% of the fluid, while the lymphatic system reabsorbs the other 15%. 3. *Not only water crosses the capillary wall. Chemicals dissolved in it are dragged along with it by bulk flow, a process called solvent drag, and they will pass the capillary if they are not too big. However, the total quantity of a substance moving in or out a capillary by net bulk water flow is small.

What are post capillary venules?

1. The smallest, most porous (even more than capillaries) type. They are the ones closest to the capillary bed. They are able to exchange fluids with the surrounding tissues through their pores. They receive blood directly from the capillaries, even though blood exchange may occur throughout the bed.

*Things to know about arteries*

1. They have a relatively strong structure that resists BP. They are designed to withstand the surges of pressure created by each heartbeat that pumps blood into them. They are also more muscular than veins, which is why they tend to have a round shape even if they're empty.

Chemicals pass through the capillary by three routes:

1: Through the endothelial cell membrane (those that are lipid-soluble) by diffusion or by transcytosis. 2. Through intercellular clefts, such as hydrophilic molecules that go through diffusion. 3. Through fenestrations (filtration pores) by diffusion.

*Things to know about capillaries*

At any given time, about 3/4 of the body's capillaries are shut down because there isn't enough blood to supply them all at the same time. Depending on the tissue activity level, capillaries are either open or close. For ex., in a more metabolically active tissue, capillaries will be open and the bed will be more perfused. During rest, there is little blood flow in the capillaries of the skeletal muscle, but it receives abundant blood flow during exercise, while those capillaries in the skin and intestines shut down to compensate.

Arterial sense organs

Certain major arteries above the heart have sensory structures in their walls that monitor blood pressure and chemistry changes. Then, they transmit this information to the brainstem (medulla and pons), which in turn regulates HR, vasomotion, and respiration.

What is the coronary sinus?

It is a large transverse vein located in the coronary sulcus, the posterior atrioventricular sulcus, which collects blood from several coronary veins. 90% of the venous blood that's been flowing around the heart tissue returns to the right atrium by the coronary sinus.

Coronary circulation

The blood flowing through the heart chambers is not immediately available for exchange with the heart tissue. Thus, the myocardium has its own supply of arteries and capillaries that deliver the blood to each muscle cell, called the coronary circulation, which is a system of blood vessels, exclusive to the heart wall, that delivers blood to the myocardium for exchange. At rest, the coronary vessels supply the myocardium with about 5% of circulating blood, even though the heart is only 0.5% of the body's weight. Thus, the heart receives a disproportionate amount of blood relative to its size, but it needs such amount to sustain its strenuous workload.

*Things to know about medium veins*

The tunica media in them is much thinner than it is in medium arteries. It consists of bundles of smooth muscle, with alternating layers of collagenous and elastic tissue. They exhibit internal valves that are directed towards the heart. Even though they work against gravity, the pressure in them is not enough to push the blood upward. Thus, when the muscles surrounding a vein contract, they force a blood through these valves, which lock when the same muscles relax, preventing the blood from falling backwards.

What are small arteries?

Their wall consists mostly of smooth muscle with relatively little elastic tissue.

What are sinusoid (discontinuous capillaries)?

They are irregular blood-filled cells in the liver, bone marrow and spleen. Their endothelial cells are separated by wide gaps and have no basal lamina, allowing large molecules and proteins to go through the fenestrations. In the liver, proteins can go through the gaps, while blood cells cannot. Thus, the liver is able to make protein pockets of albumin, which enter the blood through the capillarie's gaps. Also, newly formed blood cells from the bone marrow enter the blood through these capillaries.

What are distributing arteries?

They are medium size arteries, which distribute blood to specific organs, such as the femoral, brachial, renal and splenic arteries. Their smooth muscle constitutes about 3/4 of their wall's thickness (more than the conducting ones)

*Things to know about veins*

They are relatively thin-walled, have less muscular and elastic tissue, so they expand more easily to accommodate an increased volume of blood, thus they have a greater blood capacity than arteries do. They are so thin walled because they are subjected to relatively low blood pressure, as they are away from the ventricles of the heart, so there's no need for them to have thick walls. In large arteries, for example, BP is 90-100 mmHg and surges to 120 during systole, while veins BP is only 10 mmHg and fluctuates less. Furthermore, the blood flow in the veins is more steady, rather than pulsating with each heartbeat (like in the arteries). However, even though they can take in more blood, it's harder for them to push the blood out as they are weaker. They tend to collapse when empty, showing a flatter, irregular shape.

What are conducting arteries?

They are the biggest of all arteries. Ex., pulmonary trunk, aorta, common carotid, subclavian, common iliac. In these arteries, the tunica media consists of many perforated sheets (swiss cheese) of elastic tissue that alternate with thin layers of smooth muscle and collagen. These arteries expand during ventricular contraction to receive blood and recoil during ventricular relaxation. These arteries also exhibit partial resistance, useful because it allows for them to absorb energy from each heartbeat, expanding into a gradual and steady manner. Such expansion (systole) and recoil(diastole) is due to BP changes, while vasomotion is due to smooth muscle. The ability of these arteries to expand or recoil lesses the dramatic fluctuation in BP, as the expansion keeps the BP from surging too high, while the recoil keeps it from dropping too low.

What are chemoreceptors?

They are the carotid and aortic bodies, which are oval bodies found near the branch of the common carotid and in the aortic arch, respectively. They monitor blood chemistry changes, and transmit signals primarily to the respiratory centers of the brainstem, where they adjust breathing rate to stabilize blood pH, CO2 and O2.

What are the continuous capillaries?

They are the most abundant. Their endothelial cells, held together by tight junctions, form a continuous tube with intercellular clefts between them, which allow small molecules such as glucose to go through, but preventing larger molecules such as proteins or blood cells to go trough. However, the continuous capillaries in the brain lack these intercellular clefts, forming the blood-brain barrier.

What are the most common anastomoses?

They are venous anastomoses, where one vein empties directly into another. These provide several alternative routes of drainage from an organ, so the blockage of a vein is rarely as life threatening as the blockage of an artery.

What are muscular venules?

They are venules that receive blood from the post capillary venules.

*Things to know about large veins*

They have smooth muscle in all three tunic layers. The most important ones are venae cavae, pulmonary veins, internal jugular veins, and the renal veins.

What are baroreceptors?

They monitor BP. Found in the aortic arch and in the wall of the internal carotid artery, right above the branch where the carotid artery splits into an external and internal carotid artery, *a place called the carotid sinus.* The internal carotid artery, which leads to the brain, is an important monitor on the high BP leading to the brain. If BP increases, the internal carotid artery signals the cardiac and vasomotor centers in the medulla, causing a decrease in HR and blood vessel dilation, which increases blood flow, decreasing BP.

Typically, fluid filters out of the _______ end of a capillary and reenters the capillary at the ________ end.

arterial, venous