BIOL #18: Circulation & the Cardiovascular System

Question 1

Maximizing Surface Area For Diffusion

Answer 1

Animals maximize the surface area available for diffusion of gases, nutrients, and other key solutes in a variety of ways:

• Single-celled organisms and tiny animals have a small enough volume that diffusion over their body surface is adequate to keep them alive.
• Jellyfish and corals have a large, highly folded gastrovascular cavity that offers a large surface area for molecular exchange.
• The flattened bodies of flatworms and tapeworms give them a high surface area/volume ratio; molecular exchange thus occurs over the entire body surface.

Question 2

The Circulatory System

Answer 2

In larger animals, however, the problem of providing a large surface area for diffusion is solved by interstitial fluid and a circulatory system.
- The function of a circulatory system is to carry blood (or hemolymph) into close contact with every cell in the body – delivering nutrients and allowing gas exchange to occur.

All circulatory systems have three basic components:

• A circulatory fluid
• A heart (a muscular pump)
• A set of interconnecting vessels

Circulatory systems can be open or closed and rely on pumps that differ in structure.

Question 3

Open Circulatory System

Answer 3

Open circulatory systems are found in arthropods (e.g. crayfish, insects) and most molluscs (e.g. snails, clams)

Hemolymph, is the circulatory fluid in these system – it bathes organs directly and is not always confined to vessels, thus, it is the circulatory fluid and also the interstitial fluid.
- Hemolymph transports wastes and nutrients and may also contain oxygen-carrying pigments, some cells, and clotting agents.

Hemolymph is pumped by a heart into blood vessels that empty into an open, fluid-filled space, and hemolymph is returned to the heart when the heart relaxes and internal pressure is lower.

Question 4

Limitations of Open Circulatory Systems

Answer 4

Limitation 1: hemolymph is under relatively low pressure and, as a result, hemolymph flow rates may also be low.
- The low pressure of open circulatory systems is most suitable for relatively sedentary organisms that do not have high oxygen demands.

Insects are an exception to this rule – they overcome the limitations imposed by low hemolymph pressure via their tracheal respiratory system, which delivers oxygen directly to the tissues.

Limitation 2: without discrete, continuous vessels, the flow of hemolymph cannot be directed toward tissues that have a high oxygen demand and CO2 buildup.

Crustaceans (e.g. crayfish, lobster) are an exception to this rule – they have a network of small vessels that can preferentially send hemolymph to tissues with high oxygen demands (e.g. the muscular tail).

Question 5

Closed Circulatory System

Answer 5

Closed circulatory systems are found annelids (e.g. earthworms), cephalopods (e.g. octopi and squid), and all vertebrates

Blood flows in a continuous circuit in which blood is

• under pressure generated by a heart.
• confined to vessels

A closed system can generate enough pressure to maintain a high flow rate.
+ Allows for effective delivery of 02 to tissues in relatively large and active organisms

Blood flow can also be directed in a precise way in a closed circulatory system.

Question 6

Blood Vessels

Answer 6

Blood vessels are classified as:

• Arteries
• Capillaries
• Veins

The structure of arteries, capillaries, and veins correlates closely with their function in a closed circulatory system

Question 7

Arteries

Answer 7

Arteries are tough, thick-walled vessels that take blood away from the heart under high pressure.
- Small arteries are called arterioles.

The heart ejects blood into a large artery, usually called the aorta, which has elastic walls, allowing it to expand when blood enters it under high pressure from the heart.

Arteries and arterioles have smooth muscle fibers called sphincters wrapped around their circumference. The sphincters allow the diameter of the vessel to be carefully regulated in response to signals from the nervous system.

Question 8

Veins

Answer 8

Veins are vessels that return blood to the heart under low pressure.
- Small veins are called venules.

Veins have much thinner walls and much larger interior diameters than arteries do and contain valves, thin flaps of tissue that prevent any backflow of blood.

All veins contain some muscle fibers, which contract in response to signals from the nervous system.

Veins are compressed by muscle activity in the extremities, which helps speed the return of blood to the heart.

Question 9

Capillaries

Answer 9

Capillaries are the smallest vessels.

• Their walls are just one cell thick.
• They form an extremely dense network throughout the body passing within a few diameters of every cell within the body (capillary beds).
• They are where gases, nutrients, and wastes are exchanged between the blood and the interstitial fluid surrounding cells.

Question 10

Capillary Exchange

Answer 10

Blood plasma is 90% water and 10% ions and proteins.

Two opposing forces control the movement of fluid between the capillaries and the surrounding interstitial fluid:

1) Blood pressure tends to drive fluid out of the capillaries and into the interstitial fluid.
2) The presence of blood proteins that are too large to leave the capillaries tend to pull fluid back into the capillaries.

Blood proteins are responsible for blood’s osmotic pressure (pressure produced by the difference in solute concentration across a membrane) – the high concentration of dissolved solutes in the blood means there is less water present in the blood than in the interstitial fluid, thus water tends to flow back to the blood in the capillaries.

Question 11

Lymphatic System

Answer 11

Each day, an adult human loses ~4-8 L of fluid from capillaries to the surrounding tissue.

Any fluid that is not reclaimed in the capillaries is collected in the lymphatic system:

• Composed of thin-walled, branching tubules called lymphatic ducts or vessels.
• The fluid lost by capillaries that enters the lymphatic ducts is called lymph.

Lymphatic ducts:

1) Permeate all tissues.
2) Eventually join with one another to form larger vessels.
3) Return excess fluid, in the form of lymph, to the major veins entering the heart.

Disorders of the lymphatic system typically cause fluid to accumulate in extremities

Question 12

Heart

Answer 12

In animals with closed circulatory systems, the heart contains at least two chambers.

The atrium is a chamber that receives blood returning from circulation

The ventricle is a chamber that generates force to propel the blood through the system.

Atria are separated from ventricles by atrioventricular valves.

Question 13

Evolutionary Trends in Vertebrate Heart

Answer 13

The number of chambers and the extent to which they are separated from one another differs substantially among groups of vertebrates – these differences reflect a close fit between form and function.

There have been two distinct trends as vertebrates diversified:

1) The number of distinct heart chambers has increased.
2) In fish, the circulatory system forms a single circuit—one loop services the gills and the body (single circulation). In other lineages, there are separate circuits to the lungs and to the body (double circulation).

Fish have a two-chambered heart and single circulatory circuit to both gills and the rest of the body.

Circulation in land-dwelling vertebrates is much more affected by the force of gravity and because of this, land dwelling vertebrates evolved two separate pumping circuits:

• The pulmonary circuit is a lower-pressure circuit to and from the lungs.
• The systemic circuit is a higher-pressure circuit to and from the rest of the body.

The pulmonary and systemic circulations are completely separated in the four-chambered hearts of birds and mammals, allowing for efficient oxygen delivery.

The chambers are only partially separated in the three-chambered hearts of amphibians and the “five-chambered” hearts of turtles and lizards, which can result in mixing of oxygenated and deoxygenated blood before it is sent out to body tissues.

Question 14

The Mammalian Heart

Answer 14

Blood flows through a four-chambered heart in a specific sequence:

1) Blood returns from the body deoxygenated (C02-rich, 02 -poor), through the superior/inferior vena cava, to the right atrium.
2) Blood enters the right ventricle through the right AV valve.
3) Blood is pumped through the pulmonary (semi-lunar) valve, into the pulmonary artery, and to the lungs (where gas exchange occurs).
4) Blood returns from the lungs oxygenated (02-rich, CO2 poor), via the pulmonary veins, to the left atrium.
5) Blood enters the left ventricle through the left AV valve.
6) Blood is pumped through the aortic (semi-lunar) valve, into the aorta, and to the body to deposit O2 and pick up CO2 from body tissues.

Question 15

Function of Heart Valves

Answer 15

The one-way valves of the heart prevent backflow and ensure that blood flows in a single direction.

• Right AV (tricuspid) valve (e.g. “tri” before you “bi”)
• Pulmonary (semi-lunar) valve
• Left AV (bicuspid) value
• Aortic (semi-lunar) valve

If heart valves are damaged or defective, the resulting backflow can be heard through a stethoscope as a heart murmur.

Question 16

The Cardiac Cycle

Answer 16

The heart contracts and relaxes in a rhythmic cycle

When the atria and/or ventricles contract, the heart pumps blood and when the atria and ventricles relax, the heart chambers fill with blood

The contraction phase for the atria and the ventricles is called systole

The relaxation phase for the atria and ventricles is called diastole.

A cardiac cycle consists of one sequence of pumping and filling.

Question 17

Blood Pressure

Answer 17

Blood pressure is the force that blood exerts on the walls of arteries, capillaries, and veins.

Arterial blood pressure is highest when the ventricles contract (ventricular systole), this is called the systolic blood pressure

When the ventricles relax, the elastic walls of the arteries snap back and there is a lower, but still substantial, blood pressure called the diastolic blood pressure.

Before enough blood has flowed into the arterioles to completely relieve pressure on the arteries, the heart contracts again – this allows the arteries to remain pressurized throughout the cardiac cycle.

Question 18

Measuring Blood Pressure

Answer 18

A inflatable blood pressure cuff attached to a pressure gauge is inflated until the pressure closes the artery.

The cuff is deflated gradual until a blood pulse is heard, signifying that the pressure of the cuff is lower than that of the artery (measure of systolic pressure)

The cuff is deflated until no blood pulse is heard, signifying that the blood is flowing freely through the artery (diastolic pressure)

Blood pressure is given with systolic pressure as the numerator and diastolic pressure as the denominator:

120/80 mm Hg

People with blood pressures consistently higher than 140/90 mm Hg have high blood pressure, or hypertension, a serious health concern because it can lead to a variety of circulatory system defects.

Abnormally high blood pressure puts mechanical stress on arteries – if the walls of an artery fail, the individual may experience burst or dilated blood vessels, heart attack, stroke, and/or kidney failure.

Question 19

Maintaining the Heart’s Rhythmic Beat

Answer 19

In vertebrates, the heartbeat originates in the heart itself – some cardiac muscle cells are autorhythmic, meaning they can contract and relax without any signal from the nervous system

Each of these autorhythmic cells has its own intrinsic contraction rhythm, thus their contractions are coordinated in an intact heart by pacemaker cells that initiate contraction in the heart
- Pacemaker cells are located in a region of the right atrium called the sinoatrial (SA) node.

The SA node and the heart muscle cells receive input from the nervous system and from chemical messengers in the blood. These inputs are important for regulating the heart rate.

Question 20

Electrical Activation of the Heart

Answer 20

The electrical impulse generated by the SA node is rapidly conducted throughout the right and left atria.

The signal spreads quickly from cell to cell because cardiac muscle cells form physical and electrical connections with each other using specialized structures called intercalated discs.

Because these discs contain numerous gap junctions, electrical signals pass directly from one cell to the next.

Once the electrical signal has swept over the atria, it is conducted to an area of the heart called the atrioventricular (AV) node, which passes it on to the ventricles.

Question 21

Electrocardiogram, or ECG

Answer 21

An electrocardiogram, or ECG (aka EKG), is a graph that corresponds to the electrical activity associated with cardiac muscle contraction.

Key events in the heart’s electrical activation include:

1) The SA node originates a signal which is spread over the atria, allowing the atria to contract simultaneously and fill the ventricles.
2) The signal is conducted to the AV node
3) The signal is sent through the septum (bundle branches) of the heart to the apex.
4) The electrical impulse is rapidly transmitted through both ventricles (Purkinje fibers), causing them to contract as the atria relax.

Question 22

Patterns in Blood Pressure and Blood Flow

Answer 22

Blood pressure drops dramatically as blood moves through the capillaries because the total cross-sectional area of blood vessels in the circulatory system increases greatly in the capillaries.

The drop in blood pressure in the capillaries decreases the rate of blood flow to allow sufficient time for gases, nutrients, and wastes to diffuse between tissues and blood in the capillaries.

Question 23

Homeostatic Control of Blood Pressure

Answer 23

Blood movement is carefully regulated at an array of points throughout the circulatory system in order to maintain blood pressure within a range that allows O2-rich blood to flow to all the vital body tissues.

Physical or emotional stress can trigger nervous system and hormonal responses that:

• Can cause the smooth muscles in arteriole walls to contract, resulting in narrowing (vasocontriction) and increased blood pressure upstream of the arterioles.
• Can cause the smooth muscles in arteriole walls to relax, resulting in widening (vasodilation) and decreased blood pressure.

Signaling molecules produced in blood vessels respond to cues from the nervous and endocrine system:

• A gas, nitric oxide (NO), is a major inducer of vasodilation
• A peptide, endothelin, is a major inducer of vasoconstriction

Each kind of molecule binds to a specific receptor on smooth muscle cells, activating a signal transduction pathway that alters smooth muscle contraction, thus changing blood vessel diameter.

Often times vasoconstriction and vasodilation are coupled with changes in cardiac output to affect blood pressure
- During exercise, arterioles in working muscles must dilate to bring a greater volume of O2 to these muscles. This would normally result in a decrease in blood pressure but cardiac output (blood volume/min) increases to maintain sufficient blood pressure to deliver O2.