Mammalian circulatory system and blood vessels

In a nutshell

Large organisms have a small surface area to volume ratio, so substances cannot enter or exit in sufficient quantities or by simple diffusion. These organisms need transport systems, like the circulatory system, to move substances from exchange surfaces to cells and to remove waste.

Mammalian circulatory system

Mammals have a closed, double circulatory system. This means that blood is enclosed in vessels and that it passes through the heart twice for each complete circuit around the body.

The first passage takes deoxygenated blood from the heart to the lungs and back to the heart. The other circulates oxygenated blood around the body and back to the heart.

Biology; Exchange and transport; KS5 Year 12; Mammalian circulatory system and blood vessels

1.	Brain, head and neck	10.	Vena cava
2.	Arms	11.	Pulmonary vein
3.	Lungs	12.	Hepatic vein
4.	Heart	13.	Hepatic artery
5.	Liver	14.	Hepatic portal vein
6.	Gut	15.	Renal vein
7.	Kidneys	16.	Renal artery
8.	Pulmonary artery	17.	Blood flow from lower limbs
9.	Aorta	18.	Blood flow to lower limbs

Passage through the circulatory system

1.	Deoxygenated blood is pumped from the heart to the lungs through the pulmonary artery.
2.	Oxygenated blood leaves the lungs and travels to the heart through the pulmonary vein.
3.	Blood is pumped from the heart to the body through the aorta.
4.	Blood travels from the body to the kidneys through the renal vein.
5.	Blood travels from the kidneys towards the heart through the renal artery.
6.	Blood travels from the body to the heart through the vena cava.
7.	Deoxygenated blood is transported from the liver to the heart through the hepatic vein.
8.	Oxygenated blood is transported from the heart to the liver through the hepatic artery.
9.	Deoxygenated blood travels from the gastrointestinal tract, gallbladder, pancreas and spleen to the liver through the hepatic portal vein.

Blood vessels

There are four types of blood vessels; arteries, arterioles, veins and capillaries.

Arteries

Most arteries carry oxygenated blood from the heart to the body, with the exception of the pulmonary artery which carries deoxygenated blood. Arteries have elastic tissue in their outer walls that allow them to stretch and recoil as the heart beats. They also have a thick muscular middle layer of smooth muscle that allows the artery to constrict, dilate and withstand high blood pressure. Arteries also have a folded endothelium which allows them to stretch to accommodate blood flow.

Arterioles

Arteries eventually narrow into arterioles which direct blood to where it is needed in the body. Muscles in the arterioles are able to contract and restrict blood flow or relax to allow maximum blood flow.

Veins

Veins carry deoxygenated blood from the body to the heart, except for the pulmonary vein which carries oxygenated blood. Veins have less elastic tissue in their walls and have a thinner muscular middle layer as blood returns to the heart under low pressure. Veins have valves that prevent blood flowing backwards.

Capillaries

Arterioles narrow into capillaries which are adapted for substance exchange with cells. They are one cell thick and located near exchange surfaces which reduces the diffusion distance between the capillary and the cell. Lots of capillaries are arranged into capillary beds which increases the surface area for exchange.

Biology; Exchange and transport; KS5 Year 12; Mammalian circulatory system and blood vessels — A: Artery, 1. Elastic tissue, 2. Thick muscular layer, 3. Folded endothelium creating a smaller lumen.
B: Capillary, 4. One cell thick endothelium.

C: Vein, 5. Large lumen, 6. Endothelium, 7. Thinner muscular layer.

Tissue fluid

Definition

Tissue fluid is the watery substance that surrounds cells. It is composed of small molecules from the blood plasma.

Examples

Oxygen, water and nutrients.

The cells will take in some substances, like oxygen and nutrients, from the tissue fluid and release their metabolic waste into it. This process occurs in capillary beds and it is described below.

Biology; Exchange and transport; KS5 Year 12; Mammalian circulatory system and blood vessels — 1. Blood plasma, 2. Tissue cell, 3. Capillary, 4. Red blood cell, 5. Tissue fluid, 6. Blood from the arteriole, 7. Tissue, 8. Capillary bed, 9. Blood to the venule, 10. Water

A.	The hydrostatic pressure is greatest nearer the arteries and higher in the capillaries compared to the tissue fluid.
B.	The hydrostatic pressure difference results in the expulsion of fluid from the capillaries into the surrounding tissues. This forms tissue fluid.
C.	The expulsion of fluid lowers the hydrostatic pressure in the capillaries. This means that the hydrostatic pressure is lowest near the veins.
D.	The water potential is lower in the capillaries than in the tissue fluid. Therefore, water enters the capillaries by osmosis.
E.	Any excess tissue fluid will be drained into the lymphatic system.