Circulatory System

Question 1

What are the primary components of blood?

Answer 1

• plasma
• red blood cells
• white blood cells
• platelets

About 45% of total blood volume is due to cellular components, while the remaining 55% comes from plasma.

Question 2

What is the normal pH of blood?

Answer 2

7.4

Normal blood is slightly basic, and even minute deviations can be very harmful. Thus, pH is tightly regulated by mechanisms such as the bicarbonate buffering system.

Question 3

What is the difference between acidosis and alkalosis?

Answer 3

• Acidosis is an increase in blood acidity, occurring when blood pH drops below 7.35.
• Alkalosis is an increase in blood basicity, occurring when blood pH rises above 7.45.

Question 4

If a person were to hyperventilate continuously, would they eventually experience acidosis or alkalosis?

Answer 4

alkalosis

Hyperventilation, or quickened breathing, results in an increased loss of CO₂. The reaction proceeds according to this equilibrium:

H₂O + CO₂ ⇔ H₂CO₃ ⇔ H⁺ + HCO₃^-

When CO₂ is excreted, the equilibrium shifts to the left to regenerate it. This requires protons to combine with bicarbonate ions, decreasing the plasma H⁺ concentration. Lowered H⁺, or decreased acidity, means that pH will rise.

Question 5

Define:

plasma

Answer 5

It is the liquid component of blood in which red and white blood cells are suspended.

Though primarily composed of water, plasma also contains proteins, glucose, hormones, electrolytes, and gases.

Question 6

Which homeostatic function is most relevant to albumin?

Answer 6

osmoregulation

Albumin, the most common plasma protein, is produced in the liver and primarily maintains osmotic pressure. It also helps in buffering blood pH and acts to transport substances in the blood.

Question 7

Name three substances or mechanisms that help regulate plasma volume.

Answer 7

• osmotic pressure
• antidiuretic hormone (ADH)
• aldosterone

High plasma osmolarity draws water into the blood from the tissues; low plasma osmolarity causes water to flow out of the blood. Both ADH and aldosterone act on the kidneys, ADH by directly increasing water reabsorption and aldosterone by increasing the reabsorption of salt.

Question 8

Define:

erythrocyte

Answer 8

Also known as red blood cells, these are the most common cells found in the blood. Their main responsibility is to carry oxygen to body tissues.

Red blood cells transport oxygen using hemoglobin. For the MCAT, note that these cells do not contain nuclei (or any other membrane-bound organelles), and thus cannot undergo mitosis.

Question 9

In what part of the body are erythrocytes produced?

Answer 9

In the bone marrow from stem cells.

This process is referred to as erythropoiesis and is stimulated by decreased oxygen in the blood, which causes the kidneys to release erythropoietin (EPO).

Question 10

What organs are responsible for the destruction of old and damaged erythrocytes?

Answer 10

The spleen and liver are the principal organs that destroy erythrocytes and process their parts.

When heme is broken down, iron is first released and recycled. The remainder of the molecule is converted into bilirubin, some of which becomes a component of bile and the rest of which is immediately excreted.

Question 11

How do the kidneys respond to low oxygen levels in the blood?

Answer 11

They produce erythropoietin (EPO), which stimulates bone marrow to produce more red blood cells. This increases the oxygen carrying capacity of the blood.

Note that EPO is a peptide hormone.

Question 12

Anemia is marked by a low value of what blood-related measurement?

Answer 12

Hematocrit

Hematocrit describes the percentage of a person’s total blood volume that is composed of red blood cells. A low hematocrit signifies that fewer RBCs are being produced, generally resulting in a decreased ability to transport oxygen.

Question 13

What features characterize the structure of hemoglobin?

Answer 13

Hemoglobin has four protein subunits, each composed of a heme group and a globin protein. Together, these monomers form a hemoglobin tetramer.

Question 14

To which part of blood does oxygen bind in order to be transported to tissue?

Answer 14

It binds to the iron in the heme group of hemoglobin, which is located in red blood cells.

Oxygen can travel in and out of a red blood cell via diffusion.

Question 15

What conditions can alter the oxygen affinity of hemoglobin?

Answer 15

High temperature, low pH, and high carbon dioxide concentration lower the oxygen affinity of hemoglobin. This decrease allows oxygen to be more easily released to body tissue.

These conditions can be remembered as effects that occur during exercise, when the body is starved for oxygen. During such situations, hemoglobin must be able to readily drop off oxygen in the muscle and other tissue.

Question 16

What term describes the basic shape of the oxygen-hemoglobin dissociation curve?

Answer 16

sigmoidal

or S-shaped

This shape stems from the ability of hemoglobin to utilize cooperative binding. Once a single oxygen molecule is bound, it becomes much easier for hemoglobin to bind to additional molecules, creating a steep slope.

Question 17

What is the Bohr effect?

Answer 17

It states that the oxygen binding affinity of hemoglobin decreases as carbon dioxide concentration increases. In other words, O₂ binding and plasma [CO₂] are inversely related.

Since high CO₂ concentration results in a lowered pH, acidic blood also correlates with lower binding affinity.

Question 18

What happens to the oxygen binding curve of hemoglobin when a person exercises?

Answer 18

The curve shifts to the right.

A rightward shift represents a decrease in oxygen affinity. Exercise causes an increase in carbon dioxide, a drop in pH, and an increase in temperature, resulting in a lowered affinity for oxygen. This makes it easier to release the oxygen in muscle tissue.

Question 19

Define:

myoglobin

Answer 19

It is the iron-containing protein that binds oxygen in skeletal muscle cells. It is closely related to hemoglobin, but contains only one monomer instead of four.

Because it picks up the oxygen that hemoglobin releases in the tissues, myoglobin has a higher O₂ affinity than hemoglobin.

Question 20

What is the basic shape of the oxygen binding curve of myoglobin?

Answer 20

hyperbolic

For the MCAT, it is most important to know that unlike hemoglobin, it is not sigmoidal.

The myoglobin curve lacks a sigmoidal shape because, as a protein composed of a single subunit, it cannot undergo cooperative binding.

Question 21

How do adult and fetal hemoglobin differ in their oxygen affinity?

Answer 21

Fetal hemoglobin has a greater affinity for oxygen than adult hemoglobin.

Specifically, fetal hemoglobin must be more prone to binding oxygen than maternal hemoglobin. This phenomenon allows a developing fetus to accept oxygen from its mother’s partially deoxygenated blood.

Question 22

Define:

leukocyte

Answer 22

Also known as the white blood cell, it is an immune cell that combats infection in the body.

Unlike red blood cells, leukocytes do contain nuclei, and are also larger and less numerous.

Question 23

Which main component of blood is required for proper clotting?

Answer 23

platelets

In addition to physically blocking lacerations, they activate proteins in the blood called clotting factors.

Platelets are not full cells, but tiny cell fragments derived from larger megakaryocytes. As such, they do not contain nuclei.

Question 24

Name the primary protein responsible for blood clotting.

Answer 24

fibrin

Fibrin is derived from the soluble precursor fibrinogen.

Another protein, thrombin, cleaves fibrin into its active form. Thrombin also forms from a precursor, called prothrombin.

Question 25

Which organ is responsible for producing clotting factors?

Answer 25

liver ## Footnote These factors, including fibrinogen and prothrombin, circulate in the bloodstream and are activated by platelets after tissue damage.

Question 26

# Define: coagulation

Answer 26

It is the process by which a liquid becomes a gel. When referring to blood, this process is also called **clotting**. ## Footnote Coagulation functions to prevent immediate blood loss from a damaged vessel. It also facilitates eventual repair of the tissue.

Question 27

How does the blood respond when the endothelium of a vessel is damaged?

Answer 27

First, platelets plug the area of the wound and activate a coagulation cascade. This pathway ends in the activation of fibrin from fibrinogen. Finally, fibrin forms an insoluble crosslinked mesh that seals the laceration.

Question 28

How many oxygen molecules can be carried by one molecule of hemoglobin?

Answer 28

Hemoglobin has an iron-containing heme group for each of its four protein subunits, allowing it to carry **four** oxygen molecules total. ## Footnote Each red blood cell, then, contains around 250 million molecules of hemoglobin, resulting in a carrying capacity of around one billion O₂ molecules per erythrocyte.

Question 29

In which form is most carbon dioxide carried in the blood?

Answer 29

Most carbon dioxide (70-80%) is carried in the form of **bicarbonate ions**, HCO₃^-. ## Footnote These ions comprise an integral part of the blood buffer system. The remaining CO₂ is either bound to hemoglobin or dissolved in its original form in the plasma.

Question 30

# Define: antigen

Answer 30

It is a substance that causes an immune response. Specifically, antigens are markers that can be recognized by antibodies. ## Footnote The surface proteins that determine blood type (A and B) are examples of antigens.

Question 31

Which characteristic of an individual's blood cells is used to identify **blood type**?

Answer 31

Blood type is classified based on the surface antigens present on a person's erythrocytes. ## Footnote Human blood types include A, B, AB, and O. While A and B denote possible antigens, an O blood type implies that neither is present.

Question 32

With regard to blood type, what is the **Rhesus factor**?

Answer 32

Also known as the **Rh factor**, this refers to a surface antigen on human blood cells that was first discovered in Rhesus monkeys. ## Footnote Rh positive (Rh⁺) blood displays the Rh antigen, while Rh negative (Rh^-) blood lacks it. Rh^- blood can be given to individuals of either blood type, while Rh⁺ blood is only given to other Rh⁺ people.

Question 33

# Define: antibody

Answer 33

An immune protein that recognizes a specific, potentially harmful antigen or class of antigens. Antibodies are produced by B lymphocytes. ## Footnote For the MCAT, be sure to avoid confusion between antibodies and antigens. Antigens are attacked, while antibodies do the attacking.

Question 34

What is significant about AB⁺ blood, and how does it differ from O^- blood?

Answer 34

* AB⁺ individuals are **universal acceptors**. * Those with O^- blood are **universal donors**. ## Footnote Since AB⁺ individuals have all the major surface antigens, they produce no antibodies and can accept blood of any type. In contrast, O^- blood has no surface antigens and thus will not induce an immune response upon transfusion.

Question 35

What requirements must be met for one person to donate blood to another?

Answer 35

The donor blood must not contain surface antigens that are foreign to the recipient. ## Footnote In other words, the donated blood must not induce the production of antibodies.

Question 36

What requirements must be met for one person to receive blood from another?

Answer 36

The recipient must have all of the surface antigens that are present on the donated blood cells. ## Footnote This is necessary to prevent the production of antibodies. For example, if the recipient is AB^-, she can accept blood with either A or B antigens (or both). However, she cannot accept blood that contains the Rh factor.

Question 37

What occurs when a person is given a blood transfusion that contains foreign antigens?

Answer 37

In a process called **hemagglutination**, the recipient's antibodies cause the transfused blood to bind and clump together.

Question 38

A man has a blood type of B^-. Which antibodies could his immune system potentially produce?

Answer 38

Antibodies against the A and Rh antigens. ## Footnote An individual's blood type denotes which antigens are present on his red blood cells. Antibodies will be produced only for foreign antigens - in other words, those antigens that are **not** part of the blood type. B^- blood contains only B antigens, so all others will be considered foreign.

Question 39

What blood types could a person with A⁺ blood receive?

Answer 39

* A⁺ * A^- * O⁺ * O^- ## Footnote Simply put, an A⁺ individual can receive blood as long as it does not contain antigens other than A and Rh. The only antigen that violates this rule is B, so the person could not receive B⁺, B^-, AB⁺, or AB^- blood.

Question 40

A mother with Rh^- blood becomes pregnant with her first child, who is found to have a blood type of Rh⁺. What complications could occur?

Answer 40

The mother may develop antibodies against the Rh factor, which could affect future pregnancies. ## Footnote Since fetal and maternal blood are separated, development of antibodies usually does not occur until birth. The first baby, then, will be unaffected. However, if the mother becomes pregnant with a second Rh⁺ fetus, her antibodies may attack its cells. This is known as hemolytic disease of the newborn, or erythroblastosis fetalis.

Question 41

What are the **main functions of the circulatory system**?

Answer 41

* Circulate oxygen, nutrients, hormones, fluids, and ions to body tissues * Remove carbon dioxide * Remove metabolic wastes, such as urea * Aid in thermoregulation

Question 42

How does the circulatory system regulate body temperature?

Answer 42

Blood vessels can undergo **vasoconstriction** and **vasodilation**. ## Footnote In cold temperatures, arterioles near the skin become narrow, or constrict, to limit heat loss. In hot temperatures, arterioles widen, or dilate, to allow heat to dissipate from the body's surface.

Question 43

Which type of muscle allows for variation in blood vessel diameter depending on the ambient temperature?

Answer 43

**Smooth muscle** lining the arteries and arterioles allows vasodilation and vasoconstriction to occur. ## Footnote Since smooth muscle is controlled by the autonomic nervous system, these thermoregulatory functions are involuntary.

Question 44

On a cold winter day, a woman leaves her office and begins a long walk to her car. How would her blood vessels respond?

Answer 44

Surface arterioles would **vasoconstrict**, reducing blood flow to the skin in an attempt to conserve heat. ## Footnote Long periods of vasoconstriction are usually interrupted by brief vasodilatory cycles to avoid the excessive restriction of blood flow.

Question 45

At 1 PM on a hot summer afternoon, a teenager goes outside to sunbathe. How would his blood vessels respond?

Answer 45

Surface arterioles would **vasodilate**, carrying blood close to the external environment in an attempt to dissipate heat. ## Footnote Vasodilation can explain the flushed red face of a person who has become overheated.

Question 46

Describe the general structure of the human heart.

Answer 46

It has four chambers, with the left and the right atrium positioned above the left and the right ventricle. The atria and ventricles are separated by valves. ## Footnote The right side of the heart contains deoxygenated blood, while blood on the left side is oxygenated.

Question 47

What structural and functional differences distinguish an **atrium** from a **ventricle**?

Answer 47

* Structurally, atria are smaller and located superior to the ventricles. * Functionally, atria receive blood that returns to the heart and pump it to the ventricles, while ventricles pump blood to the rest of the body.

Question 48

In order, list the structures that blood will contact between the vena cava and the aorta. Do not include valves.

Answer 48

After returning to the heart via the vena cava, blood will contact the: 1. Right atrium 2. Right ventricle 3. Pulmonary artery 4. Lung 5. Pulmonary vein 6. Left atrium 7. Left ventricle ## Footnote From the left ventricle, it exits the heart through the aorta.

Question 49

Name the two valves in the heart that separate an atrium from a ventricle.

Answer 49

1. The tricuspid valve 2. The bicuspid or mitral valve ## Footnote The **tricuspid valve** is situated between the right atrium and the right ventricle. The **bicuspid** or **mitral valve** separates the left atrium from the left ventricle. Both of these structures are classified as atrioventricular valves.

Question 50

After blood is oxygenated in the lungs, which vessel carries it back to the heart?

Answer 50

pulmonary vein ## Footnote Unlike the vast majority of venous circulation, the pulmonary vein carries oxygenated blood. Remember that veins are defined as vessels that transport blood toward the heart, not as vessels that carry oxygen-depleted blood.

Question 51

A certain heart malformation prevents the mitral valve from closing properly. How would this condition affect blood flow?

Answer 51

Some of the blood leaving the left atrium would leak back in instead of fully progressing to the left ventricle. ## Footnote Such defects can cause the heart to work harder to counteract the defective valve, causing stress and long-term impairments in blood flow.

Question 52

# Define: systolic pressure

Answer 52

The blood pressure measured when the ventricles are contracting and blood is being pumped. ## Footnote Of the two pressure measurements, systolic pressure is larger and is given as the numerator. In the standard reading of "120 over 80," the systolic pressure is 120 mmHg.

Question 53

# Define: diastolic pressure

Answer 53

The blood pressure measured when the ventricles are relaxed and no blood is being pumped. ## Footnote Of the two pressure measurements, diastolic pressure is smaller and is given as the denominator. In the standard reading of "120 over 80," the diastolic pressure is 80 mmHg.

Question 54

Name the main components present in arterial walls.

Answer 54

Arteries have thick walls that contain: * endothelium * elastic connective tissue * smooth muscle ## Footnote The elastic fibers allow arteries to stretch in diameter, while smooth muscle functions in vasoconstriction and vasodilation.

Question 55

What are the **two major types of arteries**, and where is each type **located in the circulatory system**?

Answer 55

1. **Elastic** - those that attach directly to the heart, namely the aorta and pulmonary artery. 2. **Muscular** - are more distant from the heart and carry blood to systemic organs

Question 56

Describe the **structural differences** between elastic and muscular arteries.

Answer 56

* **Elastic arteries** contain more connective fibers, such as collagen and elastin. This allows for expansion to accomodate blood that is pumped from the heart. * **Muscular arteries** contain more smooth muscle, as they need to constrict or dilate to control blood flow to a specific region of the body.

Question 57

What is the function of **arterioles**?

Answer 57

They connect directly to arteries and control blood flow to the capillaries. ## Footnote Structurally, they are smaller than arteries but otherwise similar. Arterioles are the primary site of vasoconstriction.

Question 58

Describe the **similarities** between arterial and venous walls.

Answer 58

Like arteries, veins contain an inner lining of endothelium, as well as smooth muscle and connective tissue.

Question 59

Describe the **differences** between arterial and venous walls.

Answer 59

Venous walls are thinner than arterial walls. As a result, the lumen or interior of a vein is larger than that of an artery, allowing it to hold more blood. ## Footnote Specifically, veins contain less elastic tissue and less smooth muscle than arteries. Veins also have one-way valves, which prevent blood from traveling away from the heart.

Question 60

What is the function of **venules**?

Answer 60

They collect blood from the capillaries and carry it to the veins. ## Footnote Structurally, they are smaller than veins but otherwise similar.

Question 61

Which type of vessel connects arterioles to venules?

Answer 61

Capillaries ## Footnote Capillaries, the smallest and most numerous blood vessels, arise from arterioles and converge into venules. They form the site of gas and nutrient exchange with the tissues.

Question 62

Which process provides the pressure that propels blood into arteries?

Answer 62

Contraction of the ventricles ## Footnote Vasoconstriction can also alter blood pressure and control the flow of blood to specific tissues; however, it occurs mainly in the arterioles.

Question 63

Which process provides the pressure that carries blood back to the heart through veins?

Answer 63

The contraction of nearby skeletal muscle compresses veins, propelling blood forward. Veins can also alter their pressure by constricting or dilating. ## Footnote In the lower part of the body, gravity opposes proper venous blood flow. To prevent blood from traveling backwards, veins contain one-way valves.

Question 64

Which type of blood vessel has the lowest pressure?

Answer 64

Veins ## Footnote Pressure is highest in the arteries and decreases in the same order as the circulatory structure itself: arterioles, capillaries, venules, veins.

Question 65

In which type of vessel does blood flow the slowest?

Answer 65

In the capillaries ## Footnote According to the continuity equation, fluid velocity and cross-sectional area of a vessel are inversely related. Since all of the body's capillaries combine to have an extremely large area, blood moves through them slowly.

Question 66

A student observes a circulatory structure that is lined with endothelium, has valves, and can vasoconstrict. Which type of blood vessel is this?

Answer 66

vein ## Footnote Only veins have valves to prevent backflow. Arteries, veins, and arterioles can all vasoconstrict to some degree, and all blood vessels possess endothelium.

Question 67

A student observes a circulatory structure that directly connect to capillaries, carries blood away from the heart, and can vasoconstrict. Which type of blood vessel is this?

Answer 67

arteriole ## Footnote Only venules and arterioles directly connect to the capillaries, and only the arterial system carries blood away from the heart. Arterioles often control their diameter via vasoconstriction.

Question 68

What is the primary difference between pulmonary and systemic vessels?

Answer 68

Systemic veins carry deoxygenated blood, while systemic arteries contain blood that has been oxygenated. The reverse is true for pulmonary circulation. ## Footnote Pulmonary circulation refers to bloodflow between the heart and the lungs, while systemic circulation provides blood to the rest of the body.

Question 69

What function do **capillary beds** serve?

Answer 69

They provide a site where oxygen, carbon dioxide, nutrients, and wastes are exchanged between blood and interstitial fluid.

Question 70

Which two opposing pressures control the movement of fluid between capillaries and tissues?

Answer 70

* Hydrostatic pressure * Osmotic pressure ## Footnote Hydrostatic pressure is synonymous with blood pressure and promotes the movement of fluid out of the capillaries. Osmotic pressure is controlled by solute concentration and promotes the return of fluid from the tissues.

Question 71

What name is given to osmotic pressure that is specifically produced by proteins?

Answer 71

Oncotic pressure ## Footnote In the circulatory system, proteins have a unique ability to generate osmotic pressure, since they are too large to leave the capillaries. Albumin is a notable example of such a protein.

Question 72

Which transport method is used in the capillary exchange of most gases and solutes?

Answer 72

diffusion ## Footnote In general, diffusion is favored by small, nonpolar substances. Oxygen and nutrients flow from the blood, where their concentration is relatively high, to the interstitial fluid. Carbon dioxide and waste flow from the insterstitial fluid to the blood.

Question 73

How is the circulatory system involved in thermoregulation?

Answer 73

Vasoconstriction and vasodilation function to control body temperature. In hot conditions, vessels dilate to bring blood close to the outside environment and facilitate heat loss. Cold conditions promote constriction to keep blood close to the warmer core. ## Footnote Indirectly, plasma also provides fluid that diffuses into the interstitium and eventually comprises sweat.

Question 74

How do capillary walls differ from the walls of other types of blood vessels?

Answer 74

Capillary walls consist only of a single layer of endothelium. This thin-walled structure facilitates the diffusion of gases and solutes. ## Footnote Since capillaries do not contain smooth muscle, they cannot vasoconstrict.

Question 75

In which part of a capillary bed does hydrostatic pressure predominate?

Answer 75

At the **arterial end**, closest to the heart. ## Footnote Since hydrostatic pressure is simply another name for blood pressure, it is highest at the beginning of a capillary bed. As fluid is pushed outward, it decreases and eventually is surpassed by osmotic pressure.

Question 76

Which type of pressure would be disrupted by a condition that introduced large holes or gaps to capillary walls?

Answer 76

**Osmotic**, or **oncotic**, pressure. ## Footnote Osmotic pressure in a capillary bed is maintained by proteins, which are too large to leave the vessels. If capillary walls became more permeable to large molecules, this function would be altered, disrupting the flow of fluid back into the capillary.

Question 77

What is/are the site(s) of blood oxygenation and detoxification in the fetus, and how does this differ from adult circulation?

Answer 77

In adult circulation, gas exchange occurs within the lungs, while detoxification of blood is performed by the liver. In the fetus, both of these functions are performed by the placenta. ## Footnote For this reason, adaptations of the fetal circulatory system exist to bypass both the lungs and the liver.

Question 78

Which structures are connected by the **ductus arteriosus**?

Answer 78

the pulmonary artery to the aorta ## Footnote Much of fetal blood does not even reach the pulmonary artery, but this adaptation ensures that any blood that does reach this vessel will still bypass the lungs.

Question 79

Which structures are connected by the **ductus venosus**?

Answer 79

the umbilical vein to the inferior vena cava ## Footnote This adaptation allows oxygenated blood from the placenta to bypass the liver.

Question 80

What is the function of the **foramen ovale**?

Answer 80

This is a hole between the fetal atria that allows oxygenated blood to travel directly from the right atrium to the left atrium. (Normally, in adults, the blood in the right atrium is *de*oxygenated. In a fetus, however, this blood was already oxygenated in the placenta.) ## Footnote Along with the ductus arteriosus, the foramen ovale causes fetal blood to bypass the lungs.

Question 81

Why are adaptations to fetal pulmonary circulation necessary?

Answer 81

Fetal lungs are underdeveloped and fluid-filled, so they are bypassed by fetal circulation. Additionally, a fetus receives oxygen from maternal blood, not the external environment; this renders the lungs unnecessary.

Question 82

What may occur if the ductus arteriosus does not close properly after birth?

Answer 82

Oxygenated blood from the aorta may flow back to the lungs, resulting in high pulmonary pressure and difficulty breathing. ## Footnote This condition is known as "patent ductus arteriosus" and can lead to congestive heart failure.