Venous Blood Circulation
Venous system constitutes veins. These are the blood vessels that carry blood towards the heart from different parts of the body. Veins carry deoxygenated blood from various parts of the body except pulmonary veins. The principal veins in human body are:
A. Blood returning from the Lungs
It is the circulation from the lungs to the heart. Here, the left auricle receives oxygenated blood through two branches of pulmonary veins from each lung.
i. Left pulmonary vein- Collects oxygenated blood from left lung to the left atrium.
ii. Right pulmonary vein- Collects oxygenated blood from right lung to left atrium.
B. Venous blood returning from the different parts of body
The impure blood is collected into the right atrium from different parts of the body through two venacava - One superior venacava and one inferior venacava.
1. Venous blood collected through superior venacava
A superior venacava collects de oxygenated blood from the anterior region (head, neck and upper limb) and opens into right atrium. Each superior venacava is formed by the union of two veins:
i. Right brachiocephalic vein
ii. Left brachiocephalic vein
Each brachiocephalic vein runs along the trachea and collects the venous blood from the respective side. Each brachiocephalic vein is formed by the union of three veins:
a. External jugular vein
It collects blood from mouth, face and buccal cavity.
b. Internal jugular vein
It collects the blood from brain, eye and ear.
c. Subclavian vein
It collects blood from shoulder limbs by auxillary vein and cephalic vein.
2. Venous blood collected through inferior venacava
The inferior venacava collects the de oxygenated blood from posterior parts of the body and opens into the right atrium. It is formed by the following major veins:
a. Common iliac vein
Each common iliac vein of each side is formed by the union of two veins:
i. External iliac vein
It collects blood from outer side of leg and parts of pelvis. It is formed by anterior and posterior tibial veins, and large and small saphenous veins.
ii. Internal iliac (=hypogastric) vein
It collects blood from rectum, ureter, urinary bladder, yterus, vagina, prostrate gland, seminal vesicle, penis, scrotum in man.
b. Lumbar veins
These collect blood from lumbar region.
c. Genital vein
It collects blood from gonads. In man, right and left testicular veins, which drain the blood from male organs and inguinal region. In women, right and left ovarian veins drain blood from ovaries, uterus,etc.
d. Renal veins
These collect blood by two renal veins which drain the blood from respective kidneys. The left renal is about three times longer than the right one.
e. Supra renal veins
These drain venous blood from supra renal glands. The right supra renal is short and empties into venacava, but he left one is longer and descends down to empty into the left renal vein.
g. Hepatic veins
These are short and thick veins draining blood from liver into venacava.
h. Inferior phrenic veins
These drain the blood from lower surface of diaphragm.
Hepatic Portal System
In all parts of circulation, venous blood passes from the tissues to the heart by the most direct routes. However, the blood coming from the parts of alimentary canal do not enter directly to the heart but through another organ i.e. liver. This is called hepatic portal system.
Hepatic portal system is a system in which blood do not open directly into heart but passes via liver. It collects impure blood from various parts of alimentary canal, pancreas and spleen. It consists of a hepatic portal vein which is formed by the union of following veins:
1. Cystic vein
It collects blood from the gall bladder.
2. Pancreatic vein
It collects blood from pancreas.
3. Gastric vein
It collects blood from stomach and distal end of oesophagus.
4. Duodenal vein
It collects blood from duodenum.
5. Superior mesentric vein
It collects blood from small intestine and the proximal parts of large intestine (caecum).
6. Inferior mesentric vein
It collects blood from rectum, pelvis and descending colon of the large intestine.
7. Splenic vein
It collects blood from spleen and parts of stomach.
Significance of hepatic portal system:
1. Liver stores excess glucose in the form of glycogen. The process is called glycogenesis. When there s deficiency of glucose, then glycogen is broken into glucose (glucogenolysis) which is released into blood.
2. The fat cells are picked up by the Kupffer cells of the liver.
3. The ammonia produced during the process of protein metabolism is converted into urea, which is carried to the kidneys.
4. Undesirable substances are also excreted by the liver.