Circulatory Systems 38. Chapter 38 Circulatory Systems Key Concepts 38.1 Circulatory Systems Can Be Open or Closed 38.2 Circulatory Systems May Have Separate

Circulatory Systems

38

Chapter 38 Circulatory Systems

Key Concepts

• 38.1 Circulatory Systems Can Be Open or Closed

• 38.2 Circulatory Systems May Have Separate Pulmonary and Systemic Circuits

• 38.3 A Beating Heart Propels the Blood

• 38.4 Blood Consists of Cells Suspended in Plasma

Chapter 38 Circulatory Systems

Key Concepts

• 38.5 Blood Circulates through Arteries, Capillaries, and Veins

• 38.6 Circulation Is Regulated by Autoregulation, Nerves, and Hormones

Chapter 38 Opening Question

What are the critical factors that determine whether a person recovers from a heart attack?

Concept 38.1 Circulatory Systems Can Be Open or Closed

The function of a circulatory system is to transport substances around the body.

It consists of:

• Muscular pump—the heart

• Fluid—blood

• Series of conduits—blood vessels


Some animals do not need circulatory systems:

• Single-celled organisms exchange directly with the environment

• Structures and body shapes allow exchange between cells and the environment

• Gastrovascular systems bring the external environment inside the animal


Open circulatory system:

• The circulatory fluid—hemolymph—leaves circulatory system and moves between cells and tissues

• Flows back into heart or circulatory vessels

Open circulatory systems are found in arthropods and mollusks.

Figure 38.1 Circulatory Systems (Part 1)


Closed circulatory system—blood vessels keep circulatory fluid (blood) separate from the fluid around cells (interstitial fluid).

Blood consists of liquid blood plasma and blood cells.

Water and small molecules leak out through capillaries—blood plasma and interstitial fluid make up extracellular fluid.

Figure 38.1 Circulatory Systems (Part 2)


The closed vascular system contains:

• Arteries—carry blood away from the heart and branch into arterioles that feed the capillary beds

• Capillaries—the site of exchange between blood and interstitial fluid

• Venules—drain the capillary beds and form veins, which deliver blood back to the heart


Advantages of closed circulatory systems:

• Circulatory fluid can flow more rapidly

• Blood flow to specific tissues can be controlled by varying resistance

• Specialized cells and molecules that transport oxygen, hormones, and nutrients can be kept in the vessels

Concept 38.2 Circulatory Systems May Have Separate Pulmonary and Systemic Circuits

In fish, there is a single circuit; in birds, crocodiles and mammals, there are separate circuits:

• Pulmonary circuit—blood is pumped from the heart to the lungs and back again

• Systemic circuit—blood travels from the heart to the rest of the body and back to the heart


Fish hearts have two chambers:

• Atrium—receives blood from the body

• Ventricle—receives pumped blood from the atrium and sends it to the gills, arranged on gill arches

Blood flows through afferent arterioles into the gill arch.

Blood leaves in efferent arterioles, which join together in the aorta.

In-Text Art, Ch. 38, p. 748


Lungfish have adapted to breathe in air as well as in water.

A lung formed from the gut functions in air.

A divided atrium separates blood into pulmonary and systemic circuits—it can receive blood from either the lung or other tissues.

In-Text Art, Ch. 38, p. 749 (1)


Amphibians have three-chambered hearts.

A ventricle pumps blood to the lungs and body.

One atrium receives oxygenated blood from the lungs, a second atrium receives blood from the body.

The ventricle directs the flow to the pulmonary or systemic circuit.

In-Text Art, Ch. 38, p. 749 (2)


Reptiles have three- or four-chambered hearts and two aortas:

• The left aorta receives oxygenated blood from the left side of the ventricle and delivers it to the body

• The right aorta can receive blood from either side of the ventricle

In-Text Art, Ch. 38, p. 749 (3)


The reptilian ventricle is partly divided by a septum.

When the animal is breathing, blood flows to the pulmonary circuit.

When the animal is not breathing, blood flows to the systemic circuit.


Crocodilians, birds, and mammals have four-chambered hearts and separate pulmonary and systemic circuits.

Deoxygenated blood from the body arrives at the right atrium and flows into the right ventricle.

Right ventricle pumps blood through pulmonary arteries to lungs to pick up oxygen—then back to the left atrium of the heart through pulmonary arteries.


Oxygenated blood flows from the left atrium into the left ventricle—it contracts and sends blood through the aorta to the rest of the body.

Oxygen-depleted blood returns to the right atrium through the vena cavae, large veins.

In-Text Art, Ch. 38, p. 750


Separate circuits have advantages:

• Oxygenated blood can be distributed at higher pressure and flow than is possible in fishes.

• Blood in each system cannot mix—systemic circuit always receives blood with higher O2 content.

• Circuits can operate at different pressures.

Concept 38.3 A Beating Heart Propels the Blood

The human heart has four chambers—two atria and two ventricles.

The right atrium receives oxygen-depleted blood—it then flows through an atrioventricular (AV) valve into the right ventricle.

When the right ventricle contracts, the flaps of the AV valve close, to prevent backflow.

Blood is pumped through pulmonary artery to the lungs and pulmonary valve closes.

Figure 38.2 The Human Heart and Circulation (Part 1)




Oxygenated blood returns via pulmonary veins to the left atrium.

Blood flows into left ventricle through another AV valve.

Left ventricle contracts forcefully to send blood through the aorta—then relaxes.

The aortic valve at base of aorta then closes, to prevent backflow.


The ventricles can adjust the force of their contractions to meet demands of exercise.

The Frank-Starling law is a property of cardiac muscle cells:

• When they are stretched, as occurs when returning blood volume increases, they contract more forcefully.


The cycle of cardiac contraction and relaxation is the cardiac cycle.

Two phases:

Systole—when ventricles contract

Diastole—when ventricles relax

The atria contract just before the ventricles, to add blood volume to the ventricles.

Heart murmurs are sounds made by valves that do not close completely.

Figure 38.3 The Cardiac Cycle


Cardiac muscle functions as a pump:

Cells are in electrical contact with each other through gap junctions—spread of action potentials stimulates contraction in unison.

Some cells are pacemaker cells and can initiate action potentials without input from the nervous system.

The primary pacemaker cells are in the sinoatrial node.


Action potentials in pacemaker cells are generated by voltage-gated Ca2+ channels.

The resting membrane potential of these cells is not stable and gradually drifts upward.


Ion channels in pacemaker cells are different from other cardiac cells:

• Na+ channels are more permeable to sodium influx, so resting potential is higher.

• K+ channels that open after action potential to repolarize cell eventually close—K+ inside cell causes membrane potential to drift upwards toward threshold


Pacemaker cells initiate contractions—the heart does not need nerve signals to beat.

The nervous system controls heart rate by influencing resting potential:

• Norepinephrine from sympathetic nerves increases permeability of Na+/K+ and Ca2+ channels.

The resting potential rises more quickly and action potentials are closer together.


Opposite effect from parasympathetic nerves:

• Acetylcholine increases permeability of K+ and decreases that of Ca2+ channels.

The resting potential rises more slowly and action potentials are farther apart.

Figure 38.4 The Autonomic Nervous System Controls Heart Rate (Part 1)

Figure 38.4 The Autonomic Nervous System Controls Heart Rate (Part 2)


Heart muscle contraction is coordinated.

An action potential is generated in the sinoatrial node.

The action potential spreads through gap junctions in the atria and they contract together, but it does not spread to the ventricles.


The action potential in the atria stimulates the atrioventricular node.

The node consists of non-contracting cells that send action potentials to the ventricles via the bundle of His.

The bundle divides into right and left bundle branches that run to the tips of the ventricles and then spread throughout—called Purkinje fibers.

A contraction spreads rapidly and evenly throughout the ventricles.

Figure 38.5 The Heartbeat (Part 1)




Ventricular muscle fibers contract for much longer than skeletal muscle fibers.

Their extended action potential is due to a longer opening of voltage-gated Ca2+ channels and increased availability of Ca2+ to stimulate contraction.


An electrocardiogram (ECG or EKG) uses electrodes to record events in the cardiac cycle.

Large action potentials in the heart cause electrical current to flow outward to all parts of the body.

Electrodes register the voltage difference at different times.

Wave patterns of an ECG are labeled by letters P, Q, R, S, and T—each representing an event

Figure 38.6 The Electrocardiogram (Part 1)

Figure 38.6 The Electrocardiogram (Part 2)

Concept 38.4 Blood Consists of Cells Suspended in Plasma

Blood is a connective tissue made of cells in a liquid extracellular matrix, called plasma.

Most of the cells are erythrocytes, or red blood cells, that transport gases.

Blood also contains leukocytes (white blood cells) as well as platelets (pinched-off fragments of cells).

Figure 38.7 The Composition of Blood


Red blood cells are generated by stem cells in the bone marrow—their function is to transport respiratory gases.

Erythropoietin, a hormone released in the kidney in response to insufficient oxygen, or hypoxia, controls red blood cell production.

New blood cells contain mostly hemoglobin and circulate for about 120 days in humans.


Blood cells are broken down in the spleen, and the iron is recycled to make more hemoglobin.

A blood sample can be spun down to assess content—plasma will remain on top.

Hematocrit is the percentage of the blood made up by cells.

A low number may indicate anemia.


Bone marrow also produces megakaryocytes that release platelets.

Platelets initiate blood clotting when activated by collagen exposed in damaged blood vessels.

They release chemical clotting factors, which activate other platelets.


Steps in blood clotting:

• Cell damage and platelet activation

• Inactive enzyme prothrombin converts to active form, thrombin

• Thrombin cleaves fibrinogen and forms fibrin

• Fibrin threads form mesh that clots blood and seals vessel

Hemophilia is a genetic inability to form one clotting factor.

Figure 38.8 Blood Clotting (Part 1)



Concept 38.5 Blood Circulates through Arteries, Capillaries, and Veins

Different types of blood vessels help control blood flow.

Arteries must withstand greater pressure than veins—walls have elastin and collagen that allow them to stretch and recoil and move blood forward.

Smooth muscle cells in the walls allow them to dilate or constrict, which changes volume of blood flow.

Figure 38.9 Anatomy of Blood Vessels (Part 1)


Blood pressure and flow is lower through the capillaries—each artery supplies many capillaries, which have an enormous surface area.

Capillary walls are a single layer of endothelial cells and have tiny holes called fenestrations.

Capillary beds are permeable to water, ions, and small molecules, but not to large proteins.

Figure 38.9 Anatomy of Blood Vessels (Part 2)


Capillary beds are variable in their permeability to large ions.

In tissues other than brain, most molecules can pass through—brain capillaries have no fenestrations.

The blood-brain barrier is due to this low permeability of brain capillaries, which helps protect the brain from toxins.

Figure 38.10 A Narrow Lane


Starling’s forces are two opposing forces that maintain water balance in the capillaries:

• Blood pressure—forces water and small solutes out

• Osmotic pressure—pulls water back into the capillaries


Blood pressure is higher at the arterial end of the capillary bed and drops at the venous end.

Osmotic pressure is constant along the capillary.

If blood pressure is higher than the osmotic pressure, fluid leaves the capillary—if blood pressure is lower, fluid returns to the capillary.

Figure 38.11 Starling’s Forces (Part 1)

Figure 38.11 Starling’s Forces (Part 2)


Edema is accumulation of fluid in extracellular space due to:

• Fall in blood protein levels from disease

• Histamine release—increases capillary permeability, relaxes smooth muscle in arterioles and raises blood pressure in the capillaries


Veins have a high capacity to stretch and store blood.

Blood returning from below the heart is assisted by skeletal muscle contractions that squeeze the veins.

One-way valves in the veins prevent backflow.

Both leg muscle contractions and breathing actions help return venous blood to the heart.

Figure 38.12 One-Way Flow


The lymphatic system returns interstitial fluid to the blood.

When the fluid enters the vessels it is called lymph.

Lymph capillaries are blind-ended and continually take up excess fluid.

They ultimately merge into two thoracic ducts—these empty into veins in the neck.

Lymph nodes produce lymphocytes that screen lymph fluid for pathogens.

Figure 38.13 The Human Lymphatic System

Concept 38.6 Circulation Is Regulated by Autoregulation, Nerves, and Hormones

Blood flow depends on pressure.

Mean arterial pressure (MAP) is determined by cardiac output (CO) and total peripheral resistance (TPR).

MAP = CO x TPR

Heart rate (HR) and stroke volume (SV) are also important.

MAP = HR x SV x TPR


Autoregulatory mechanisms are local actions in the capillary bed that cause the arterioles to constrict or dilate.

Smooth muscle “cuffs” or precapillary sphincters can shut off blood flow from an arteriole to a capillary bed.

Relaxation of the smooth muscle causes increased blood flow.

Figure 38.14 Local Control of Blood Flow


Autoregulation depends on smooth muscle being sensitive to its chemical environment.

Low O2 and high CO2 levels cause smooth muscle to relax, increasing blood supply and bringing in O2 and decreasing CO2.

Other by-products of metabolism also cause arterioles to dilate.


Arteries and arterioles are innervated by the sympathetic division of the autonomic nervous system.

The neurotransmitter norepinephrine causes arterioles to constrict and elevates TPR.

Heart rate and stroke volume are also raised by sympathetic activation, resulting in elevated MAP.

This enables rapid delivery of oxygen and fuel where needed.


Sympathetic activation is coordinated by the medulla—receives information from baroreceptors (stretch receptors) that monitor blood pressure changes.

Chemoreceptors send information about blood composition.

Figure 38.15 Regulating Cardiac Output


Increased activity of the stretch receptors signals rising blood pressure.

Medulla issues two sets of commands:

• Inhibition of sympathetic nervous system, causing arterioles to dilate

• Parasympathetic nerves slow pacemaker cells

The result is lowered blood pressure.


Hormones regulate arterial pressure:

• Epinephrine—released from adrenal medulla in response to a fall in arterial pressure or the “fight-or-flight” response; arterioles contract

• Angiotensin—produced when blood supply to the kidneys falls; reduces flow to peripheral tissues and directs it to essential organs


Antidiuretic hormone (ADH), or vasopressin, is released in response to low baroreceptor activity:

• Causes the kidneys to absorb more water and increases blood pressure

Figure 38.16 Influences of Local and Systemic Mechanisms on Blood Pressure

Answer to Opening Question

To survive a heart attack the brain must have blood supply restored.

To recover from a heart attack, high cardiac output must be sustained to the organs.

After a heart attack, sympathetic activity will increase the rate and strength of contractions—if the heart cannot pump the returning blood, blood and fluids will accumulate in the body, and oxygen levels will be low.

It is essential that the kidneys function to remove the excess fluid.

Documents

Circulatory Systems 38. Chapter 38 Circulatory Systems Key Concepts 38.1 Circulatory Systems Can Be Open or Closed 38.2 Circulatory Systems May Have Separate