MCAT Biology Review

Chapter 7: The Cardiovascular System

Conclusion

The cardiovascular system is one of the most commonly tested MCAT topics. You should be familiar with its basic structure: a system with two pumps in series. The right ventricle pumps blood into the pulmonary circulation, while the left ventricle pumps blood into the systemic circulation. We discussed the myogenic activity of cardiac muscle and the pathway that electricity follows in the heart through the SA node, AV node, bundle of His, and Purkinje fibers. The movement of blood through the vascular system is a function of the heart’s pumping to generate pressure. Blood pressure is a measure of the blood’s force per unit area on the vessel walls and is recorded as a gauge pressure (pressure above and beyond atmospheric pressure). We discussed the differences in structure between arteries, capillaries, and veins, and how these anatomical differences are reflective of their different functions. We then reviewed the composition of blood along with the three major blood cell types. We examined the ABO and Rh antigen systems, which frequently appear on the MCAT due to their widespread clinical relevance. The blood’s ability to carry oxygen and carbon dioxide was also described; recall that carbon dioxide is primarily carried as bicarbonate ions in the blood. The conversion of carbon dioxide to and from this ion is accomplished by the enzyme carbonic anhydrase.

In this chapter, we focused on the functions of red blood cells, plasma, and platelets. We briefly examined the immune system, which is primarily driven by the actions of white blood cells and their products. Immunology is considered one of the most challenging courses in medical school, as you’ll learn about dozens of cytokines, clusters of differentiation (CD), and specialized cell types. In the next chapter, we’ll focus on the basics of immunology, discussing the major components of the innate (nonspecific) and adaptive (specific) immune responses.

Concept Summary

Anatomy of the Cardiovascular System

· The cardiovascular system consists of a muscular four-chambered heart, blood vessels, and blood.

· The heart is composed of cardiac muscle and supports two different circulations: the pulmonary circulation and the systemic circulation.

o Each side of the heart consists of an atrium and a ventricle.

o The atria are separated from the ventricles by the atrioventricular valves (tricuspid on the right, mitral [bicuspid] on the left).

o The ventricles are separated from the vasculature by the semilunar valves (pulmonary on the right, aortic on the left).

o The pathway of blood is:

o The left side of the heart contains more muscle than the right side because the systemic circulation has a much higher resistance and pressure.

o Electrical conduction of the heart starts at the sinoatrial (SA) node and then goes to the atrioventricular (AV) node. From the AV node, electrical conduction goes to the bundle of His before traveling through the Purkinje fibers.

o Systole refers to the period during ventricular contraction when the AV valves are closed. During diastole, the heart is relaxed and the semilunar valves are closed.

o The cardiac output is the product of heart rate and stroke volume.

o The sympathetic nervous system increases the heart rate and contractility. The parasympathetic nervous system decreases heart rate.

· The vasculature consists of arteries, veins, and capillaries.

o Arteries are thick, highly muscular structures with an elastic quality. This allows for recoil and helps to propel blood forward within the system. Small muscular arteries are arterioles, which control flow into capillary beds.

o Capillaries have walls that are one cell thick, making them so narrow that red blood cells must travel through them in single-file lines. Capillaries are the sites of gas and solute exchange.

o Veins are inelastic, thin-walled structures that transport blood to the heart. They are able to stretch in order to accommodate large volumes of blood but do not have recoil capability. Veins are compressed by surrounding skeletal muscles and have valves to maintain one-way flow. Small veins are called venules.

· A portal system is one in which blood passes through two capillary beds in series.

o In the hepatic portal system, blood travels from the gut capillary beds to the liver capillary bed via the hepatic portal vein.

o In the hypophyseal portal system, blood travels from the hypothalamus to the anterior pituitary.

o In the renal portal system, blood travels from the glomerulus to the vasa recta through an efferent arteriole.

Blood

· Blood is composed of cells and plasma, an aqueous mixture of nutrients, salts, respiratory gases, hormones, and blood proteins.

· Erythrocytes (red blood cells) lack mitochondria, a nucleus, and organelles in order to make room for hemoglobin, a protein that carries oxygen. Common measurements include hemoglobin concentration and hematocrit, the percent of blood composed of erythrocytes.

· Leukocytes (white blood cells) are formed in the bone marrow. They are a crucial part of the immune system.

o Granular leukocytes such as neutrophils, eosinophils, and basophils play a role in nonspecific immunity.

o Agranulocytes, including lymphocytes and monocytes, also play a role in immunity, with lymphocytes playing a large role in specific immunity.

· Thrombocytes (platelets) are cell fragments from megakaryocytes that are required for coagulation.

· Blood antigens include the surface antigens A, B, and O, as well as Rh factor (D).

o The I^A (A) and I^B (B) alleles are codominant, while the i (O) allele is recessive. An individual has antibodies for any ABO alleles he or she does not have.

o Positive Rh factor is dominant. An Rh-negative individual will only create anti-Rh antibodies after exposure to Rh-positive blood.

Physiology of the Cardiovascular System

· Blood pressure refers to the force per unit area that is exerted on the walls of blood vessels by blood. It is divided into systolic and diastolic components.

o It must be high enough to overcome the resistance created by arterioles and capillaries, but low enough to avoid damaging the vasculature and surrounding structures.

o It can be measured with a sphygmomanometer.

o The blood pressure is maintained by baroreceptor and chemoreceptor reflexes. Low blood pressure promotes aldosterone and antidiuretic hormone (ADH or vasopressin) release. High blood osmolarity also promotes ADH release. High blood pressure promotes atrial natriuretic peptide (ANP) release.

· Gas and solute exchange occurs at the level of the capillaries and relies on the existence of concentration gradients to facilitate diffusion across the capillary walls. Capillaries are also leaky, which aids in transport of gases and solutes.

o Starling forces consist of hydrostatic pressure and osmotic (oncotic) pressure. Hydrostatic pressure is the pressure of the fluid within the blood vessel, while osmotic pressure is the “sucking” pressure drawing water toward solutes. Oncotic pressure is osmotic pressure due to proteins. Hydrostatic pressure forces fluid out at the arteriolar end of a capillary bed; oncotic pressure draws it back in at the venule end.

o Oxygen is carried by hemoglobin, which exhibits cooperative binding. In the lungs, there is a high partial pressure of oxygen, resulting in loading of oxygen onto hemoglobin. In the tissues, there is a low partial pressure of oxygen, resulting in unloading. With cooperative binding, each successive oxygen bound to hemoglobin increases the affinity of the other subunits, while each successive oxygen released decreases the affinity of the other subunits.

o Carbon dioxide is largely carried in the blood in the form of carbonic acid, or bicarbonate and hydrogen ions. Carbon dioxide is nonpolar and not particularly soluble, while bicarbonate, hydrogen ions, and carbonic acid are polar and highly soluble.

o A high P_aCO₂, high [H⁺], low pH, high temperature, and high concentration of 2,3-BPG can cause a right shift in the oxyhemoglobin dissociation curve, reflecting a decreased affinity for oxygen.

o In addition to the opposites of the causes of a right shift, a left shift can also be seen in fetal hemoglobin compared to adult hemoglobin.

o Nutrients, wastes, and hormones are carried in the bloodstream to tissues for use or disposal.

· Coagulation results from an activation cascade.

o When the endothelial lining of a blood vessel is damaged, the collagen and tissue factor underlying the endothelial cells are exposed. This results in a cascade of events known as the coagulation cascade, ultimately resulting in the formation of a clot over the damaged area.

o Platelets bind to the collagen and are stabilized by fibrin, which is activated by thrombin.

o Clots can be broken down by plasmin.

Answers to Concept Checks

· 7.1

1. 

2. Sinoatrial (SA) node → atrioventricular (AV) node → bundle of His (AV bundle) and its branches → Purkinje fibers

3. 

4. The right side of the heart pumps blood into a lower-resistance circuit and must do so at lower pressures; therefore, it requires less muscle. The left side of the heart pumps blood into a higher-resistance circuit at higher pressures; therefore, it requires more muscle.

5. If all autonomic innervation to the heart were lost, the heart would continue beating at the intrinsic rate of the pacemaker (SA node). The individual would be unable to change his or her heart rate via the sympathetic or parasympathetic nervous system, but the heart would not stop beating.

· 7.2

1. Plasma is an aqueous mixture of nutrients, salts, respiratory gases, hormones, and blood proteins (clotting proteins, immunoglobulins, and so on).

2. A B⁺ person could receive blood from a B⁺, B^–, O⁺, or O^– person. A B⁺ person could donate blood to a B⁺ or AB⁺ person.

3. Hematocrit measures the percentage of a blood sample occupied by red blood cells. It is measured in percentage points.

4. Lymphocytes are involved in specific immune defense.

5. Platelets are cellular fragments or shards that are given off by megakaryocytes in the bone marrow.

6. Only leukocytes (including neutrophils, eosinophils, basophils, monocytes/macrophages, and lymphocytes) contain nuclei. Erythrocytes and platelets do not.

· 7.3

1. Opening up more capillary beds (which are in parallel) will decrease the overall resistance of the circuit. The cardiac output will therefore increase in an attempt to maintain constant blood pressure. This is a risk to the heart because the increased demand on the heart can eventually tire it, leading to a heart attack or a precipitous drop in blood pressure.

2. The bicarbonate buffer system equation is CO₂ (g) + H₂O (l) ⇌ H₂CO₃ (aq) ⇌ H⁺ (aq) + HCO^–₃ (aq). The combining of carbon dioxide and water is catalyzed by carbonic anhydrase.

3. The amount of oxygen delivery can be seen as a drop in the y-value (percent hemoglobin saturation) on an oxyhemoglobin dissociation curve. For example, if the blood is 100% saturated while in the lungs (at 100 mmHg O₂) and only 80% saturated while in tissues (at 80 mmHg O₂), then 20% of the oxygen has been released to tissues.

4. A right shift can be caused by increased CO₂, increased [H⁺], decreased pH, increased temperature, or increased 2,3-BPG. A left shift can be caused by decreased CO₂, decreased [H⁺], increased pH, decreased temperature, decreased 2,3-BPG, or in fetal hemoglobin (in comparison to adult hemoglobin).

5. The coagulation cascade can be started by exposure of collagen and tissue factor. The clot is stabilized by fibrin.

Equations to Remember

(7.1) Cardiac output: CO = HR × SV

(7.2) Ohm’s law applied to circulation: ΔP = CO × TPR

Shared Concepts

· Biochemistry Chapter 9

o Carbohydrate Metabolism I

· Biology Chapter 6

o The Respiratory System

· Biology Chapter 8

o The Immune System

· General Chemistry Chapter 12

o Electrochemistry

· Physics and Math Chapter 4

o Fluids

· Physics and Math Chapter 6

o Circuits