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Anatomy and Physiology Unit 5: Cardiovascular and respiratory systems

The heart pumps blood through vessels to deliver oxygen, and the lungs load blood with oxygen and remove carbon dioxide.

Covers: Heart anatomy, blood flow, blood vessels, blood components, cardiac output, lungs and gas exchange

Study guide

The heart is a muscular pump with four chambers: two upper atria that receive blood and two lower ventricles that pump it out. Valves prevent backflow. The right side pumps oxygen-poor blood to the lungs (pulmonary circuit), and the left side pumps oxygen-rich blood to the body (systemic circuit). The left ventricle has the thickest wall because it pumps blood the farthest. Blood flows: body → vena cavae → right atrium → right ventricle → pulmonary arteries → lungs → pulmonary veins → left atrium → left ventricle → aorta → body. The SA node is the heart's natural pacemaker, starting each beat.

Arteries carry blood away from the heart under high pressure and have thick, elastic walls. Veins carry blood toward the heart under low pressure and have valves. Capillaries are tiny vessels with walls one cell thick where gases, nutrients and wastes are exchanged with tissues. Blood pressure is written as systolic over diastolic, such as 120/80.

Blood is a fluid connective tissue. Plasma, mostly water, carries nutrients, hormones and wastes. Red blood cells carry oxygen bound to hemoglobin, white blood cells fight infection, and platelets help clot blood. Blood types (A, B, AB, O and Rh factor) matter for transfusions.

Cardiac output is the amount of blood the heart pumps per minute: heart rate × stroke volume, about 5 liters per minute at rest. It increases during exercise.

The respiratory system brings in oxygen and removes carbon dioxide. Air travels through the nose, pharynx, larynx, trachea, bronchi and bronchioles to the alveoli, tiny air sacs surrounded by capillaries where gas exchange happens by diffusion: O₂ moves into the blood and CO₂ moves out. Breathing is driven by the diaphragm: contracting it enlarges the chest cavity and pulls air in. Breathing rate is controlled mainly by CO₂ levels in the blood.

The SA node in the wall of the right atrium starts each heartbeat. The signal spreads across the atria to the AV node, which delays it briefly. It then travels down the bundle of His and the bundle branches to the Purkinje fibers, which make the ventricles contract. On an ECG, the P wave is atrial depolarization, the QRS complex is ventricular depolarization, and the T wave is ventricular repolarization. The tricuspid valve sits between the right atrium and right ventricle, and the bicuspid (mitral) valve between the left atrium and left ventricle. Semilunar valves guard the exits to the pulmonary trunk (pulmonary valve) and the aorta (aortic valve). The "lub-dub" sound is the AV valves closing and then the semilunar valves closing.

Gases diffuse from areas of higher partial pressure to lower partial pressure, so O₂ moves from the alveoli into the blood and CO₂ moves out. Most CO₂ travels in plasma as bicarbonate ions, made in red blood cells with the help of carbonic anhydrase. That's why CO₂ levels affect blood pH: more CO₂ makes the blood more acidic. Surfactant lowers surface tension in the alveoli so they don't collapse. Quiet exhalation is passive: the diaphragm relaxes, the chest cavity shrinks, and air flows out. In obstructive diseases like asthma and COPD, narrowed airways make it hard to get air out, while restrictive diseases make it hard for the lungs to expand.

Respiratory centers in the medulla and pons set the breathing rhythm. Central chemoreceptors respond mainly to rising CO₂ (and the drop in pH that comes with it), while peripheral chemoreceptors in the carotid and aortic bodies also sense low O₂. Faster breathing removes CO₂ and raises blood pH.

Key ideas

• Blood path: body → right atrium → right ventricle → lungs → left atrium → left ventricle → body.
• Arteries carry blood away from the heart; veins carry it back.
• Capillaries are where exchange happens.
• Red blood cells carry O₂ on hemoglobin; white cells fight infection; platelets clot.
• Gas exchange happens in alveoli by diffusion.

Formulas and rules

Common mistakes

• Saying arteries always carry oxygen-rich blood. Pulmonary arteries carry oxygen-poor blood.
• Mixing up which side of the heart pumps to the lungs (the right).

Words to know

Left ventricle

The heart chamber that pumps blood to the body.

Artery

A vessel that carries blood away from the heart.

Alveoli

Tiny air sacs in the lungs where gas exchange happens.

Cardiac output

Heart rate × stroke volume.

Red blood cell

Carries oxygen using hemoglobin.

Vein

A vessel that carries blood toward the heart.

Atrium

An upper heart chamber that receives blood.

Ventricle

A lower heart chamber that pumps blood out.

Pulmonary circuit

Carries blood between the heart and lungs.

Systemic circuit

Carries blood between the heart and body.

SA node

The heart's natural pacemaker.

Capillary

A tiny vessel where exchange with tissues happens.

Blood pressure

Systolic over diastolic, like 120/80.

Plasma

The liquid part of blood.

Platelets

Cell fragments that help blood clot.

Hemoglobin

The protein in red blood cells that binds oxygen.

Diaphragm

The muscle that drives breathing.

Trachea

The windpipe.

Sample practice questions

15 more questions like these, from level 1 to 5, are in the practice set.