Question

Trace the pathway of blood to and from the brain in the systemic circuit. (pages $102-04$ )

Short answer

The pathway of blood to and from the brain in the systemic circuit begins at the left ventricle, which pumps oxygen-rich blood into the aorta. From the aorta, blood flows to the brain through the common carotid arteries, which divide into internal and external carotid arteries. The internal carotid arteries supply blood directly to the brain and form the Circle of Willis, an arterial network ensuring blood flow to vital brain areas. Nutrient exchange occurs in the brain's capillary beds, and the now deoxygenated blood returns to the heart through veins like the internal jugular veins. These veins merge and form the superior vena cava, which drains the blood into the right atrium. Lastly, the blood is pumped to the lungs for oxygenation and returns to the left atrium, completing the blood circulation cycle.

Step-by-step solution

Beginning of the systemic circuit at the left ventricle

The systemic circuit starts from the left ventricle of the heart, which pumps oxygen-rich blood received from the lungs through pulmonary veins into the aorta.

Blood flows through the aorta

The blood ejected from the left ventricle enters the aorta, the largest artery in the body. Aorta carries oxygen-rich blood to various parts of the body, including the brain, through its branches.

Blood reaches the brain via carotid arteries

From the aorta, blood flows to the brain through the common carotid arteries. These vessels further divide into the internal and external carotid arteries. The internal carotid arteries supply oxygen-rich blood directly to the brain.

Blood reaches the Circle of Willis

The internal carotid arteries enter the cranial cavity through the carotid canal and give rise to the Circle of Willis (an arterial network). The Circle of Willis consists of various arteries that provide blood supply to different parts of the brain, ensuring that even if one artery is blocked, there is still blood flow to vital brain areas.

Nutrient exchange in the capillary beds of the brain

Within the different areas of the brain, the arteries divide into capillaries for optimal nutrient exchange. In the capillary beds, oxygen and nutrients (e.g. glucose) are delivered to brain cells, and waste products like carbon dioxide (CO2) and other cellular metabolites are collected.

Blood leaves the brain via veins

After exchanging nutrients and waste products in the brain, the now deoxygenated blood starts its return journey back to the heart through veins. Multiple small veins merge to form larger veins, such as the internal jugular veins, which drain the deoxygenated blood from the brain.

Blood returns to the heart

The internal jugular veins merge with subclavian veins, forming brachiocephalic veins. Eventually, the brachiocephalic veins unite and form the superior vena cava, which drains the deoxygenated blood into the right atrium of the heart.

Blood is pumped to the lungs for oxygenation

The right atrium pushes the deoxygenated blood into the right ventricle, which then pumps it to the lungs through the pulmonary arteries for oxygenation. Oxygen is added to the blood, and CO2 is removed in the lung capillaries. The now oxygen-rich blood returns to the left atrium of the heart through pulmonary veins, completing the blood circulation cycle.

Key concepts

Systemic Circuit

The systemic circuit is a crucial part of our circulatory system that specifically deals with the flow of oxygen-rich blood from the heart to the rest of the body and back again. It begins in the left ventricle of the heart, where blood is pumped into the aorta, the largest artery in the body.

This oxygen-rich blood travels from the aorta to various parts of the body, including the brain. The main goal is to deliver oxygen and nutrients to body tissues and return deoxygenated blood to the heart.

• The left ventricle pumps blood into the aorta.
• Oxygen-rich blood is distributed to the body, including organs like the brain.
• Blood eventually returns to the heart after dropping off oxygen and picking up waste products.

This circuit is essential for maintaining the oxygen and energy supply that our organs need to function properly, particularly the brain, which consumes a significant amount of the body's oxygen.

Carotid Arteries

The carotid arteries are major blood vessels in the neck that supply the brain with essential oxygen-rich blood. Starting from the aorta, the blood destined for the brain passes through the common carotid arteries, which then branch into internal and external carotid arteries.

The internal carotid arteries are specifically responsible for providing the brain with a steady flow of oxygen and crucial nutrients. Their role is vital because the brain is one of the organs that require a constant supply of oxygen to function properly.

• Common carotid arteries branch from the aorta.
• These arteries divide into internal and external carotid arteries.
• Internal carotid arteries supply blood directly to the brain.

These pathways ensure that oxygen and nutrients are efficiently delivered to the brain, enabling cognitive functions and overall brain health.

Circle of Willis

The Circle of Willis is an important arterial network located at the base of the brain. It plays a crucial role in maintaining adequate blood flow to the brain.

Formed by the joining of the internal carotid arteries and several other arteries that supply blood to the brain, the Circle of Willis ensures that, even if one pathway is obstructed, blood can still reach vital areas of the brain through alternate routes.

• Internal carotid arteries feed into the Circle of Willis.
• Consists of interconnected arteries, creating redundancy.
• Ensures consistent blood supply to critical brain areas.

This redundancy is particularly important in preventing strokes and ensuring that the brain continues to receive the blood it needs, even in the event of an arterial blockage.