Chapter 8: Transport in Mammals

Circulatory system can either be open, for instance in insects, or closed, like in fish and mammals where the blood is confined to blood vessels only.  The circulatory system is made up of blood vessels that carry blood away from and towards the heart. Arteries carry blood away from the heart and veins carry blood back to the heart. The circulatory system carries oxygen, nutrients, and hormones to cells, and removes waste products, like carbon dioxide.

In open circulatory system, the blood vessels open into spaces and not into capillaries, so that blood comes in direct contact with tissues. Whereas, in closed circulatory system, blood flows through capillaries connected by arteries and veins.   

Types of blood vessels:

Arteries

  • Arteries are blood vessels that carry blood at high pressures away from the heart
  • Arteries have relatively thick walls which allow them to withstand the high pressure of blood as it surges through with each ventricular contraction of the heart
  • The walls of arteries are composed of elastic and muscular tissue, as well as collagen fibers.

Capillaries

  • Arterioles branch into the smallest blood vessel – the capillaries – which form networks throughout most tissues of the body (where they are described as capillary beds)
  • Capillaries have a diameter of between 5-10 μm and most cells of the body are no more than a few μm from one
    • The diameter of a typical red blood cell is 7 μm
  • Blood flowing through the capillaries is brought close to the cells of the body to allow efficient exchange of materials (particularly the diffusion of oxygen)

Veins

  • Capillaries join together to form larger blood vessels called venules which join to form veins
  • The outer layer of the veins is relatively tough, composed largely of collagen fibers.
  • Conversely, the middle layer of the veins is relatively thin in comparison and contains only a small amount of smooth muscle and elastic fiber.

Blood

Blood is a specialized body fluid. It has four main components: plasma, red blood cells, white blood cells, and platelets. Blood has many different functions, including: transporting oxygen and nutrients to the lungs and tissues. Forming blood clots to prevent excess blood loss.

  • Your blood is made up of liquid and solids. The liquid part, called plasma, is made of water, salts, and protein. Over half of your blood is plasma. The solid part of your blood contains red blood cells, white blood cells, and platelets. Red blood cells (RBC) deliver oxygen from your lungs to your tissues and organs. Red blood cells contain hemoglobin, a protein with a quaternary structure that contains haem iron groups which can bind reversibly to oxygen

White blood cells are part of the body’s immune system. They help the body fight infection and other diseases. Types of white blood cells are granulocytes (neutrophils, eosinophil, and basophils), monocytes, and lymphocytes (T cells and B cells).

Blood Tissue Fluid and Lymph

As blood passes through capillaries, some plasma leaks out through gaps in the walls of the capillary to surround the cells of the body. This results in the formation of tissue fluid.

Blood is both a tissue and a fluid. … It is a tissue because it is a collection of similar specialized cells that serve particular functions. These cells are suspended in a liquid matrix (plasma), which makes the blood a fluid. 

The tissue fluid which is not passed through capillaries is carried back through lymphatic system. This forms lymph. Lymph is a fluid similar in composition to blood plasma. It is derived from blood plasma as fluids pass through capillary walls at the arterial end. As the interstitial fluid begins to accumulate, it is picked up and removed by tiny lymphatic vessels and returned to the blood.

Hemoglobin

Hemoglobin (Hb) is a protein found in the red blood cells that carries oxygen in your body and gives blood its red color. Hemoglobin levels vary from person to person. Men usually have higher levels than women. The affinity for oxygen of Haemoglobin varies depending on the partial pressure of oxygen which is a measure of oxygen concentration.

Each hemoglobin molecule is made up of four heme groups surrounding a globin group, forming a tetrahedral structure. Heme, which accounts for only 4 percent of the weight of the molecule, is composed of a ring like organic compound known as a porphyrin to which an iron atom is attached. It is the iron atom that binds oxygen as the blood travels between the lungs and the tissues.

Hemoglobin develops in cells in the bone marrow that become red blood cells. When red cells die, hemoglobin is broken up: iron is salvaged, transported to the bone marrow by proteins called transferrins, and used again in the production of new red blood cells.

Mammalian Heart and Cardiac Cycle

  • The human heart has a mass of around 300g and is roughly the size of a closed fist
  • The heart is a hollow, muscular organ located in the chest cavity.
  • It is protected in the chest cavity by the pericardium, a tough and fibrous sac.
  • In humans, the heart is about the size of a clenched fist; it is divided into four chambers: two atria and two ventricles.
  • There is one atrium and one ventricle on the right side and one atrium and one ventricle on the left side.

Coronary arteries

  • The heart is a muscle and so requires its own blood supply for aerobic respiration
  • The heart receives blood through arteries on its surface, called coronary arteries
  • It’s important that these arteries remain clear of plaques, as this could lead to angina or a heart attack (myocardial infarction).

 

Valves of Heart

  • Tricuspid valve: located between the right atrium and the right ventricle.
  • Pulmonary valve: located between the right ventricle and the pulmonary artery.
  • Mitral valve: located between the left atrium and the left ventricle.
  • Aortic valve: located between the left ventricle and the aorta.

Cardiac Cycle

The cardiac cycle is the performance of the human heart from the beginning of one heartbeat to the beginning of the next. It consists of two periods: one during which the heart muscle relaxes and refills with blood, called diastole, following a period of robust contraction and pumping of blood, called systole.

  • The contraction of the heart is called systole, while the relaxation of the heart is called diastole
  • Atrial and Ventricular diastole is the period when chambers are relaxed and filling with blood.
  • Atrial systole is the period when the atria are contracting and ventricular systole is when the ventricles are contracting
  • Ventricular systole happens around 0.13 seconds after atrial systole
  • During ventricular systole, blood is forced out of the pulmonary artery (to the lungs) and aorta (to the rest of the body)
  • One systole and diastole make a heartbeat and last around 0.8 seconds in humans. This is the cardiac cycle.