2.10 Pulmonary and Systemic Circulation
The circulatory system ensures the distribution of oxygen and nutrients and the removal of waste products throughout the body. Blood follows distinct pathways, broadly categorized into pulmonary and systemic circulation, with specialized circuits for specific organs.
1. Pulmonary Circulation
This circuit is responsible for oxygenating the blood. It transports deoxygenated blood from the heart to the lungs and returns oxygenated blood to the heart.
Path of Blood
- Deoxygenated blood from the body enters the right atrium via the superior and inferior vena cavae.
- From the right atrium, blood flows into the right ventricle.
- The right ventricle pumps blood into the pulmonary arch (trunk), which divides into two pulmonary arteries.
- Each pulmonary artery carries deoxygenated blood to one lung for gas exchange.
- In the lungs, blood picks up oxygen and releases carbon dioxide.
- Oxygenated blood returns from the lungs to the left atrium through two pairs of pulmonary veins.
- From the left atrium, blood flows into the left ventricle, ready for systemic circulation.
Key Feature
Pulmonary arteries carry deoxygenated blood, and pulmonary veins carry oxygenated blood – opposite to the general pattern in the systemic circuit.
2. Systemic Circulation
This circuit distributes oxygenated blood from the heart to all body tissues and returns deoxygenated blood to the heart. It encompasses all arteries and veins not involved in the pulmonary circuit.
Path of Blood
- The left ventricle pumps oxygenated blood into the aorta (the largest artery).
- Branches from the aorta deliver blood to various organs and major body regions.
- After delivering oxygen and nutrients, and picking up waste, deoxygenated blood is collected by veins.
- The superior vena cava collects blood from the head, chest, and arms.
- The inferior vena cava collects blood from the lower body regions.
- Both the superior and inferior vena cavae empty their deoxygenated blood into the right atrium, completing the circuit.
Major Vessels
The aorta and the vena cavae are the principal pathways. Arteries and veins serving the same organ typically share the same name.
3. Coronary Circulation
This is a specialized part of the systemic circulation that supplies blood directly to the myocardium (heart muscle) itself, meeting its vital metabolic needs.
Path of Blood
- The myocardium is supplied with oxygenated blood by the right and left coronary arteries, which branch off the aorta.
- From the capillaries within the myocardium, deoxygenated blood enters the cardiac veins.
- These cardiac veins converge to form the coronary sinus (a channel on the posterior surface of the heart).
- The coronary venous blood then empties into the right atrium through a dedicated opening.
4. Hepatic Portal System
A portal system is a unique vascular arrangement where a capillary bed drains into another capillary bed, without passing through the heart. The hepatic portal system connects the digestive system to the liver.
Definition of a Portal System
A vascular system that begins and ends with capillary beds and lacks a pumping mechanism (like the heart) between them.
Path of Blood
- Veins from the digestive system (collecting nutrient-rich, but often deoxygenated, blood) unite to form the hepatic portal vein (the largest vein of this system).
- The hepatic portal vein carries this blood to the liver.
- Within the liver, the blood flows through a series of dilated capillaries (sinusoids).
- These liver capillaries process the absorbed nutrients and detoxify substances.
- Blood is then collected by hepatic veins.
- The hepatic veins join the inferior vena cava, returning blood to the general systemic circulation.
5. Renal Circulation
This circuit supplies blood to and drains blood from the kidneys, essential for filtering waste products from the blood and maintaining fluid balance.
Path of Blood
- The renal artery (branching off the aorta) enters the kidney, carrying oxygenated blood.
- It gives rise to branches that pass through the medulla and further divide into afferent glomerular arterioles in the cortex.
- Blood enters the glomerulus (capillary network), then the peritubular capillaries and vasa recta (other capillary networks surrounding renal tubules). This is where filtration and reabsorption occur.
- From these capillary networks, blood is drained through veins.
- These veins converge to form a single renal vein.
- The renal vein leaves the kidney, carrying deoxygenated, filtered blood, and empties into the inferior vena cava.
