Single circulation is found in fish and some amphibians. In this system, blood passes through the heart only once during each complete circuit of the body.

Mechanism

1. The heart pumps deoxygenated blood through the ventral aorta to the gills.
2. In the gill capillaries, blood picks up oxygen and releases carbon dioxide (gas exchange occurs).
3. Oxygenated blood then flows directly from the gills to the body tissues (systemic vessels).
4. After delivering oxygen, the blood returns to the heart through the dorsal aorta.

Characteristics

• Blood flows in series through gills and body organs
• Blood pressure falls significantly after passing through gill capillaries
• Flow is slower and under lower pressure compared to double circulation
• Oxygenated and deoxygenated blood can mix slightly

Mammals and birds have double circulation, where blood passes through the heart twice during each complete circuit. This system is more efficient and maintains higher blood pressure.

Two Circuits

1. Pulmonary Circulation (Lung Circuit)

• Path: Right ventricle → Pulmonary artery → Lungs → Pulmonary veins → Left atrium
• Deoxygenated blood is carried from the heart to the lungs
• Oxygenated blood returns from lungs to the heart

2. Systemic Circulation (Body Circuit)

• Path: Left ventricle → Aorta → Body arteries → Capillaries → Veins → Right atrium
• Oxygenated blood is distributed to all body tissues
• Deoxygenated blood returns to the heart

Advantages of Double Circulation

• High pressure maintained in systemic circulation for efficient delivery to distant tissues
• Complete separation of oxygenated and deoxygenated blood
• Supports high metabolic rates in birds and mammals
• Body organs arranged in parallel, allowing independent regulation of blood flow

The foetus obtains oxygen and nutrients from the mother through the placenta, not through its own lungs. The foetal circulatory system has special adaptations:

Key Structures

1. Umbilical vein: Carries oxygenated blood from placenta to foetus
2. Ductus venosus: Bypasses the foetal liver, connecting umbilical vein to inferior vena cava
3. Foramen ovale: Opening between right and left atria allowing blood to bypass lungs
4. Ductus arteriosus: Connection between pulmonary artery and aorta, shunting blood away from non-functional lungs

Blood Flow in the Foetus

1. Oxygenated blood arrives via umbilical vein → ductus venosus → inferior vena cava → right atrium
2. Most blood flows through foramen ovale directly to left atrium → left ventricle → aorta → body
3. Some blood goes to right ventricle → pulmonary artery → ductus arteriosus → aorta (bypassing lungs)
4. Deoxygenated blood returns to placenta via umbilical arteries

Foetal Haemoglobin

Foetal haemoglobin (HbF) has higher affinity for oxygen than adult haemoglobin (HbA), enabling the foetus to extract oxygen efficiently from maternal blood.

When the newborn starts breathing:

1. Foramen ovale closes due to increased left atrial pressure (becomes fossa ovalis)
2. Ductus arteriosus constricts within hours (becomes ligamentum arteriosum)
3. Ductus venosus closes (becomes ligamentum venosum)
4. Pulmonary circulation becomes functional
5. Umbilical vessels thrombose

Question: Describe the path a red blood cell takes starting from the right atrium through one complete double circulation in a mammal.

Answer:

1. Deoxygenated blood enters right atrium from vena cava
2. Flows through tricuspid valve into right ventricle
3. Pumped through pulmonary semilunar valve into pulmonary artery
4. Travels to lungs for oxygenation (pulmonary capillaries)
5. Returns via pulmonary veins to left atrium
6. Flows through bicuspid (mitral) valve into left ventricle
7. Pumped through aortic semilunar valve into aorta
8. Distributed through systemic arteries to body tissues
9. Returns via systemic veins to right atrium (cycle complete)

Understanding blood circulation is important for Tanzanian health workers and students. In Tanzania, conditions like hypertension (high blood pressure) are common, and knowing how double circulation works helps explain why measuring blood pressure in the aorta (systemic circulation) is clinically important. For instance, a patient at Mnazi Mmoja Hospital presenting with a blood pressure of 160/100 mmHg indicates that their systemic circulation is under high pressure—knowledge essential for proper diagnosis and treatment in our local healthcare system.