The Circulatory System

Name the three blood components that carry out transport, defence, and clotting functions, and state what each of them does.

Explain what blood plasma is, what blood looks like when its components are separated, and cite two types of substances that travel dissolved in it.

Explain what platelets are and describe, step by step, what happens when we get a small cut in the skin until it stops bleeding.

Explain what white blood cells are and reason why their number in the blood increases when we have an infection.

A blood donation removes about $450\ \text{mL}$. If an adult has about $5\ \text{L}$ of blood, calculate what percentage of their blood they donate and explain why the body can replace that amount within a few weeks without problems.

Look at the cross-sectional image of the heart. State which numbers point to the two atria and which point to the two ventricles, and explain one difference between their walls.

Name the four chambers of the heart and state through which one oxygen-poor blood from the body enters and from which one oxygen-rich blood leaves towards the whole organism.

In the image of the heart, the numbered valves between the chambers can be seen. Explain the function of these valves and reason what problem a person would have if the valve between the left atrium and the left ventricle did not close properly.

A child takes their pulse at the wrist and counts $72$ beats in one minute; after running during break time, they count $115$ beats in one minute. Explain why the heart rate increases during exercise, linking it to the needs of muscle cells.

The heart is said to actually be "two pumps in one". Explain what this expression means, which internal wall separates the two halves, and what problem there would be if the blood from both sides mixed.

Complete the sentence with the correct terms: "$\dots$ carry blood from the heart to the organs, $\dots$ return it from the organs to the heart, and $\dots$ are very thin vessels where the exchange of substances with tissues takes place."

State whether these statements are true or false and justify your answer: a) all the blood flowing through the arteries is rich in oxygen; b) capillaries connect arteries with veins; c) the heart valves prevent blood from flowing backwards.

Arteries have thick, elastic walls, and veins have thinner walls. Relate this difference to the pressure at which blood travels in each type of vessel, and explain what would happen if a vein had to withstand the pressure of an artery.

Explain what blood pressure is, which two values are measured when it is taken and in which type of vessel it is usually measured, and reason why keeping it constantly high ends up damaging the blood vessels.

A student attempts to check if their partner has a pulse by firmly placing their fingers on a visible vein on the back of the hand. Sensing no pulsation at all, they state with concern: "There is no pulse, so blood is not circulating through this vein."

1. a) Refute the student's conclusion by explaining what mechanical phenomenon precisely causes the pulse wave and why this pulse disappears as it passes through the capillaries and reaches the veins, even though blood continues to flow.
2. b) Justify what structural difference in the elastic layer of artery walls (compared to veins) allows them to withstand and dampen this continuous pressure pulse without rupturing.

Members of the royal guard must stand completely straight and motionless for hours. Quite often, some suffer from sudden dizziness and even fainting. In contrast, if they are allowed to rhythmically contract their calf muscles by raising their heels without moving from their spot, fainting is completely prevented.

1. a) Explain how gravity, leg muscle contraction, and the semilunar valves in veins interact to ensure blood returns to the heart.
2. b) Justify why standing still reduces blood flow to the brain, causing fainting, and predict what long-term damage venous valves will suffer if a person works standing still for many hours over years (development of varicose veins).

Describe the complete pathway of blood through the two circuits of the circulatory system: the pulmonary circuit (or minor circuit), indicating which heart chamber it begins in, which vessel carries it out, what happens to it in the lungs, and through which vessel and to which chamber it returns; and the systemic circuit (or major circuit), indicating which chamber it leaves from, through which major artery, to which parts of the body it carries blood, and through which major vein it returns to the heart.

Follow the path of a glucose molecule from the moment it is absorbed in the small intestine until it reaches a cell in the thigh, indicating which vessels it travels through, which heart chambers it passes through, and what compulsory "detour" it makes before reaching its destination.

A patient goes to the doctor and is diagnosed with severe pulmonary hypertension (the resistance of the vessels leading to the lungs is abnormally high), while the pressure in the rest of the body is normal. Two years later, an ultrasound reveals that the muscular wall of one of their two ventricles has doubled in thickness to compensate for this extra workload.

1. a) Reasonably justify which of the two ventricles will have increased in thickness.
2. b) Predict what consequences it would have for gas exchange in the alveoli and for the body if that ventricle were to eventually fail due to muscular exhaustion.

If a person's heart expels about $70\ \text{mL}$ of blood with each beat and beats $75$ times per minute, calculate the volume of blood it pumps in one day, express it in liters, and comment on why the figure seems so large given that the body only contains about $5\ \text{L}$ of blood.

A student raises the following question in class: "The heart's chambers are constantly filled with oxygen-rich blood coming from the lungs. Therefore, it makes no sense that a blockage in the coronary arteries causes a heart attack, since the cardiac muscle could absorb nutrients directly from the blood it pumps."

1. a) Refute this argument by justifying why myocardial cells cannot obtain nutrients directly by diffusion from inside the heart chambers and depend exclusively on the coronary arteries.
2. b) Explain the "domino effect" that occurs in the rest of the body when an area of the heart muscle undergoes necrosis, comparing it to what would happen if the blocked artery were in an arm.

At a medical center, the cases of three patients who present with an identical symptom are analyzed: they suffer from extreme muscle fatigue and become breathless at the slightest physical exertion.

• Patient A: Normal red blood cells and hemoglobin, but their heart pumps at half capacity due to heart failure.

• Patient B: Heart and lungs in perfect condition, but suffers from severe anemia with a hemoglobin level of $6.5\ \text{g/dL}$ (the normal value is $12\text{--}15\ \text{g/dL}$).

• Patient C: Normal heart and blood test results, but suffers from a condition that drastically reduces nutrient absorption in the small intestine.

1. a) Justify why all three patients show the exact same muscle fatigue, identifying which cellular chemical process is being limited in the mitochondria of their muscles in each case.
2. b) Explain why patient B's body tries to compensate for their condition by increasing heart rate and respiratory rate.

At rest, both a sedentary person and an elite athlete need their heart to pump the exact same total volume of blood per minute: $4.8\ \text{L/min}$ ($4800\ \text{mL/min}$).

• The sedentary person has a resting heart rate of $80$ beats/min.

• The athlete has a resting heart rate of $48$ beats/min.

1. a) Calculate the volume of blood ejected by each person's ventricle in a single beat (express it in $\text{mL}$).
2. b) Based on your calculations, justify what changes the athlete's myocardium has undergone through training to become more efficient.
3. c) Predict what will happen to each person's heart rate if they start running at a speed that requires a cardiac output of $14\ \text{L/min}$, and reason who will reach their fatigue limit sooner.

A blood clot blocking a cerebral artery causes a stroke, while if it blocks a coronary artery it causes a myocardial infarction. In both cases, emergency services state that "time is tissue" and that every minute of delay severely worsens the prognosis.

1. a) Explain the sequence of events at the cellular level (from the lack of oxygen and glucose to cell death or necrosis) suffered by the affected cells (neurons or cardiomyocytes) following the obstruction.
2. b) Justify why administering a drug that dissolves the clot within the first hour can even completely restore organ function, but that same treatment progressively loses effectiveness and, after several hours, can no longer recover tissue that has already died.
3. c) Reason why the sequelae of a stroke (such as loss of speech or mobility) are usually irreversible or very difficult to recover from, relating this to the capacity for division and regeneration of nervous tissue compared to other tissues.