BIOL 1202 - Chapter 42: Circulation and Gas Exchange

CH 42 Learning Objectives

Compare and contrast the structure and functions of the major classes of circulatory systems in animals.
Describe the role of the major structures of the heart during contraction and relaxation.
Identify the types of blood vessels and their functions in maintaining and regulating blood flow.
List the components of mammalian blood and their functions in exchange, transport, and defense.
Compare and contrast the respiratory systems of aquatic animals, insects, and mammals.
Describe the mechanisms for ventilating bird and human lungs, as well as the feedback pathways regulating human breathing.
Use examples to describe the adaptive properties of respiratory pigments in gas exchange between body cells and the environment.

Trading Places

Every organism must exchange substances with its environment.
Exchanges ultimately occur at the cellular level by crossing the plasma membrane.
In unicellular organisms, these exchanges occur directly with the environment.
For most cells of multicellular organisms, direct exchange with the environment is not possible.
Gills are an example of a specialized exchange system in animals (oxygen IN, carbon dioxide OUT).
Internal transport and gas exchange are functionally related in most animals.

Concept 42.1: Circulatory systems link exchange surfaces with cells throughout the body

Small molecules can move between cells and their surroundings by diffusion.
Diffusion is only efficient over small distances because the time it takes to diffuse is proportional to the square of the distance.
In some animals, many or all cells are in direct contact with the environment.
In most animals, cells exchange materials with the environment via a fluid-filled circulatory system.

Gastrovascular Cavities

Some animals lack a circulatory system.
Some cnidarians have elaborated gastrovascular cavities that function in both digestion and distribution of substances throughout the body.
The body wall that encloses the gastrovascular cavity is only two cells thick.
Flatworms have a gastrovascular cavity and a flat body that minimizes diffusion distances.

Open and Closed Circulatory Systems

A circulatory system has a circulatory fluid, set of interconnecting vessels, & a muscular pump (heart).
The circulatory system connects the fluid that surrounds cells with the organs that exchange gases, absorb nutrients, and dispose of waste.
Circulatory systems can be open or closed.
In insects, other arthropods, and some molluscs, circulatory fluid called hemolymph bathes the organs directly in an open circulatory system.
In a closed circulatory system, blood is confined to vessels and is distinct from the interstitial fluid (in the annelids, cephalopods, and vertebrates).

Organization of Vertebrate Circulatory Systems

Humans and other vertebrates have a closed circulatory system called the cardiovascular system.
The three types of blood vessels are arteries, veins, and capillaries; blood flow is one way in these vessels.
Arteries branch into arterioles and carry blood away from the heart to capillaries.
The capillary beds are the sites of chemical exchange between the blood and interstitial fluid.
Venules converge into veins and return blood from capillaries to the heart.
Arteries and veins are distinguished by the direction of blood flow, not by O2 content.

Single Circulation

Vertebrate hearts contain two or more chambers.
Blood enters through an atria and is pumped out through ventricles.
Bony fishes, rays, and sharks have single circulation with a two chambered heart.
In single circulation, blood leaving the heart passes through two capillary beds before returning.

Double Circulation

Amphibians, reptiles, and mammals have double circulation.
Oxygen-poor and oxygen-rich blood are pumped separately from the right and left sides of heart
In reptiles and mammals, O2-poor blood flows through the pulmonary circuit to pick up O2 in the lungs.
In amphibians, O2-poor blood flows through a pulmocutaneous circuit to pick up O2 using lungs/skin.
O2-rich blood delivers O2 through the systemic (entire body) circuit.
Double circulation maintains higher blood pressure in the organs than does single circulation.

Evolutionary Variation in Double Circulation

Some vertebrates with double circulation are intermittent breathers.
EX: Amphibians and many reptiles may pass long periods without gas exchange or relying on gas exchange from another tissue; usually the skin.
Frogs and other amphibians have a three-chambered heart: 2 atria & 1 ventricle.
A ridge in the ventricle diverts most of the oxygen-rich blood into the systemic circuit and most oxygen-poor blood into the pulmocutaneous.
When underwater, blood flow to the lungs is nearly shut off.
Turtles, snakes, and lizards have a 3-chambered heart: 2 atria and 1 ventricle, partially divided by an incomplete septum.
In alligators and other crocodilians, a septum divides the ventricles, but pulmonary and systemic circuits connect where arteries exist the heart.
Mammals and birds have a 4-chambered heart with 2 atria and 2 ventricles.
The left side of the heart pumps/receives O2-rich blood, the right side receives/pumps O2-poor blood.
Mammals and birds are endotherms and require more O2 than ectotherms.

Concept 42.2: Coordinated heart contractions creates double circulation in mammals

The mammalian cardiovascular system meets the body’s continuous demand for O2.
Contraction of the right ventricle pumps blood to the lungs via pulmonary arteries.
The blood flows through capillary beds in the left and right lungs and loads O2 and unloads CO2.
Oxygen-rich blood returns from the lungs via the pulmonary veins to the left atrium of the heart.
Oxygen-rich blood flows into the left ventricle and is pumped out to body tissues via the systemic circuit.
Blood leaves the left ventricle via the aorta.
The first branches are the coronary arteries, supplying the heart muscle; further branches lead to capillary beds in the abdominal organs and hind limbs.
O2 diffuses from blood to tissues, and CO2 diffuses from tissues to blood.
Capillaries rejoin, forming venules, moving blood to veins.
Oxygen-poor blood from the head, neck, and forelimbs is channeled into the superior vena cava.
The inferior vena cava drains blood from the trunk and hind limbs.
The two venae cavae empty their blood into the right atrium from which the oxygen-poor blood flows into the right ventricle.

The Mammalian Heart: A Closer Look

The human heart is about the size of a clenched fist and consists mainly of cardiac muscle.
The two atria have relatively thin walls and serve as collection chambers for blood returning to the heart.
The two ventricles have thicker walls and contract much more forcefully.
The heart contracts and relaxes in a rhythmic cycle called the cardiac cycle.
The contraction, or pumping, phase is systole.
The relaxation, or filling, phase is diastole.

FLOW OF A DROP OF BLOOD (assume you start in the Right Atrium

Right Atrium
Right Ventricle
Pulmonary Arteries (to the Lungs)
Interacts with Lung Tissue to Re-Oxygenate cells
Pulmonary Veins (from the Lungs)
Left Atrium
Left Ventricle
Aorta (to body tissues)
Body Tissues
Superior and Inferior Vena Cava (from body tissues)
Process begins over again

The Mammalian Heart: A Closer Look (cont’d.)

The cardiac output is the VOL/min of blood pumped
The heart rate is the number of beats per minute
The stroke volume is the amount of blood pumped in a single contraction
Four valves prevent backflow of blood in the heart
The atrioventricular (AV) valves separate each atrium and ventricle
The semilunar (SL) valves control blood flow to the pulmonary artery
Backflow of blood through a defective valve causes a heart murmur

Maintaining the Heart’s Rhythmic Beat

Some cardiac muscle cells are autorhythmic, meaning they contract without any signal from the nervous system.
The sinoatrial (SA) node, or pacemaker, sets the rate and timing at which cardiac muscle cells contract.
Impulses that travel during the cardiac cycle can be recorded as an electrocardiogram (ECK/EKG)
- Tachycardia= increased heart rate
- Bradycardia= decreased heart rate
- Normal heart rate (pulse or bpm) varies depending on age and health.
Impulses from the SA node travel to the atrioventricular (AV) node
Here, the impulses are delayed and then travel to the Purkinje fibers that make the ventricles contract
The pacemaker is regulated by two portions of the nervous system: the sympathetic and parasympathetic divisions
The sympathetic division speeds up the pacemaker
The parasympathetic division slows down the pacemaker (normal rate is about 70 BPM)
The pacemaker is also regulated by hormones and temperature

Concept 42.3: Patterns of blood pressure and flow reflect the structure and arrangement of blood vessels

The vertebrate circulatory system relies on blood vessels that exhibit a close match of structure & function
These vessels vary in their diameter and elasticity (as you get older, elasticity decreases)

Blood Vessel Structure and Function

All blood vessels contain a central lumen lined with an epithelial layer that lines blood vessels
This endothelium is smooth & minimizes resistance
Capillaries are only slightly wider than a red blood cell
Capillaries have thin walls, the endothelium plus its basal lamina, to facilitate the exchange of materials
Arteries and veins have an endothelium, smooth muscle, and connective tissue
Arteries have thick, elastic walls to accommodate the high pressure of blood pumped from the heart
Unlike arteries, veins contain valves to maintain unidirectional blood flow

Blood Flow Velocity

Physical laws governing movement of fluids through pipes affect blood flow and blood pressure
Velocity of blood flow is slowest in the capillary beds as a result of the high resistance and large total cross-sectional area
Blood flow in capillaries is necessarily slow for exchange of materials

Blood Pressure

Blood flows from areas of higher pressure to areas of lower pressure
Blood pressure is a force exerted in all directions, including against the walls of blood vessels
The recoil of elastic arterial walls plays a role in maintaining blood pressure.
The resistance to blood flow in the narrow diameters of tiny capillaries and arterioles dissipates much of the pressure

Changes in Blood Pressure During the Cardiac Cycle

Systolic pressure is the pressure in the arteries during ventricular systole; it is the highest pressure in the arteries
A pulse is the rhythmic bulging of artery walls with each heartbeat
Diastolic pressure is the pressure in the arteries during diastole; it is lower than systolic pressure (doctors worry more about diastolic pressure)
“Normal” ranges Vary widely Systolic: 140-90 Diastolic: 60-90

Regulation of Blood Pressure

Homeostatic mechanisms regulate arterial blood pressure by altering the diameters of arterioles
Vasoconstriction is the contraction of smooth muscle in arteriole walls; it increases blood pressure
Vasodilation is the relaxation of smooth muscles in the arterioles; it causes blood pressure.
Nitric oxide (NO) is a major inducer of vasodilation
The peptide endothelin is a potent inducer of vasoconstriction
Vasoconstriction and vasodilation are often coupled to changes in cardiac output that affect blood pressure

Blood Pressure and Gravity

Blood pressure (BP) is generally measured for an artery in the arm at the same height as the heart
BP for a healthy 20-year-old human at rest is ~120 mm Hg at systole and ~70mm Hg at diastole
Gravity has a significant effect in blood pressure
Fainting is caused by inadequate blood flow to the head
Animals with long necks require a very high systolic pressure to pump blood a great distance against gravity
Because blood pressure is low in veins, one-way valves in veins prevent systolic backflow of blood
Return of blood is also enhanced by contraction of smooth muscle in venule walls and skeletal muscle contraction

Capillary Function

Blood flows through only 5–10% of the body’s capillaries at any given time
Capillaries in major organs are usually filled to capacity
Blood supply varies in many other sites
Two mechanisms regulate distribution of blood in capillary beds:
1. Constriction or dilation of arterioles that supply capillary beds
2. Precapillary sphincters that control flow of blood between arterioles and venules
Sphincters relaxed – Blood flow
Sphincters contract – No blood flow

Capillary Function (cont’d.)

Blood flow is regulated by nerve impulses, hormones, and other chemicals
The exchange of substances between the blood and interstitial fluid takes place across the thin endothelial walls of the capillaries
Blood pressure tends to drive fluid out of capillaries, and blood proteins tend to pull fluid back
These proteins are responsible for much of the blood’s osmotic pressure
On average, there is a net loss of fluid from capillaries

Fluid Return by the Lymphatic System

The lymphatic system returns fluid that leaks out from the capillary beds
Fluid lost by capillaries is called lymph
The lymphatic system drains into veins in the neck
Valves in lymph vessels prevent the backflow of fluid
Edema is swelling caused by disruptions in the flow of lymph
Lymph nodes are organs that filter lymph and play an important role in the body’s defense
When the body is fighting an infection, lymph nodes become swollen and tender

Concept 42.4: Blood components function in exchange, transport, and defense

The closed circulatory systems of vertebrates contain a more highly specialized fluid called blood
Blood in vertebrates is a connective tissue consisting of different of cells suspended in a liquid matrix called plasma, accounts for 55% of the volume of blood
Cells (RBC and WBC) and cell fragments (platelets) occupy about 45% of the volume of blood
RBC’s are anucleate and have no mitochondria

Plasma

Plasma contains inorganic salts as dissolved ions, sometimes called electrolytes
Plasma proteins influence blood pH and help maintain osmotic balance between blood and interstitial fluid
Certain plasma proteins function in lipid transport, immunity, and blood clotting
Plasma is similar in composition to interstitial fluid, but plasma has a much higher protein concentration

Cellular Elements

Suspended in blood plasma are two types of cells:
- Red blood cells (RBC, erythrocytes) transport O2
- White blood cells (WBC, leukocytes) function in defense
- Platelets are fragments of cells used in clotting
Red blood cells, or erythrocytes, are the most numerous cells
They contain hemoglobin, the iron-containing protein that transports O2
Each molecule of hemoglobin binds up to four molecules of O2
In mammals, mature RBC lack nuclei & mitochondria

Cellular Elements (cont’d.)

Sickle-cell disease (SSD) is caused by abnormal hemoglobin proteins that that form aggregates
The aggregates can deform an erythrocyte into a sickle shape
Sickled cells can rupture or can block blood vessels
There are five major types of white blood cells, or leukocytes
They function in defense either by phagocytizing bacteria and debris or by mounting immune responses against foreign substances
They are found both in and outside of the circulatory system (too much red blood cells makes blood thicker)

Stem Cells and the Replacement of Cellular Elements

Erythrocytes, leukocytes, and platelets all develop from a common source of stem cells in the red marrow of bones, especially ribs, vertebrae, sternum, & pelvis
The hormone erythropoietin (EPO) stimulates erythrocyte production when O2 delivery is low
Physicians can use recombinant EPO to treat people with conditions such as anemia

RBC levels controlled by Negative Feedback

Oxygen deficiency stimulates Erythropoietin production by the kidneys causes Red blood cell production in the bone marrow

Blood Clotting

Coagulation is the formation of a solid clot from liquid blood
A cascade of complex reactions converts inactive fibrinogen to fibrin, forming a clot
A blood clot formed within a blood vessel is called a thrombus and can block blood flow

Cardiovascular Disease

Cardiovascular diseases are disorders of the heart and the blood vessels
One type of cardiovascular disease, atherosclerosis, is caused by the buildup of fatty deposits (plaque) within arteries
Cholesterol is a key player in the development of atherosclerosis
Low-density lipoprotein (LDL) delivers cholesterol to cells for membrane production (we want less of this)
High-density lipoprotein (HDL) scavenges excess cholesterol for return to the liver (we want more of this)
Risk for heart disease increases with a high LDL:HDL
A heart attack, or myocardial infarction, is the damage or death of cardiac muscle tissue resulting from blockage of one or more coronary arteries
A stroke is the death of nervous tissue in the brain, resulting from rupture/blockage of arteries in the head
Angina pectoris is chest pain caused by partial blockage of the coronary arteries

Risk Factors and Treatment of Cardiovascular Disease

The proportion of LDL relative to HDL can be decreased by diet, exercise, & not smoking
Drugs called statins reduce LCL levels & heart attacks
Inflammation plays a role in atherosclerosis and thrombus formation
Aspirin inhibits inflammation and reduces the risk of heart attacks and strokes
Hypertension, or high blood pressure, also contributes to heart attack and stroke
Hypertension can be controlled by diet & exercise &/or medicaiton

Concept 42.5: Gas exchange occurs across specialized respiratory surfaces

Gas exchange is the uptake of O2 from the environment and the discharge of CO2 to the environment
Partial pressure is the pressure exerted by a particular gas in a mixture of gases
Partial pressures also apply to gases dissolved in liquids such as water
O2 is much less soluble in water than in air

Respiratory Media and Surfaces

Breathing air is relatively easy and need not be very efficient
In a given volume, there is less O2 available in water than in air
Obtaining O2 from water requires greater efficiency than air breathing
Gas exchange across respiratory surfaces takes place by diffusion
Respiratory surfaces vary by animal and can include the skill, gills, tracheae, & lungs

Gills in Aquatic Animals

Gills are outfoldings of the body that create a large surface area for gas exchange
Ventilation moves the respiratory medium over the respiration surface
Aquatic animals move through water or move water over their gills for ventilation

Gills in Aquatic Animals, Continued

Fish gills use a countercurrent exchange system; blood flows in the opposite direction to water parring over the skills
Blood is always less saturated with O2 than water
In fish gills, more than 80% of the O2 dissolved in the water is removed as water passes over the respiratory surface

Tracheal Systems in Insects

The tracheal system of insects consists of a network of branching tubes throughout the body
The tracheal tubes supply O2 directly to body cells
The respiratory and circulatory systems are seprate
Larger insects must ventilate their tracheal system to meet O2 demands

Mammalian Respiratory Systems: A Closer Look

A system of branching ducts conveys air to the lungs
Air inhaled through the nostrils is filtered, warmed, humidified, and sampled for odors
The pharynx directs air to the lungs and and food to the stomach
Swallowing moves the larynx upward and tips the epiglottis over the glottis in the pharynx to prevent food from entering the trachea, or windpipe
Air passes through the pharynx, larynx, trachea, bronchi, and bronchioles to the alveoli, where gas exchange occurs

Mammalian Respiratory Systems: A Closer Look, Continued

Exhaled air passes over the vocal cords in the larynx to create sounds
Cilia and mucus line the epithelium of the air ducts and move particles up to the pharynx
This “mucus escalator” cleans the respiratory system and allows particles to be swallowed into the esophagus
Gas exchange takes place in alveoli, air sacs at the tips of bronchioles
Oxygen diffuses through the moist film of the epithelium and into capillaries

Mammalian Respiratory Systems: A Closer Look, Continued

CO2 diffuses from the capillaries across the epithelium and into the air space
Alveoli lack cilia and are susceptible to contamination
Secretions called surfactants coat the surface of the alveoli
These surfactants prevent the alveoli from collapsing and “sticking to themselves”
Preterm babies lack surfactant and are vulnerable to respiratory distress syndrome; treatment is provided by artificial surfactants

Concept 42.6: Breathing ventilates the lungs

The process that ventilates the lungs is breathing, the alternate inhalation and exhalation of air
An amphibian such as a frog ventilates its lungs by positive pressure breathing, which forces air down the trachea
Birds have air sacs that function as bellows that keep air flowing through the lungs
Air passes through the lungs in one direction only
Passage of air through the entire system of lungs and air sacs requires two cycles of inhalation and exhalation
Ventilation in birds is highly efficient

How a Mammal Breathes

Mammals ventilate their lungs by negative pressure breathing, which pulls air into the lungs
Lung volume increases as the rib muscles and diaphragm contract
The tidal volume is the volume of air inhaled with each breath (normal volume with no stress)
The maximum tidal volume is the vital capacity
After exhalation, a residual volume of air remained in the lungs
Inhalation:
- Actively
- Air moves in
- Rib cage expands
- Lungs expand
- Diaphragm contracts downward
Exhalation:
- Passively
- Air moves out
- Rib cage contracts
- Lung's compress
- Diaphragm moves up

Control of Breathing in Humans

Breathing is regulated by involuntary mechanisms
The breathing control centers are found in the medulla oblongata of the brain
The medulla regulates the rate and depth of breathing in response to pH changes in the cerebrospinal fluid
Sensors in the aorta and carotid arteries monitor O2 and CO2 concentrations in the blood
These signal the breathing control centers, which respond as needed
Additional modulation of breathing takes place in the pons, next to the medulla

Concept 42.7: Adaptations for gas exchange include pigments that bind and transport gases

The metabolic demands of many organisms require that the blood transport large quantities of O2 and CO2 (not the only 2 gases)
During inhalation, fresh air mixes with air in the lungs
The resulting mixture has a higher O2 pressure than the blood flowing through alveolar capillaries
In the alveoli, O2 diffuses into the blood and CO2 diffuses into the air
There, exchange occurs across the alveolar capillaries, resulting in exhaled air enriched in CO2 and partially depleted of O2

Respiratory Pigments

Respiratory pigments, proteins that transport oxygen, greatly increase the amount of oxygen that blood can carry
Arthropods and many molluscs have hemocyanin, with copper as the oxygen-binding component
Most vertebrates and some invertebrates use hemoglobin
In vertebrates, hemoglobin is contained within erythrocytes
Hemoglobin: 4 Subgroups each with a heme w/ iron
Not all blood is the same color

Respiratory Pigments, Continued

A single hemoglobin molecule can carry four molecules of O2, one molecule for each iron-containing heme group
The hemoglobin dissociation curve shows that a small change in the partial pressure of oxygen can result in a large change in delivery of O2
CO2 produced during cellular respiration lowers blood pH and decreases the affinity of hemoglobin for O2; this is called the Bohr shift
Hemoglobin plays a minor role in transport of CO2 and assists in buffering the blood
Hemoglobin retains less O2 at lower pH (Higher CO2 concentration)

Carbon Dioxide Transport

CO2 is transported in the blood in three ways
1. As bicarbonate ions (70%)
2. Bound to hemoglobin (20%)
3. Dissolved in plasma as CO2 (10%)
Bicarbonate ions $(HCO_3^-)$
are formed in RBCs when CO2 combines with water using carbonic anhydrase
CO2 + H2O \à H^+ + HCO_3^-
The reaction is reversed as the blood flows through capillaries surrounding the alveoli, where CO2 is low:
H^+ + HCO3^- \à CO2 + H_2O
The CO2 then diffuses into the air in the alveoli, which is exhaled from the lungs, the H2O stays in the blood

Respiratory Adaptations of Diving Mammals

Diving mammals have evolutionary adaptations that allow them to perform extraordinary feats
Weddell seals in Antarctica can remain underwater for 20 minutes
The Cuvier’s beaked whale can dive to 2,900 m and stay submerged for more than 2 hours

Respiratory Adaptations of Diving Mammals, Continued

These animals have a high blood to body volume ratio
Deep-diving air breathers' stockpile O2 and use it slowly
Diving mammals can store oxygen in their muscles in myoglobin proteins
Diving mammals also conserve oxygen by:
- Changing their buoyancy to glide passively
- Routing blood to vital tissues
- Deriving ATP in muscles from fermentation once oxygen is depleted